PRINCIPLES AND METHODS
Prof.MVDr Václav
K o u b a, DrSc.
Prague
2008
P R E F A C E
The objectives of modern action-oriented epizootiology education/training consist in
achievement that the learners:
will know the characteristics of
health importance of animal populations, etiological agents and environment,
sources and ways of transmission of etiological agents, infection and epizootic
processes, influencing factors, diseases common to man and animals and
consequences of animal population health and diseases;
will be able to trace disease
outbreaks, investigate, analyze, monitor and survey animal population health
situation, elaborate strategy and identify measures for animal population
health protection and recovery, investigate outbreaks and apply effective
measures, elaborate and organize animal health programmes and evaluate their
results.
Epizootiology principles and methods are applicable on any specific health and
disease of any animal population species, herd, flock and group at any place,
time and level.The structure of the textbook is based on three main components
- principles of animal population health/disease processes, diagnostic/analytic
methods and practical preventive and control methods.
The theoretical study must be combined with
practical field training under the real or simulated conditions. Practical activities
start and end in the field and the main criterion is final result at the grass-root level of animal populations.
The textbook represents important component
of a set of teaching and self-study materials for undergraduate and
postgraduate courses. Examples of successful practical application of the epizootiology
principles and methods can be found also in http://vaclavkouba.byl.cz. In
order to facilitate the training and problem solutions a special software EPIZOO
has been developed for health/disease analyzing, planning and problem
solution at animal population level.
Prof.MVDr Vaclav K o u b a , DrSc. *)
-------------------
*) Former:
Chief, Animal Health
Service, Food and Agriculture Organization of the United Nations (FAO), Rome;
Veterinary Public Health Expert, World Health Organization (WHO), Geneva;
Informatics Expert, International Office of Epizootics (OIE), Paris; Editor-in-Chief,
global FAO/OIE/WHO Animal Health Yearbook; Member of Education Committee, World
Veterinary Association (WVA), Madrid; National Chief Epizootiologist and
Technical Director of Veterinary Services, Prague; Professor of Epizootiology,
University of Veterinary Sciences, Brno; Visiting Professor of Havana, Mexico
City, Kosice and Prague University
TABLE OF CONTENTS
1. Epizootiology: definition, objectives,
objects
and methods
2. Animal population and its characteristics of epizootiological
importance
3. Animal population disease resistance
4. Animal population health and disease
5.
Animal population collective health
6.
Animal population morbidity and mortality
7.
Animal population epizootiological structure
8.
Etiological agents of animal population diseases
9.
Sources of biological etiological agents
10. Transmission of biological etiological
agents
11. Natural environmental factors
12. Interaction animal-etiological agent-environment
13.
Epizootic process
14.
Animal population disease nidality
15.
Diseases common to man and animals
16.
Economic and social factors influencing epizootic process
17. Consequences
of animal population health and
diseases
18.
Investigation of epizootiological situation
19.
Epizootiological information system
20.
Analysis of epizootiological situation
21.
Epizootiological monitoring and surveillance
22.
Epizootiological theory, experiments and studies
23.
Epizootiological strategy and measures
24.
Active creation of animal population health
25.
General preventive measures in animal population
26.
Protection of animal population specific health
27. Epizootiological protection of country
territory
28.
Animal population general health recovery
29.
Animal population specific health recovery
30.
Measures against diseases common to man and animals
31.
Epizootiological sanitation
32.
Planning of epizootiological measures
33.
Organization of epizootiological activities
34.
Results and efficiency of epizootiological programmes
B i b l i o g r a p h y
A n n e x : Selected
epizootiological indicators
1.
EPIZOOTIOLOGY:
DEFINITION, OBJECTIVES, OBJECTS AND METHODS
=============================================================
1.1 Epizootiology definition:
Epizootiology is the
science which studies origin, frequency, distribution, development and
extinction of animal health and disease at populations, herds and flocks levels
as well as causes and influencing factors and based on their analyses defines
the methods for creation, protection, improvement and recovery of collective
health by reducing, eliminating and eradicating common diseases.
Notes: Name is from Greek: epi=upon;
zoon=animal. Syn.: veterinary epidemiology, animal epidemiology, animal
population medicine, etc.
1.2 Epizootiology objectives
a) Only healthy animal populations, herds and
flocks create the conditions for effective and wholesome production, for the
protection of human population against diseases from animals, for desirable
animal exploitation and welfare. The main objective of epizootiology is to
provide the methods contributing as much as possible to these conditions.
b) Following groups of objectives can be
distinguished:
- to create
actively diseases free animal populations, herds, flocks, groups and
territories;
- to
protect infectious diseases free populations, herds, flocks, groups and
territories against the introduction of these diseases;
- to
protect non-infectious diseases free populations, herds, flock and groups
against etiological agents and factors causing these diseases;
- to
protect human population against diseases transmissible from animals;
- to
improve and recover the health of animal populations, herds, flocks and groups;
- to
reduce, eliminate and finally eradicate specific mass diseases of animals
following the order of priorities and respecting feasibility aspects.
1.3 Epizootiology objects
a) The main objects of epizootiology are
- animal
populations, herds, flocks and groups
- etiological
agents and factors of mass diseases
- environmental
factors
and their
interactions determining the origin, development and extinction of animal
health and diseases.
These three main
objects are called "epizootiological triad".
b) Epizootiology studies
aa) animal
population characteristics of epizootiological importance such as species
and categories structure, functions, ethology, development, resistance and
susceptibility, collective health, morbidity, mortality etc.;
bb) etiological agents
and their complexes from the point
of view of their characteristics of epizootiological importance such as
specificity, pathogenity, tenacity, sources, ways of transmission, etc.;
cc) environmental
factors and their complexes influencing animal populations and etiological
agents as well as their interactions;
dd) interactions
between animal population and etiological agents under different external
influences being reflected in collective health, morbidity and mortality,
epizootic processes and their characteristics including the nidality as well as
in the biological, economic, public health and social consequences;
c) Epizootiology covers
the population health and disease processes related to any species of
animals, to any species of etiological agents and to any
influencing environmental factors.
1.4 Epizootiology methods
a) Diagnostic methods reveal the
epizootiological reality detecting healthy and diseased animals, groups, herds,
flocks and populations, etiological agents and their sources and influencing
environmental factors in space and time.
b) Descriptive methods consist of the
collection, compilation and processing results of the diagnostic activities,
available data on animals, etiological agents and external factors.
c) Analytical methods are based upon the
results of epizootiological investigations and relevant data processing making
possible the evaluation of the true epizootiological situation and its
development. The analyses are the bases for decision making on epizootiological
strategy, programmes and measures.
d) Statistical methods applying
essentially the principles of probability calculations are necessary for
processing and interpretation of data of epizootiological importance. The backbone
of these methods is the complex of epizootiological indicators.
e) Experimental methods are used for the
confirmation of the epizootiological hypotheses and for new methods development
and testing.
f) Theoretical methods are based mainly upon the
generalization of scientific finding and practical experience as well as
modelling and simulation of epizootic processes.
g) Action field methods for creation,
protection, improvement and recovery of collective health by reducing,
eliminating and eradicating common diseases.
1.5 Epizootiology division
a) General epizootiology
concerns the study of the origin, development and extinction of animal
population general (crude) health and general (crude) morbidity as well as the
common factors and principles related to general epizootiological strategy and
measures. It studies general methodologies applicable on any specific animal
health and disease(s).
b) Special epizootiology concerns the
study of the origin, development and extinction of animal population specific
health and specific diseases as well as the factors and principles related to
specific epizootiological strategy and measures. It studies specific
methodologies applicable on particular health and diseases.
c) Epizootiology can be divided according to other
criteria such as:
- transmissibility
of etiological agents (epizootiology of transmissible diseases, epizootiology
of no-transmissible diseases);
- etiological
groups and complexes;
- animal
species and categories;
- environmental
factors;
- characteristics
and phases of epizootiological processes;
- space
and time;
- diagnostic
procedures (e.g.,sero-epizootiology, clinical epizootiology);
- genetic
aspects, molecular biology, etc.
1.6 Epizootiology relations to other
sciences
a) Epizootiology studies the reality, formulates
theories and principles related to mass phenomena in animal population
health and disease influencing many other scientific branches. On the other
hand it is supported by a series of knowledge and methods of other scientific
branches creating interdisciplinary approach and facilitating the synthesis of
all relevant aspects for epizootiological diagnosis, analysis and actions. The
health/disease mass phenomena are based on individual animals' phenomena which
represent integral components of the populations and their characteristics.
b) From the methodological point of view, the human
epidemiology is the closest scientific branch. Both have many common
principles of study and measures. Homo sapiens belongs
from biological point of view also to animal kingdom. Study of diseases common
to man and
animals and the measures taken against them which should be perfectly
coordinated to achieve common objectives.
c) Among the other related scientific
branches belong: biology, zoology, morphology, physiology, immunology,
haematology, reproduction, genetics, ethology, etiology, microbiology,
parasitology, entomology, toxicology, traumatology, pathology, clinical
'propedeutics', preventive medicine, internal medicine, pharmacology, ecology,
animal husbandry, nutrition, zoohygiene, food hygiene, food technology,
climatology, geography, statistics, economics, sociology, etc.
2. ANIMAL POPULATION
AND ITS CHARACTERISTICS OF EPIZOOTIOLOGICAL IMPORTANCE
===========================================================================
2.1 Introduction
Animal population and its characteristics and
dynamics create the background upon which health and disease processes
develop. Animal population development changes, horizontal and vertical
movements influence significantly the location, development and dynamics of
population health and disease.
2.2 Population numbers, species and
categories
a) First step in
analyzing animal population is to identify their size in a given space
and time. Number of animals represents the basic denominator for the majority
of health and disease epizootiological indicators.
Note: Animal population at epizootiological
risk is often the denominator in measurement of disease frequency relative
indicators.
b) The total number of
animals can be subdivided according to species (or group of species), categories
(breed, strain, sex, age, weight, physiological, genetic and nutritional status,
yielding, etc.), territorial and time distributions, etc.
2.3 Space aspects
a) Space aspects create
the framework for disease space distribution and propagation and their
analysis.
b) Location
requires to identify the places according to geographical,
ecological, organizational and administrative structures using relevant
statistical data and territory maps.
c) Density
(dispersion) represents the number of animals per one surface measure unit
(km2, ha, etc.) in a given territory. Concentration represents the
number of animals per one space unit of breeding/production establishment
(stable, farm, ranch, etc.). For this purpose there is a need for data on the
number, size and distribution of herds, flocks, farms, ranches, zones, etc.
using clear-cut definitions.
On the other hand the average value of
surface or space measure units per one animal (e.g. stocking rate - ha/animal,
m2/animal) is also of epizootiological importance. Increasing animal density
and concentration create better conditions for transmissible diseases
propagation and vice versa.
d) Fluctuation
of wild animal populations density reflects in
fluctuation of many diseases with natural nidality. Some of them need for their
propagation a minimal (critical) value of susceptible animals
density (e.g. rabies in fox population).
2.4 Time aspects
a) Time
identification is a must to avoid misunderstanding when evaluating any
aspects of animal populations. It must be considered that the number and
characteristics of animal populations are changing continuously.
b) The number of
animals must be distinguished according to time factors: existing at a moment,
during a period or in average.
c) Duration of
the existence of animals is used in other group of epizootiological parameters
based on "animal-time" units needed for certain types of morbidity
rates.
2.5 Ethological behaviour
Ethological behaviour of animal population
such as intra and inter-species relations, cooperation, commensalism,
mutualism, competition, predacity etc. is of particular epizootiological
importance in wild animals.
2.6 Animal population dynamics
a) Animal populations,
herds, flocks and groups are characterized also by permanent changes,
development and horizontal/vertical movement influencing the dynamics of animal
health and disease.
b) Animal population development
has different forms. Among them belong genetic, age, morphological,
physiological, resistance/susceptibility and ethological changes contributing
to the infinite variation of animal disease pictures, courses, and impacts.
Among the development criteria can be mentioned: age (at weaning, at a given
weight, at first parturition, at market maturity, etc.), weight (weight at
birth, at weaning, total litters weight at birth, at a given age, market
weight, initial and final weight in feedlots, etc.), growth rate (weight gain
to weaning, daily or total live weight gain, rate of gain, etc.).
c) Changing number
of animals in population can be expressed in following indicators:
chronological time series, comparative indexes (current and/or chain), average
of changing numbers during sub-periods, seasonality, changing number in
reproduction and/or production cycle, tendency (linear, cyclic, ascending,
descending, continuing), etc. A
measure of population numeric growth (without migration) comprises addition of
new born animals to the population and subtraction of deaths (natural and
artificial).
d) Horizontal
movement of population is characterized by the number of moved
(transferred) animals, direction (origin and destination places), distance and
velocity (duration). A particular role has international export/import having
caused the introduction of many so called "exotic diseases" into
specific disease free countries. In wild animals an important role has the migration
which could be within the local environment or within major territorial
framework up to intercontinental one.
e) Vertical movement
is characterized
by continuing replacement of one generation by a new one. Following criteria
are used: viability index (new born/dead ratio), replacement rate (new
born/total number), survival rate (numbers at end/beginning), etc. This
movement facilitates the transmission of many infectious diseases from elder
generation to a new one within reproduction and production cycles. For reproduction process
evaluation following criteria (indicators) are used: fertility (new
born/females of reproduction age), natality (new born/total number), weaned
rate (weaned/new born), conception rate, gestation length, parturition
intervals, service period, etc.
2.7 Production systems and other
external factors
a) Animal population
characteristics, development and dynamics are influenced by different external
factors such as breeding, production, marketing and slaughter systems as well
as by different ecological factors. Each of them creates different conditions
for the origin, development and results of animal diseases.
b) Among the breeding
and production systems are: primitive natural breeding, extensive breeding and
production in pastures, intensive breeding and production, production of
industrial type as well as mixed systems. Among other factors belong:
concentration, specialization, type and grade of exploitation, technology based
on grazing or foraging or other feeding system, confined or non-confined
systems, type of mating, genetic programme, hygiene, grade
of exposition to wild animals, mechanization, management, grade of
commercialization, etc.
c) Organization and
structure of breeding and production have also the influence on animal
health/disease. Organization with open
system requiring to introduce animals from other
localities facilitates the introduction of diseases. This is not the case in closed
system being
self-sufficient and without a need to introduce animals.
d) Marketing
systems define the modes and ways of selling animals and their products
creating the routes for their movement and places of their concentration
(market places) which represent also the routes and concentration of many animal
diseases.
e) Animal populations
are integral parts of respective ecosystems. Therefore following factors
must always be taken into consideration: atmospheric (climate, air temperature,
rainfall, etc.), geospheric (soil, geomorphological relief, etc.), hydrospheric
(water distribution, arid zones, humid zones, etc.) and biospheric (flora,
fauna). The biocenosis where the relationship among animals is linked by food
chains defines the variety of animals and their populations
structures in a particular area.
Note: More information in
chapters 11 and 16.
3. ANIMAL POPULATION DISEASE RESISTANCE.
=========================================
3.1 Introduction
a) Animal population
disease resistance is represented by the collective body mechanisms which
interpose barriers to the progress of invasion or multiplication of infectious
agents or to damage by their toxic products in animals at population and
herd/flock levels. The collective resistance is based upon the resistance mechanisms of
individual animals creating a defense complex of animal population.
b) Animal population
disease resistance (lack of susceptibility) contributes often decisively
to the restriction up to interruption of etiological agents circulation and
thus to the restriction or interruption of the epizootiological chain, i.e.
epizootic process.
c) Resistant animals
possess specific protective antibodies or cellular immunity as a result
of previous infection or immunization, or is so conditioned by such previous
specific experience as to response adequately with production of antibodies
sufficient to prevent clinical disease following exposure to the specific
infectious agents.
d) Susceptible animals
do not possess sufficient resistance against particular etiological agents to
prevent contracting a disease if or when exposed to these agents.
e) Immunity is
relative. An ordinarily effective protection may be overwhelmed by an
excessive dose of the infectious agents or via an unusual portal of entry; may
also be impaired by immune-suppressive drug therapy or concurrent disease.
f) Degree of
specific resistance varies according to the type of etiological agents and
their immunizing capacity, the dose of infection, the antibody-forming capacity
of the animals, etc.
3.2 Factors of general resistance
a) General
(non-specific) resistance is based on a series of morphological and
physiological barriers creating defense lines.
Primary defense serves to hinder or prevent the passage of
etiological agents into the animal tissue. Secondary or parenteral defenses
deal with such agents which have managed to enter the tissue in spite of
primary defense of the animal organism.
b) Cutaneous system. Before the tissue of the body can be reached
one of the epithelial coverings, skin or mucous membranes must be penetrated.
These integuments, the skin particularly, serve as effective physical and
chemical barriers. The sweat contains anti-microbial substances reducing up to
destroying many pathogenic microorganisms.
c) Mucous membranes
are protected by the mucus trapping small particles, including microbial
pathogens and exposing them to lysozyme and local antibodies such as
immunoglobulin in respiratory secretions and faeces. In the digestive tract
mucus gathers into masses which are then carried through the canal with its
content. In the respiratory tract most of etiological agents are either coughed
up or swept up into the pharynx by the action of the ciliated epithelium and
swallowed. Protective role have also mucous membranes secretions and
excretions of the conjunctiva, urinary tract, vagina, stomach, intestines,
etc.
d) Among parenteral
defense belong substances such as lysozyme,
properdin, interferon and the complex of phagocytosis and
reticulo-endothelial system (RES).
More information see in immunology textbooks.
3.3 Inherent specific resistance
a) Inherent specific
resistance (natural innate immunity) is an ability to resist disease independent
of antibodies or of specifically developed tissue response; it commonly
resides in anatomic or physiologic characteristics of the host and may be
genetic or acquired, permanent or temporary. This resistance is more often
genetic in origin and is not dependent on previous contact with etiological
agents by the individual or its parents.
b) Species-related
differences vary greatly
in their resistance to etiological agents
whether infectious or
non‑infectious. Monohostal infections are known only in one species
(e.g. equine infectious anaemia in horses). Polyhostal infections are known in
many species (e.g. salmonellosis, tuberculosis,etc.).
c) Breed-related
differences in resistance. Some breeds may adapt better to certain
environments while others fail to thrive
under similar conditions. (E.g., the breed N'dama is much
more resistant against trypanosomiasis than other breeds).
d) Age-related
differences in response to infectious diseases may be due to maternal
antibodies, previous exposure, etc. (E.g. gastrointestinal and respiratory
diseases are in calves with much higher frequency than in adult cattle).
e) Physiological and
production stages. All animals go through several stages throughout
their lives which
may be associated
with increased risk
of disease occurrence. (E.g. in
general, the higher producing cows are more likely to develop mastitis due to
reduced resistance) than the low producing animals.
f) Differences of individuals.
Many individual animals under the same conditions and exposure to etiological
agents do not become diseased at all when other animals of the same species,
category and age can under the same conditions and exposure become diseased. Physiological and
anatomic factors resulting
in conformational differences may be
risk factors for disease. (E.g. dairy cattle
with large udders are at greater risk of
developing chronic mastitis).
3.4 Acquired specific resistance
a) Acquired specific
resistance (immunity) is a result of previous exposure to an antigen - a
natural pathogen or foreign substance for the host. Antigen is a substance
(protein, polysaccharide, glycolipid, tissue transplant, etc.) that is capable
of inducing specific immune response. Induction of antigen may be by the
invasion of infectious organisms, immunization, inhalation, ingestion, etc.
b) Immunity is
the resistance usually associated with possession of antibodies having a
specific action on the microorganism concerned with a particular infectious
disease or on its toxin.
c) Specific immunity
is a state of altered responsiveness to a specific substance acquired through
immunization or natural infection. For certain diseases this protection lasts
for the life of the individual.
d) Passive immunity
is conferred by antibodies produced by another host and acquired naturally by a
new born from its mother - maternal transfer (via the placenta before birth,
the colostrum just after birth, via the egg yolk in birds and reptiles)
or artificially by administration of an antibody-containing preparation
(convalescent or hyperimmune serum, or immune globulin). This immunity is of
brief duration (days to month).
e) Active immunity
is the resistance developed in response to stimulus by an antigen (infecting
agent or vaccine) and usually characterized by the presence of antibodies
produced by the host. Active immunity is
attained either naturally by infection with or without clinical manifestations,
or artificially by inoculation of fractions or products of the infectious agent
or of the agent
itself in killed, modified or variant form. (See chapter 26). Active immunity
depends on cellular immunity which is conferred by T-lymphocyte
sensitization, and humoral immunity which is based on B-lymphocyte
response. Active immunity following infection or immunization can last for
several months, years or even lifelong, depending on the agent or
immunogen administered.
Individual immunity is relative and
not absolute. It depends on the nature of the agents (particularly their
virulence and
invasiveness), the dose of the infectious agents, the route of infection, the environment and
physiological condition of the host.
f) Antibodies
are protein molecules formed by exposure to a "foreign" or extraneous
substance, e.g. invading microorganism responsible for infection, or active
immunization. They may also be present as a result of passive transfer from
mother to new born animal, via immune globulin, etc. Antibodies have the
capacity to bind specifically to the foreign substance (antigen) that elicited
its production, thus supplying a mechanism for protection against infectious
diseases.
g) Herd immunity differs from
individual host immunity in that the former is dependent on the composition of
the population, i.e. on the proportion of infected, incubating, diseased,
convalescent, vaccinated, recovered and susceptible animals, and on the
presence of alternative hosts and carrier animals within the herd. Since these proportions are dynamic, herd
immunity can vary considerably over time, while host immunity usually is
present or absent and remains fairly stable over time, particularly in the case
of actively acquired immunity.
The value of herd immunity depends also on the population
dynamics, (number of births, deaths, removals and additions) as well as on the
degree of contact between individual animals.
Herd immunity level of
70‑80% may well
provide the degree of resistance needed to prevent epizootic
occurring.
h) The graduation
from susceptibility to resistance lead to a weak
to strong inhibition of multiplication of the infectious agents which have got
through. The maximum possible defense capacity is resistance which excludes the
animal from epizootiological chain formations. The distribution of susceptible
or resistant individuals with their transition forms, consequently, has a
decisive influence on the development of epizootiological chains. Many
susceptible animals lead to a rapid or even explosive development of these
chains through favourable contact possibilities between etiological agents
and the individuals. Conversely, an epizootiological chain formation can be
weakened or completely extinguished if a larger part of the population is not
susceptible, thereby causing the chance of contact between etiological agents
and susceptible animals to become very limited.
3.5 Animal population resistance structure and indicators
a) Population
structure according to resistance degree is composed of resistant
animals, susceptible animals and animals resistance-indifferent.
b) For their measuring
following indicators are mainly used: resistant animals
rate as the proportion of these animals in total number and susceptible
animals rate as the proportion of these animals in total number. In concrete cases time factor must be
respected, i.e. these general rates to be expressed as prevalence or incidence.
See chapter 20.
c) Other useful
indicator is immunized animals rate (vaccination coverage of population) which
could be combined
with herds or
territory vaccination coverage.
The immune‑effectiveness of vaccines used should be always taken into
consideration.
4. ANIMAL POPULATION HEALTH AND DISEASE DEFINITIONS
================================================
a) To use principal
epizootiological characteristics, such as animal population collective health
and morbidity in a uniform and comparable form, there is a need for clear cut
definitions of these phenomena. The definitions of the limits between the
animals considered as healthy (non-affected) and diseased
(non-healthy, affected, sick, ill, etc.) are of particular importance.
b) The general
definitions of animal population health and disease depend on the concepts
of understanding. There are many different criteria based on biological,
economical, public health and social aspects. Within the biological
criteria are differences among epizootiological, clinical,
morphological, microbiological, serological, parasitological, etc. aspects. The
animal considered as healthy from one aspect can be considered as
non-healthy from other aspect. The
problems are quite complicated if we consider that this criteria can be applied
in combined form.
c) Unfortunately, until
today does not exist any internationally accepted and used general definition
of animal health and disease. Therefore, in the literature many different
definitions can be found. (E.g. the
health is a state of dynamic balance in which an individual's or group's
capacity to cope with all the circumstances of living is at an optimum level).
d) In human medicine
for disease is understood any deviation or interruption of the normal structure
or function of any part of an organ or functional group of organs that is
manifested by symptoms and signs and for health the state of optimal physical,
mental, and social well-being and not merely the absence of disease (WHO).
e) There are different
approaches to general definitions of animal population health and disease
depending on the position of the evaluating specialists. There are often
differences evaluating the same animals and their groups, herds, flocks and
populations between the state and private veterinarians, between specialists of
importing and exporting countries, etc. The animal considered as healthy by one
specialist can be considered as non-healthy by other one.
f) From practical point
of view the most important aspect is that which respects first the
interest and welfare of human society. This anthropocentric approach
means to use first the criterion of animal population utility for the man.
However, the other criteria such as biological, ecological and economic ones
must be taken into consideration as well.
g) Therefore, the
animal population health is based not only on absence of clinical signs but
also on animal utility (performance) at least at the standard or expected
desired level corresponding to a given species and category under the given
conditions. The relativity of different criteria for the limits between
the health and disease under the changing conditions requires to use official
or consented definitions.
h) From epizootiological
point of view as healthy animals are understood those which are not only free
of clinical symptoms of specific disease(s) but also free of etiological
agents and living in disease free area without exposition to animals
with transmissible disease or to their sources during a particular period.
i) For transmissible
diseases of major importance national and international organizations issue
specific definitions mainly for the trade and control programmes purposes.
These definitions are linked with standard diagnostic methods procedure
and results interpretation.
j) Animal health and
disease have their structure and grading according to the levels of different
qualitative and quantitative criteria. (E.g. animals investigated serologically
are subdivided not only according to result positivity and negativity but also
according to the level of antibodies titles).
k) Animal population epizootiological
structure and grading can be expressed in groups of disease free animals
not at risk, disease free animals at risk (indirect, direct), suspect animals
(epizootiologically indeterminate), affected animals without clinical symptoms
(carriers, eliminators of etiological agents), affected animals with abortive
clinical symptoms and affected animals with manifest clinical symptoms.
l) Taking into
consideration the above mentioned criteria, following definitions can be
deduced:
aa) Animal
population h e a l t h is a biological system of dynamic and
multiform process in which the animals are free of undesirable
morphological and physiological deviations as well as of etiological agents
threatening health of other animals or man in the way that it makes possible to
utilize the animals and their products at least at the "normal" level
corresponding to the given species, race and category under the given
conditions.
bb) Animal
population d i s e a s e is a biological system of dynamic and
multiform process in which the animals are not free of undesirable
morphological or physiological deviations or of etiological agents threatening
health of other animals or man in the way that it does not make possible to
utilize the animals and their products at least at the "normal" level
corresponding to the given species, race and category under the given
conditions.
Note:
International definitions of specific
animal health (specific disease free status) related to the export/import
policy are described in "OIE International Animal Health Code".
International definitions of specific animal diseases of major importance for
the export/import policy are described in "OIE Recommended Diagnostic
Techniques and Requirements for Biological Products". Both publications
are subject of current updating.
5. ANIMAL POPULATION COLLECTIVE HEALTH.
===================================
5.1 Introduction
a) Animal population collective health is a
biological and dynamic system and process based on equilibrated interaction of
animals and environment without the interferences of etiological agents and
factors.
This process is realized within normal
physiological limits under favorable or supportable external conditions without
participation of agents or factors able to cause a disease.
b) Collective health is of primary importance
not only from preventive medicine but also from economic and social point of
view. Only healthy animals are capable to fulfil the existence purpose of a
particular population, especially in qualitative and quantitative terms of
production of food of animal origin.
c) Animal population
collective health process represents relative contrast to epizootic process.
Quantitative characteristics of collective health expressed in
"salubrity" indicators (proportions of healthy animals) are opposite
to animal population morbidity indicators. Increasing the values of
"salubrity" is accompanied by decreasing of morbidity and vice versa.
d) Animal population
collective health is not a simple sum of individual animals' health but an integral
phenomenon behaving as living being with all the attributes of biological
complex with internal and external interactions and relations.
5.2 Collective health process
5.2.1 Types and stages
a) Animal population
collective health process as equilibrated interaction between animals and
external factors consists in the origin, development and extinction of
healthy animal groups. This process maintains as unaffected the animals
born as healthy or animals recovered from a disease. These animals continue
living under favorable environmental conditions or are able to adapt themselves
to less favorable conditions.
b) Collective health
process from the etiological point of view can be differentiated basically in general
(covering all etiological aspects) and specific (related to specific
etiological agents).
As examples of g e n e r a l
collective health can be mentioned specific pathogen free breeding of
birds, infectious diseases free herds or flocks. As examples of s p e c i f i c collective health can be mentioned
brucellosis free herds of sheep, Newcastle disease free flocks of poultry,etc.
c) Collective health
process has its stages similarly as other biological systems. After the
initial stage commencing with minimum value of "salubrity", the
process continues through increasing stage to culminating stage with maximum
value of 'salubrity' which can be followed by decreasing stage up to eventual extinction
stage. The variability of this phenomenon is opposite to the stages of
epizootic processes.
5.2.2 Collective health forms
a) Clinical or
manifest form corresponds with the "normal" morphological and
physiological status.
b) Epizootiological
form is characterized by absence of etiologic agents in animals, i.e. not
presenting any epizootiological risk for other animals and man. For the
declaration that the animals are epizootiologically healthy, they have to
originate from healthy parents, disease free herds and zones having no contact
and exposure to animals with relevant infectious diseases.
c) Diagnostic
form is based on favorable results of health/disease investigations of animal
population (e.g. cattle herds with negative brucellosis serological tests).
Often before declaring a herd as disease free a specific test must be repeated
in defined intervals combining different investigation methods.
d) Production
and reproduction forms are based on yield criteria, i.e. as healthy animal
population and herds are understood those with
"normal" productivity and reproductivity.
e) There are other
criteria as mentioned in chapter 4. In concrete cases it is recommended to
combine more aspects in evaluation and declaration of collective animal health.
5.2.3 Collective health
occurrence
a) Population range
(width, size, extensity) of collective health is projected in the number and
proportion of healthy animals in total. For measuring prevalence rates
are used.
b) Population
collective health process intensity is projected in the number and
proportions of new healthy animals in total. For measuring incidence
rates are used.
c) Range and intensity
values of population collective health process are projected in sporadic,
localized, dispersed and total occurrence grades.
5.2.4 Collective health space
aspects
a) Space territorial delimitation
is linked with the localization of healthy population, herds, flocks and groups
using topographic criteria, natural (ecological) or artificial (economical,
organizational, administrative) ones.
b) Territorial range
(size) of healthy animal population and herds and flocks is based on
surface measure units in absolute and relative terms.
c) Territorial
intensity is reflected in the density and concentration of healthy
animals expressing the average number of healthy animals per surface or volume
measure units.
d) For geographical distribution
of disease free herds, flocks, zones, etc. epizootiological maps are used.
e) Healthy animals movements
can be at local, zone, province, national or international levels influencing
in the values of population "salubrity" in origin and destination
places.
5.2.5 Collective health time factors
a) For time
delimitation of collective health process the evaluation criteria such as
the moment, period, duration of the process and its stages as well as
time of the beginning and the end are used.
b) The variability
of this process is reflected also in healthy animals frequency and eventual
periodicity in form of cycles and seasonality.
5.3 "Salubrity" of
animal population
a) The
"salubrity" of animal population means the grade of health in
population, i.e. the proportion of healthy animals in total and their
relations with animals with other epizootiological characteristics. All
indicators of this kind can be divided according to the quality, content and
form of animal health (e.g., clinically healthy, epizootiologically healthy,
etc.). (The principles of the rates used below see in chapter 20 and the
formulae in Annex).
b) General indicator of
the "salubrity" is the healthy animals rate expressing the
relation between the number of healthy and total animals in a given space and
time. However, in concrete cases must be expressed as prevalence, incidence or
extinction rates.
c) Prevalence rates
of healthy animals mean the relations between the number of existing
healthy and total number of animals. In concrete cases time aspect of
denominator must be well defined (point prevalence rate at a given moment,
period prevalence rate during a given period or average prevalence rate during
a given period).
c) Incidence rates
of healthy animals mean the relations between the number of new
healthy animals in a given period and total number of animals. In concrete
cases time aspect of denominator must be well defined (total number of all
existing animals in the period, at a moment or in average).
d) Extinction rates
of healthy animals are opposite to
incidence rates.
e) Replacement rates
of healthy animals mean the relation of new healthy animals (born,
introduced, recovered) to the total number of healthy animals.
5.4 Viability of animal population
a) Animal population
viability or survivorship means the grade of the ability to survive and
reproduce. It can be expressed as the proportion of new born and surviving from
the total number of animals. Viability combined with mortality offers the
opportunity to evaluate population replacement and "vertical movement".
b) Animal population viability
index means the relation between natality and mortality, i.e. the relation
of the number of new born animals in a given period to the number of naturally
dead and slaughtered animals in the same period.
c) Conception rate
means the number of animals that conceive during breeding from the total
females of reproductive age inseminated or fertilized (naturally or
artificially).
d) Animal population natality
rate (crude live birth rate) means the relation of the number of live
births in a population over a given period to the total number of animals. In
concrete cases time aspects of the denominator must be well defined (existing
in the period, at initial moment or in average).
e) Animal population fertility
rate means the relation of the number of live births in a population over a
given period to the total number of females of reproductive age. In concrete
cases time aspects of the denominator must be well defined (existing in the
period or in average).
f) Animal population survival
rate (survived animals rate) means the proportion of survived animals at
the end of a given period from the total of animals existing at the beginning.
g) Animal survival-to-weaning
rate (weaned new born animals rate)
means the proportion of weaned animals from total new born animals.
h) Breeding animals
survival rate means the proportion of bred animals at the end of a given
period from the total of animals existing at the beginning of the breeding
period.
i) Fattening animals
survival rate means the proportion of fattened animals at the end of
fattening period from the total animals existing at the beginning of fattening
period.
Note: Formulae of selected indicators of animal population health see in
Annex.
6. ANIMAL POPULATION
MORBIDITY AND MORTALITY
============================================
6.1 Animal population morbidity
6.1.1 Introduction
a) Animal morbidity
representing the grade of disease in populations, herds and flocks is
the most important negative
epizootiological characteristics. It means the relation of the number of
diseased (non-healthy, affected, infected, invaded, sick, ill) to total
number of animals.
b) Animal population
morbidity can be:
aa) general (crude)
including animals affected by diseases without specifying the etiology, i.e. by
all diseases existing in the given population; it does not take any cause or
specific factor into account;
bb) specific
including animals affected by specific etiological agents or by a specific
etiological complex;
cc) according to non-etiological
aspects, i.e. clinical criteria considering symptoms only, age, etc.;
dd) combined,
e.g. cause and age specific morbidity rate measuring the morbidity in a certain
age group due to a certain cause;
ee) cumulative,
i.e. sum of subperiods' morbidity values.
c) In concrete cases
also the form of disease must be defined: acute, chronic, manifest,
abortive, subclinical, etc.
d) Morbidity indicators
are used for measuring disease frequency to analyze: space and time
distribution, spread, trends, etc. of the disease; implementation and
efficiency of programmes and measures to prevent and eradicate disease.
e) Morbidity indicators
have as nominator the number of diseased animals as a fraction of
the number of animals biologically capable of experiencing the event. These
animals are considered the population-at-risk. Total population is not always
at risk. Therefore, in concrete cases the denominator must be well defined
i.e. if representing population at risk or total population or only those
free from disease at the beginning of the period. In concrete cases this
phenomena must be clearly indicated to avoid confusion.
6.1.2 Rates of diseased animals
a) The general basis of
these indicators is the relation of the number of diseased animals to the total
number in the given space (place) and time. Three groups of these indicators
can be distinguished: prevalence, incidence and extinction. The basic difference
between prevalence and incidence is that the first is based on the number of existing
diseased animals and the latter on the number of new diseased animals (this
difference is often confused).
More information see in chapter
20 and in Annex.
b) Incidence is a
dynamic measurement and prevalence is static. If the disease course is short or
fatal the incidence is higher than prevalence. If the disease is chronic or not
very infectious the prevalence is higher than incidence. Incidence reflects
risk or the likelihood of contracting the disease in a given period. Prevalence
reflects the risk of having the disease in a specific time.
c) Prevalence rates
of diseased animals mean the relations of the number of diseased animals
(regardless of when that disease began) e
x i s t i n g at a given moment or
during a given period or in average from total number of animals existing at the
same moment or during the same period or in average in the same space:
- point (moment) prevalence rate of
diseased animals;
- period prevalence rate of diseased
animals (e.g. annual
prevalence);
- average prevalence rate of diseased
animals.
Apparent prevalence rate includes
only affected animals with clinical symptoms.
d) Incidence rates
of diseased animals mean the relations of newly diseased animals - n e w cases during a period to a total number of
animals taking into consideration the time factors:
- incidence rate of diseased animals to
existing total;
- incidence rate of diseased animals to
average total;
- incidence rate of diseased animals to
initial total.
aa) Sometimes the
incidence rate including all manifest and inapparent cases is called case
rate.
bb) Cumulative
incidence rate is the sum of subperiods' incidence values.
cc) Attack rate
is a specific incidence rate used in outbreak investigations. It is the
proportion of the new cases from the population-at-risk in the outbreak at the
beginning of the exposure. It is assumed all animals are specifically healthy
(i.e. without the given disease) at the beginning of the outbreak. Attack rate
is a cumulative incidence rate often used for particular group, observed for
limited periods and under special circumstances, as in an epizootic.
dd) Secondary attack
rate is the number of cases among contacts occurring within the accepted
incubation period following exposure to a primary case, in relation to the
total of exposed contacts; the denominator may be restricted to susceptible
contacts when determinable.
c) Extinction rates
of diseased animals mean the relations of
e x t i n c t diseased
animals (due to death, slaughter, remove, recovery) during a period to a total
number of animals taking into consideration the time factors:
- extinction rate of diseased animals to
existing total;
- extinction rate of diseased animals to
average total;
- extinction rate of diseased animals to
initial total.
Cumulative extinction
rate is the sum of subperiods' extinction values.
6.1.3 Other
indicators related to diseased animals
a) There is an other
group of indicators based on the relation of the number of diseased animals
to the number of animals of other epizootiological characteristics
such as healthy, investigated, resistant, susceptible, intrafocal, etc.
animals.
b) Diseased animals
replacement rate is the relation of new cases of diseased animals to
existing total diseased animals.
c) If the number of new
cases (events) occurring during a specified period is divided by the sum of the
animal-time units at risk for all animals during the period, the result is the animal-time
incidence rate (syn.: interval incidence density). This is a measurement
combining animals and time, used also in animal-time incidence and mortality
rates.
6.2 Animal population mortality
6.2.1 Introduction
a) Animal population
mortality means the relation of the number of naturally and artificially
dead animals in a given period to total number of animals in a given space and
time.
b) Animal population
mortality in a wide sense includes not only animals naturally dead (due
to diseases) but also artificially dead due to a slaughter (killing).
c) Therefore, the
indicators of animal mortality have to be distinguished in three group - total
(crude) mortality including all the forms of death, natural
mortality and artificial mortality.
d) All the above
mentioned forms of mortality can be subdivided in:
aa) general
mortality (crude death rate) which does not take any cause or specific factor
into account;
bb) specific
mortality (cause specific death rate) due to a specific cause including animals
affected by specific etiological agent or by a specific etiological complex
only;
cc) according to non-etiological
aspects, i.e. using clinical criteria considering symptoms only, age
specific death rate, etc.;
dd) combined
mortality, e.g. cause and age specific death rate measuring the mortality in a
certain age group due to a certain cause.
ee) cumulative
mortality is the sum of subperiods' mortality values.
6.2.2 Indicators of animal population total mortality
a) Animal population mortality rates are based
on the relation of the total number of dead animals to the total number
of animals in a given space and time. In concrete cases time factors must be
well defined distinguishing following indicators:
- animal mortality
rate to existing total (during a period)
- animal mortality rate
to average total (during a period)
- animal mortality rate
to initial total.
b) Proportional mortality rate expresses
the proportion of the deaths due to a particular cause from the total number of
deaths in a given space and period.
6.2.3 Indicators of
natural mortality
a) Animal population natural mortality rates are
based on the relation of the total number of naturally dead animals (due
to diseases, advanced age, hunger, etc.) to the total number of animals
in a given space and time. In concrete cases time factors must be well defined
distinguishing following indicators:
- natural mortality
rate to existing total (during a period)
- natural mortality
rate to average total (during a period)
- natural mortality
rate to initial total.
b) Proportional natural mortality rate
expresses the proportion of the number animals
naturally dead due to a particular cause from the total number of
naturally dead in a given space and period.
c) Neonatal mortality rate (the age range
for neonates must first be decided) expresses the proportion of the number of
neonates dying in a given age range from the total new born animals.
d) Abortion rate expresses the relation
of the number of abortions to the total number of females of reproductive age
in a given space and period.
6.2.4 Indicators of lethality (case fatality)
Animal population lethality rates are based
on the relation of the total number of naturally dead animals due to
specific disease to the total number animals diseased by the same cause in
a given space and time. In concrete cases time factors must be well defined
distinguishing following indicators:
- animal lethality
rate to existing total
- animal lethality rate
to average total
- animal lethality rate
to initial total.
6.2.5 Indicators of
artificial mortality
a) Animal population artificial mortality rates
are based on the relation of the total number of slaughtered animals
during a given period to the total number of animals in a given space
and time. In concrete cases time factors must be well defined distinguishing
following indicators:
- slaughtered
animals rate to existing total
- slaughtered animals
rate to average total
- slaughtered animals
rate to initial total.
b) Proportional artificial mortality rate
expresses the proportion of the number animals
slaughtered due to a particular
cause from the total number of slaughtered in a given space and period.
c) Condemned animals rate expresses the
proportion of the number of slaughtered or sanitary killed animals condemned
and destroyed to the total number of animals in a given space and period.
Note: Formulae of selected indicators of morbidity and mortality see in
Annex.
7. ANIMAL POPULATION
EPIZOOTIOLOGICAL STRUCTURE
===============================================
7.1 Introduction
a) Animal population
epizootiological structure is the composition of this population according
the size and proportions of its parts of different epizootiological characteristics.
b) The grades of
"salubrity" and morbidity are bases for this structure.
c) Each population
has its own epizootiological structure according to animal health/disease
situation and development related to changes in the characteristics of animals,
etiological agents and environment. The structure is a relatively dynamic
phenomenon.
d) The skeleton of the
structure is relative and it must be considered as artificial (based on
professional consensus); in reality it goes smoothly from the minimum to maximum
values of respective measure units.
e) Different grades of
population health/disease and their forms require different approach to a given
situation and ask for different measures.
7.2 Etiological,
clinical and morpho-physiological aspects
7.2.1 Etiological
aspects
a) The structure
according to specificity of etiological agents:
- general including all the aspects
of etiological factors, i.e. applying general (crude)"salubrity" and
general (crude) morbidity;
- specific based on specific "salubrity"
and specific morbidity.
b) The structure according
to the quantity of specific etiological agents:
- animals without
specific etiological agents
- animals with limited
quantity of these agents
- animals with high
quantity of these agents.
This
criterium is applied mainly in parasitic diseases such as helminthiases
combining aspects of their relative range (proportion of affected animals) with
intensity (number of helminths).
7.2.2 Clinical
aspects
a) Clinical criterion
is widely used for dividing animal population in a structure according
clinical signs:
- animals clinically
healthy
- animals clinically
indeterminate (including suspect animals)
- animals clinically
diseased (with clinical signs).
This is applicable for general aspects as
well as for specific disease aspects.
b) This structure based
on clinical investigation only cannot provide true epizootiological
picture because of covering only an apparent part of affected animals and not
all.
c) Clinical criterion
has major importance in diseases with high grade of clinical manifestation.
7.2.3 Morphological
and physiological aspects
a) According to
morphological and physiological criteria (reflected to some extent also in
clinical manifestation) animal population structure can be composed basically
in:
- animals without
morphological/physiological changes (general or specific)
- animals indeterminate
from the morphological and physiological point of view
- animals with
morphological/physiological changes (general or specific).
b) There are many
criteria of this kind, e.g.:
- levels of specific antibodies
titles
- levels of
haematological values
- levels of biochemical
values
- levels of
histological changes
- levels of specific
allergy reactions, etc.
7.3 Epizootiological
aspects
According to epizootiological criteria
animal population structure is based on the skeleton composed from animals
epizootiologically healthy, indeterminate and non-healthy.
It has to be
distinguished if the structure is based on general or specific health/disease.
7.3.1 Part of epizootiologically healthy animals
a) Animals
epizootiologically healthy are free of etiological agents and having no
any direct or indirect contact with diseased animals or other sources of
etiological agents.
b) According to the
grade of exposure to the sources of etiological agents (diseased
animals, foci, etc.) this part is subdivided in:
- animals
epizootiologically healthy non exposed
- animals
epizootiologically healthy exposed (at risk).
c) Animals exposed (at risk) are subdivided in:
- animals
epizootiologically healthy directly exposed
- animals
epizootiologically healthy indirectly exposed.
7.3.2 Part of
epizootiologically indeterminate animals
a) As animals epizootiologically indeterminate
are considered those which have not yet been identified if they are healthy or
non-healthy. In this part suspect and dubious animals are also included.
Suspect animals are those which health history and symptoms suggest that it may
have or be developing some communicable disease.
This part is artificial, transitional and
temporal used only up to the final decision.
b) Indeterminate animals are subdivided
according to the relation to diseased animals and other disease sources in:
- animals-direct
contacts
- animals-indirect
contacts.
c) Indeterminate animals are subdivided
according to clinical manifestation in:
- animals with
clinical symptoms
- animals without
clinical symptoms.
7.3.3 Part of epizootiologically non-healthy animals
a) Epizootiologically
non-healthy animals are those which cannot be included in the two
above-mentioned parts, i.e. animals showing clinical signs of disease or so
called subclinical cases determined by laboratory tests.
b) Non-healthy
(diseased, infected, invaded, affected, sick, ill) animals are subdivided
according to the presence/absence of etiological agents in:
- non-healthy animals
being hosts of etiological agents
- non-healthy animals free
of etiological agents.
c) This part of animals
can be subdivided according to elimination of etiological agents in:
- non-healthy animals -
eliminators of etiological agents
- non-healthy animals -
non-eliminators of etiological agents.
d) Non-healthy animals
are subdivided according to clinical criteria in:
- non-healthy animals with
clinical symptoms
- non-healthy animals without
clinical symptoms (which can be detected only by particular laboratory or
allergic investigations).
e) Non-healthy animals
without clinical symptoms are subdivided in:
- animals affected subclinically with inapparent
morphological or physiological changes
- animals-simple latent carriers of
etiological agents.
7.4 Structure evaluation
a) Animal population
epizootiological structure can be evaluated in absolute data or in relative
data (proportions).
b) Using absolute
data the structure is based on simple sizes (number of animals) of partial
components of different epizootiological characteristics.
c) Using relative
data the structure is based on the rates (proportions) of the
sub-parts of animals with different epizootiological characteristics (e.g.
prevalence rate of healthy animals, prevalence rate of indeterminate animals
and prevalence rate of diseased animals).
8. ETIOLOGICAL
AGENTS OF ANIMAL POPULATION DISEASES
===================================================
8.1 Introduction
a) Etiological
agents (causing agents of disease) are the factors, such as microorganisms,
chemical substances, physical phenomena, etc., whose presence, excessive
presence, or (in deficiency diseases) relative absence is essential for the
occurrence of a disease.
b) A disease may have a
single agent, several independent alternative agents (at least one must be
present), or a complex of two or more factors whose combined presence is
essential for the development of the disease.
c) Majority of
epizootiological techniques which has been originally developed for the study
of animal infectious diseases is also suited for non-infectious diseases.
d) In this chapter only
the characteristics of epizootiological importance are described.
8.2 Biological
etiological agents
8.2.1 General aspects
a) Biological
etiological agents are an animal or vegetable organisms that live on or in
animal macroorganism and derives its nourishment therefrom.
An obligate pathogen-parasite is one
that cannot lead an independent non-parasitic existence. A facultative
pathogen-parasite is one that is capable of either parasitic or independent
existence.
b) Among biological etiological agents
(infectious agents) belong pathogenic viruses, bacteria, rickettsia, fungi,
protozoa and helminths that are
capable of producing infection or infectious disease. Among biological
etiological agents belong also some species of pathogenic arthropods.
8.2.2 Specificity
a) Specificity of
etiological agents is fundamental for
differentiation of specific diseases and epizootiological situation
based on morphological, physiological, genetic, biochemical, etc. features of
these agents.
b) Specificity has
different levels of biological classification within the respective hierarchy.
The levels begin with biological class, being subdivided according to
biological order, family, genus and terminate with biological species
eventually subspecies (types, subtypes).
c) There are many
thousands of etiological agents species already known. The complexity is
stressed by permanent changes, the origin of new species and subspecies
combined by disappearance of others.
d) If we consider this enormous
spectrum of etiological agents and their dynamic changing properties, then
it can be appreciated the complexity of epizootiological problems and
difficulty to decide about the most effective practical preventive and control
measures.
e) On the other hand
the knowledge of specificity facilitates not only etiological diagnosis but
also to trace the route of specific agents (types, subtypes). (E.g.
"molecular fingerprinting" permits to identify associations
between various subtypes
of agents and
certain host species).
8.2.3 Pathogenity
a) Pathogenity is the capability
of infectious agents to cause disease in a susceptible host.
b) Pathogenity of
agents depends on the ability to invade
the tissues (invasiveness) or to produce toxins (toxigenicity).
c) Infectivity
is the characteristics of the infectious agents that embodies capability of
enter, survive, and multiply in the host.
d) The degree of pathogenity
is measured by the virulence.
Pathogenic organisms may be
of high,
moderate or low virulence.
The virulence of the same pathogenic organism may be increased by passage in the
susceptible host or may be decreased by
passage in resistant host or cultivation
under unfavorable conditions whereby it may loose its pathogenity completely
and becomes avirulent.
e) Changes in pathogenity may be
phenotypic or genotypic in nature. Phenotypic are transient and confined
to the one generation only while genotypic changes result from a
change in the genome as a result of mutation, conjugation, or transformation.
f) In general the higher dose of the agent whether living or non‑living
the more likely is the host to be affected as measured by morbidity or
mortality. In experiments this can be
calculated as infection dose
or lethal dose to infect 50% (ID 50) or
kill 50% (LD 50) of the exposed animals.
g) Some of the agents
can be conditionally pathogenic (opportunistic agents) acting only under
abnormal conditions (e.g. stress of animals).
8.2.4 Tropism
a) Tropism of
etiological agents is specific tendency to penetrate only or mainly into
determinate issues or organs of animal macroorganism.
b) Pantropism
consists in etiological agent ability and tendency to penetrate into the whole
body of affected animals while organotropism means the tendency to
penetrate into particular organs (e.g. enterotropism, neurotropism,
pneumotropism, dermotropism, epitheliotropism, etc.).
8.2.5 Selectivity
a) Selectivity of
etiological agents is natural affinity for particular species of animal
hosts (host range), i.e the causing organism may be specific to a certain
host(s).
b) Some etiological
agents may affect only one animal species (monohostal agents) while
others may infect a wide range of hosts (polyhostal agents).
c) The same agents may
be pathogenic to a certain host i.e. susceptible but be non‑pathogenic to another host which
is resistant on the basis of innate or acquired immunity.
d) Within a genus there
may be variation in host range according to
species. Within a species there may
be various subtypes such
as phage or plasmid types which show a
predilection for certain hosts. In general, the wider the host range, the more
likely are the agents to spread and resist to control measures.
8.2.6 Adaptability
a) Etiological agents adaptability is their ability
to adapt themselves to changing conditions of the host and or
environment. The adaptation can conduce to increased or decreased pathogenity
changing agents impact on epizootic triad among agents-animals-environment.
b) Usually external influences of longer
duration may cause the adaptation with the tendency of etiological agents to
better survive and/or reproduce. (E.g. resistance of some pathogens against
some antibiotics being used for a longer period).
8.2.7 Reproducibility
a) Reproducibility of
etiological agents is their ability to be reproduced keeping the strain
alive and ready for further actions in epizootiological chain.
b) The speed of
reproducibility plays important role in affecting the host and in spreading the
particular disease. This influences the amount of infection doses and the
intensity of agents elimination.
c) Some etiological
agents can reproduce only within the macroorganism of the host, some only
outside of animal body (e.g. parasites with more complex vital cycle)
and some under both conditions.
8.2.8 Heritability
Heritability of etiological agents is their ability
to transfer specific genes (features) of the species upon new
generations keeping the basic innate characteristics.
8.2.9 Immunogenicity
a) Immunogenicity
(antigenicity) is the ability of etiological agents to produce a
systemic or a local immunological reaction in the host.
b) Antigenic drift - evolutionary
changes take place in the molecular structure of DNA/RNA in microorganism
during their passage from one host to another. It may be due to recombination,
deletion or insertion of genes, to point mutation, or to several of these
events. This process leads to alteration (usually slow and progressive) in the
antigenic composition, and thus in the immunologic responses of individuals and
populations to exposure to the microorganism concerned.
c) Antigenic shift - mutation,
i.e. sudden change in molecular structure of DNA/RNA in microorganisms,
especially viruses, which produces new strains of the microorganism. Hosts
previously exposed to other strains have little or no acquired immunity against
the new strain.
8.2.10 Tenacity
a) Tenacity (viability)
is the ability of etiological agents to survive outside the host.
Some agents can produce spores which are highly resistant to adverse
environmental conditions, while other organisms are extremely fragile outside
the host.
b) Therefore the period
of surviving of particular etiological agents in different substances and
conditions is extraordinary important for application of sanitation measures
and for declaring a surface (area, zones) as free of these agents.
8.2.11 Transmissibility
a) Transmissibility is the ability of
etiological agents to be transmitted from one host to other.
b) According to transmissibility etiological
agents can be divided in:
- contagious
propagating through direct or indirect contacts among healthy and diseased animals
- non-contagious
propagating indirectly through intermediate animals - carrier of lower
biological classification (e.g. arthropods) or through non-alive substances in
which etiological agents are able to survive and reproduce (e.g. in soil).
8.2.12 Life cycle
a) Life cycle of
etiological agents is their development process during one generation.
b) The majority of
microorganism - etiological agents (viruses, bacteria) has their life cycle within
the body of a macroorganism - host.
c) A part of
etiological agents require for the life cycle to pass through more than one
host (inter-hosts) composing different life stages (within the
body of animals of lower biological classification such as arthropods, etc.).
d) Other etiological
agents are able to reproduce exclusively or also outside of animal body
(e.g. in soil, water,etc.).
8.2. Variability
Variability of etiological agents is their ability
to change their characteristics under changing conditions. These changes
can be temporal or lasting. This is valid for all above mentioned features.
8.2.14 Other characteristics
a) In many diseases
there is a need for interaction of etiological agents. More than one
agent is necessary to produce disease in
a susceptible host (e.g. respiratory syndrome disease). Other agents may
depress the immune system such that a second agent causes serious
disease (e.g. virus diarrhoea infection
followed by a severe Salmonella
infection).
b) Non‑living
agents may also interact with living agent (e.g. intoxication may predispose to
infection by certain infectious agents).
8.2.15 Higher parasites
Some higher parasites act only as
etiological agents (e.g. warble fly, screwworm) and some also as vectors
of different lower etiological agents (e.g. ticks and tick-borne diseases).
8.2.16 Conditionally pathogenic and saprophytic microbes
a) Some microbes living under normal conditions
inside of animal body are harmless. However, under the worsening conditions of
animals (stress, hunger, heat, frost, etc.) they can become pathogens causing
diseases. Similar consequences can have complex of microbes living as
saprophytes.
b) Some agents may also
cause disease only when the host
resistance is lowered such as may occur during drug therapy or intercurrent
disease. These agents are called conditional or opportunistic
pathogens and may be endogenous in origin.
b) In this context is
not recommendable to mix groups of animals originating from different
environmental conditions, i.e. with different internal and external microflora.
8.3 Genetic etiological factors
Genetic etiological factors cause hereditary
diseases (defects) being transmitted from one to other generation. These
defects can be lethal, semilethal or non lethal.
These abnormalities can be morphological,
functional, trophic, neoplastic, etc.
8.4 Chemical etiological agents
Among chemical etiological agents belong
poisons from toxic plants, snakes or insect, toxic chemicals such as
pesticides, insecticides or heavy
metals.
These agents cause toxicoses, intoxications and
toxic residues in products of affected animals.
8.5 Physical etiological factors
Among physical etiological agents belong
mechanical actions, bites and extreme values of temperature (heat,cold),
atmospheric pressure, sunlight, electricity, radiation, etc.
8.6 Deficiency diseases etiological factors
Nutritional diseases are caused by the
excess or deficiency of macro‑or micro‑elements in the
diet. Also lack of sufficient amount of drinking water can cause serious
disease and death.
8.7 Complexes of etiological agents
Relatively often etiological agents act as
etiological complexes composed from different agents combining different
biological agents with or without chemical and/or physical factors causing
diseases with high variability of clinical and epizootiological picture.
9. SOURCES OF
BIOLOGICAL ETIOLOGICAL AGENTS
===========================================
9.1 Introduction
a) Sources of biological etiological (infectious)
agents are animals, human beings, arthropods, plants, soil or inanimate matter
in or on which these agents survive and
from which they can be transmitted to a host.
b) Particular types of
sources are reservoirs of biological etiological agents in which
infectious agents normally live and multiply, and on which they depend
primarily for survival and reproduce themselves in such manner that they
can be transmitted to a susceptible host.
9.2 Characteristics
of sources
9.2.1 Epizootiological
importance of sources
a) The importance of sources depends first on
epizootiological importance of respective etiological agents. Other
factors are:
- time of etiological
agents surviving and their ability to reproduce in or on the source
- external influences
on etiological agents in or on the source (changes in pathogenity, contagiousness,etc.)
- source movement
in space and time
- source location
in relation to susceptible hosts.
b) Grades of epizootiological importance:
aa) primary sources
are represented by those in or on which etiological agents can be maintained,
survive, reproduce, revitalize and reinforce their pathogenity (virulence)
(e.g. diseased animals);
bb) secondary
sources are those in or on which
etiological agents are only able to survive for some period and serve as their
vehicle - intermediary transmission factor (e.g. infected products of animal
origin, contaminated objects and substances); some etiological agents are able
to reproduce also outside of animal body (e.g. in organic substances).
9.2.2 Quantity and
extension of sources
a) Quantity of sources can be evaluated
in their number, i.e. absolute number of specifically diseased animals, size or
weight measure units of contaminated substances and pieces of contaminated
objects.
b) Relative range (width, 'extension') of
sources can be evaluated as their affected proportion within the total number
of sources units (e.g. percentage of diseased animals from total number,
percentage of contaminated part of objects or substances from total number of
respective measure units).
9.2.3 Quality of
sources
a) Epizootiological
quality of sources of etiological agents depends mainly on their specificity,
pathogenity and external influencing factors.
b) According to
specificity they can subdivided in mono-etiological containing only one
species of etiological agents and poly-etiological containing more than
one species.
c) Epizootiological intensity
(capacity) of sources expresses the quantity of etiological agents in infectious
doses (per weight, per volume, etc.) or in higher parasites in absolute or
average values of their number.
d) The environment of
etiological agents in or on the source can reanimate or devitalized them.
9.2.4 Space aspects
a) Localization of
etiological agents in or on sources has significant influence on their
epizootiological importance. These agents can be distributed in all parts of
the source, or in a part only, on the surface or inside the source (with or
without access to exterior).
b) The distance of
sources and susceptible host has decisive role in propagating respective
etiological agents. It is logical that direct contact gives better chance for
continuing of epizootiological chain than indirect contact or threat to far
living animals.
c) Fundamental
importance has the localization of the sources in relation to epizootiological
situation. The source of particular etiological agents in disease free
space (herd) is much more important than the same source in space (herd)
already affected by the same etiological agents.
9.2.5 Time aspects
a) All above mentioned
characteristics of the sources change during the time.
b) Period of
surviving of etiological agents defines the duration of a given source.
Period of elimination (period of infectiousness) of etiological agents
defines the duration of transmission danger from diseased animals.
9.3 Type of
etiological agents sources
9.3.1 Animals as
hosts of etiological agents
a) Infected animal
is an animal who harbours etiological agents and who has either manifest
disease or inapparent infection.
b) Infectious animal
is one from whom the etiological agents can be naturally acquired. The presence of etiological agents on a body
surface (contamination) can be also included in this category.
c) Carrier is an
infected animal (or man) that harbours a specific etiological agents in the
absence of discernible clinical disease and serves as a potential source of
infection. The carrier state may occur in an individual with an infection that
is inapparent throughout its course (commonly known as healthy or asymptomatic
carrier), or during the incubation period, convalescence, and
post-convalescence (commonly known as incubatory carrier or convalescence
carrier). Under either circumstance the carrier state may be a short or long
duration (temporary or transient carrier or chronic carrier).
d) Animal-reservoir
is defined as the macroorganisms in which etiological agents normally live and
multiply (where they maintain and perpetuate themselves) and from which they
can be transmitted. The reservoirs can be main (primary), secondary, active,
passive, opportunistic, etc.
9.3.2 Humans as
hosts of etiological agents
Man can have the role of source of
etiological agents threatening animal population if he is affected by common
disease (zoonosis). There can be different forms such as manifest cases
eliminating the etiological agents or asymptomatic carriers.
9.3.3 Vectors of
etiological agents
a) Vectors are invertebrate
animals having the role of intermediator facilitating the transmission of
etiological agents between host-vertebrate animals. Vectors have extraordinary
importance in diseases with natural nidality.
b) Vectors' species are
reservoirs of etiological agents only when the agent undergoes transovarial or
transtadial passages (e.g. overwintering of arboviruses).
c) Vectors can be
subdivided in biological (active) vectors representing conditions for
obligatory or occasional phase of
respective etiological agents life cycle and mechanical (passive)
when etiological agents do not entry in biological interaction. The vectors can
be also classified as main (primary), secondary, opportunistic, potential, etc.
d) Among the vectors
belong different species of arthropods, molluscs and helminths.
9.3.4 Infected and
contaminated products of animal origin
Product of diseased animals can be
originally infected or additionally contaminated during their processing,
storing or transport. Infected or contaminated meat, milk, eggs etc. play
important role as sources of etiological agents having wide territorial
distribution.
The body of animals dead due to infectious
disease represent also an important source, mainly in case of sporogenic
etiological agents.
Also excretions and secretions
of diseased animals have the role of sources.
9.3.5 Contaminated
substances and objects
Contamination is the presence of etiological
agents on or in bedding, equipment, instruments, or other inanimate articles or
substances including water, feed, soil, air etc.
These substances and
objects play the role of passive (mechanical) transmission factors depending on
etiological agents species, epizootiological situation, movement and distance
of and relation to susceptible hosts.
9.3.6 Biological
pathogens products
Among the sources belong also the cultures
of pathogenic strains and biological products such as vaccines and serums if containing alive
microorganisms able to cause a disease.
10. TRANSMISSION OF
BIOLOGICAL ETIOLOGICAL AGENTS
=================================================
10.1 Introduction
a) Transmission of
biological etiological agents from one host - diseased animal to the other host
- susceptible animal represents the basis of epizootiological chain and thus of
epizootic process of transmissible diseases.
b) The basic factors
for disease transmission are etiological agents sources, portals of exit, modes
of transfer, portals of entry and susceptible hosts.
c) Epizootiological
importance of transmission ways depends on their consequences i.e. how far they
worsen epizootiological situation and influence epizootic process.
10.2 Characteristics
of etiological agents transmission
10.2.1 Consequences of the transmission
The consequences of etiological agents
transmission can be as follows:
- creation of primary outbreaks in
distant disease free territories (intercontinental, international,
interprovincial etc. transmission)
- creation of
secondary outbreaks in affected territory
- creation of secondary
outbreaks in perifocal zone
- spread of a disease
within the outbreak area
- superinfection of
diseased animals
- contamination
or super-contamination of products of animal origin, substances and objects
- zero, i.e. without
any epizootiologically important consequences representing "blind
branch" of epizootiological chain (majority of the cases due to the fact
that from enormous quantity of eliminated and transferred etiological agents
only very few have the chance to facilitate the continuation of
epizootiological chain).
10.2.2 Primary and secondary ways of transmission
a) Primary ways
of transmission are the principal ways and are represented by the transmission
by primary sources of etiological agents (e.g. animal-reservoirs movement,
direct contact between diseased and healthy animals in contagious diseases,
transmission by vectors in vector-borne diseases, etc.).
b) Secondary ways
of transmission are not principal ways and are represented by the transmission
by the secondary sources of etiological agents.(See chapter 9).
10.2.3 Relative range of transmission
Relative range (width) of transmission
represents the proportion of infected or contaminated part of sources
(e.g. diseased animals among the total, contaminated part from total quantity
of meat, etc.) being transferred from one place to the other one. This width
varies from maximum (e.g. all transferred animals are specific etiological
agents reservoirs) to minimum.
10.2.4 Intensity of transmission
The intensity of transmission represent the
quantity of etiological agents being transferred which can be expressed in
infectious doses eventually in helminths and arthropods in absolute or average
numbers.
10.2.5 Specificity of transmission
a) According to epizootiological
difference between the origin and destination places the transmission can
be:
- from affected places,
animals or other sources to non-affected places, healthy animals or other
potential sources worsening epizootiological situation
- between places,
animals or other sources with the same epizootiological characteristics without
major change in epizootiological situation.
b) According to etiological
structure the transmission can be:
- mono-etiological
transferring only one species of etiological agents
- poly-etiological
transferring more than one species of etiological agents.
10.2.6 Space aspects
of transmission
a) According to the distance
the transmission can be divided in: local (e.g. within animal group, herd,
flock, village), regional, provincial, national, international
(intra-continental, trans- continental).
b) According to the size
of origin and destination herds/flocks the transmission can be roughly
divided in between the herds of the same size, from a grand herd to a small
herd and from a small herd to a grand herd.
c) According to the branching
form the transmission can be without dispersion and conversion (from only one
place to other one place), with dispersion (from one place to more places) and
with conversion (from more than one place to only one place).
d) According to number
of host species the transmission can be
to one single host
species or to more (multiple) host species.
10.2.7 Time aspects of transmission
a) According to the duration
the transmission or length of communicability period can be of a moment (e.g.
direct contact - instantaneous transmission), short period (hours, days), long
period (weeks, months, years) depending on etiological agents and given
conditions for their transfer.
Usually short-time transmission corresponds
with high communicability disease and long-time transmission with low
communicability disease.
b) Speed of the
transmission expresses the relation between the distance and time. This factor
is of extraordinary importance in so called emergency diseases with rapid
spreading (e.g. foot-and-mouth disease).
c) According to the frequency
the transmission can be continuous (without interruption e.g. contamination of
meat by contaminated processing equipment), interrupted or only once.
10.2.8 Natural and
artificial transmission
Natural transmission is carried out without
any human interfering whereas artificial transmission is realized consciously
or not consciously by the man.
10.2.9 Horizontal and vertical transmission
a) Horizontal
transmission is the transfer of etiological agents within the same generation
of animals. The disease is spread from one individual to another. There are
three types of horizontal transmission: contact, vehicle and vector-borne.
b) Vertical
transmission is the transfer of etiological agents from one generation to
another, e.g. through the placenta
(transplacental transmission), milk (transmammary transmission) or eggs
(transovarial transmission). Vertical transmission not only occurs with
infectious diseases, but with non-infectious as well (e.g. genetic).
c) Vertical-horizontal
transmission combines both forms of transfer of etiological agents.
10.2.10 Active and passive transmission
a) Active
transmission is the transfer of etiological agents by their hosts being in
interaction with them.
b) Passive
transmission is the transfer of etiological agents by the animals or man as
mechanical carriers, by the contaminated products, substances and objects.
10.2.11 Direct transmission
Direct transmission is essentially immediate
transfer of infectious agents to a receptive portal of entry through which
infection of animals may take place. This may be direct physical contact by
touching or sexual intercourse, or by the direct projection of droplet spray
(droplet spread) onto the conjunctiva or onto the mucous membranes of the nose
or mouth during sneezing, coughing or by biting, etc.
10.2.12 Indirect
transmission
a) Indirect
transmission is carried out through secondary (passive) intermediatory vehicles
such as inter-hosts, biological vectors, infected or contaminated products of
animal origin, substances and objects.
b) Indirect
transmission can be of two (biphasic) or more stages (polyphasic).
c) Indirect
transmission can be mechanical (etiological agents survive only without
reproduction), propagative (etiological agents survive with
reproduction), cyclic (etiological agents pass one stage of development
in or on the transmission vehicle) and cyclopropagative combining the
two previous ones.
10.3 Transmission by
animals
10.3.1 Prenatal
transmission
Prenatal transmission can be carried out
through placenta (intraplacentary) or fertile eggs (transovarial).
10.3.2 Direct
postnatal transmission
a) Ingestion of
animals-carriers of etiological agents transmissible by susceptible carnivore.
b) Simple contact
mainly in skin infections and ectoparasitoses.
c) Other forms of
direct transmission are through biting, sucking, coitus, scraping and
licking between healthy and diseased animals.
10.3.3 Transmission
by animal movement
a) The movement
(natural, forced) and transport of animals-carriers of etiological agents
represents the most important way of disease transmission. Its importance
increases by the distance between the origin and destination places.
b) When importing
animals in spite of veterinary certificates the risk of transmission exists due
to the fact that also clinically healthy animals can be dangerous carriers of
etiological agents (not all affected animals is possible to detect clinically
or using indirect diagnostic methods).
c) Among wild animals
the migration is a problem which is difficult to control. It is obvious
that among these animals are also diseased ones moving freely from one to
another place (migrating birds are able to fly very grand distances).
10.3.4 Passive transmission by animals
Passive transmission by animals is the
transfer of etiological agents on the surface of non susceptible animals
or through
gastroenteric tract without any interaction between them. This form is
important in outbreaks of a very contagious disease (e.g. foot-and-mouth
disease).
10.3.5 Transmission
through animals - intermediate hosts
Transmission through intermediate hosts is
based on etiological agents transfer between the definitive hosts passing
through the body of other animal species in which these agents have one part of
their evolution cycle. These interhosts can be vertebrates or invertebrates.
10.3.6 Elimination
of etiological agents
a) Etiological agents
escape from animals-reservoirs through portals of exit. Ease of escape
determines the importance of a reservoir.
b) Portals of exit can
be respiratory, alimentary, urogenital, percutaneous, mammary or multiple (in
diseases with more than one portal of exit).
c) Among the primary
vehicle such as excretions, secretions and other body fluids or tissues
of diseased animals are faeces, urine, ocular, nasal, vaginal and preputial
discharges, saliva, sputum, exhaled air, vomits, blood, skin scabs or ulcers
and foetal fluids.
Fecal-oral transmission is based on ingestion of fresh faeces
through fecal splash-droplets or coprophagous activity.
10.4 Transmission by
humans
a) The man can have a role in transmitting
etiological agents of zoonotic diseases to animals. Active role have the
clinically diseased persons or persons-carriers of zoonotic agents eliminating
them in an environment with susceptible animals. The man in many zoonotic
disease with natural nidality has the role of final host, i.e. final link of
epizootiological/epidemiological chain branch. Enormous increase of human
migration and tourism stresses the importance of animal disease propagation by
man.
b) Passive role
have the healthy persons carrying on their body or on their clothes, footwear,
etc. etiological agents of animal diseases (e.g. virus of foot-and-mouth
disease).
10.5 Transmission by
vectors
a) A vector is a
living invertebrate carrier of a disease causing agents. Vector-borne
transmission can be mechanical or biological.
b) Mechanical
transmission includes simple mechanical carriage by a crawling or flying
insect through soiling of its feet or proboscis, or by passage of organisms
through its gastrointestinal tract. This does not require multiplication or
development of the organism.
With mechanical transmission, the agent does
not undergo any change while associated with the vector. The transmission
interval is usually short and depends on the survival time
of the agents on the
body or mouthparts of the vector.
c) Biological
transmission based on propagation (multiplication), cycle development or a
combination of these (cyclopropagation) is required before the arthropod can
transmit the infective form of the agents to susceptible animals. An extrinsic
incubation period is required following infection before the vector becomes
infective. Transmission may be by saliva during biting, or by regurgitation or
deposition on the skin of faeces or other material capable of penetrating
subsequently through the bite wound or through an area of trauma from
scratching or rubbing.
d) In biological
vectors the agent undergoes some changes. These may be: multiplication,
maturation of a phase in the life cycle, sexual reproduction, maturation and
multiplication. Biological vectors cannot transmit the agents immediately
after becoming infected. A prepatent
period is required while development occurs in the vector or multiplication
occurs to provide enough of the agents to infect a new host.
e) Some agents are very
well adapted to their vectors. In transovarial
transmission the agents are transmitted from the female vector to the eggs
(occurs especially in arthropods). In transstadial transmission the
agents survive through various stages of nymph development (e.g. occurs with many arthropod-borne diseases).
f) Flying
vectors (e.g. mosquitoes) can actively seek out their vertebrate hosts; the
flight range of the vector and its biting patterns may determine the extent and
rapidity of spread of an infection.
g) Non-flying
vectors (e.g. ticks, lice, mites and snails) are dependent on passive contact
with host; to overcome this disadvantage many
have developed transovarial and transstadial transmission.
Water-inhabiting vectors may release infectious organisms into the fluid medium
enabling them to be passively disseminated.
h) Vectorial
capacity means the grade of vector's efficiency in etiological agents
transmission.
10.6 Transmission by
products of animal origin
a) Infected, invaded
and contaminated products of animal origin such as milk, meat, eggs, semen,
hides, etc. can serve as the vehicles of etiological agents facilitating their
transmission to new susceptible host animals.
b) The circulation
of these products almost continuously from the origin place through the
processing and storage places to distribution network and enormous numbers of
households and other consumption establishments represent a very important
epizootiological and epidemiological risk.
c) National and
international trade with these commodities, if in crude form,
facilitates the propagation of many food-borne diseases in man and feed-borne
diseases in animals.
d) Feeding the
animals with the products of diseased or epizootiologically uncertain
animals and mainly with their wastes in crude form represent important way of transmission of
many infectious and parasitic diseases.
10.7 Transmission by contaminated substances
and objects
a) Vehicle-borne
transmission occurs by inanimate objects that become contaminated such as
equipment, instruments, transport means, water, feed or any substances serving
as an intermediate means by which etiological agents are transported and
introduced into a susceptible host through a suitable portal of entry. The
agents may or may not have multiplied or developed in or on the vehicle before
being introduced into animal.
b) Air-borne
transmission is the dissemination of
microbial aerosol containing etiological agents to a suitable portal of
entry, usually the respiratory tract. Microbial aerosols are suspensions in the
air of articles consisting partially or wholly of microorganisms. Particles in
the 1 to 5 micron range are easily drawn into the alveoli of the lungs and may
be retained there; many are exhaled from the alveoli without deposition. They
may remain suspended in the air for long periods, some retaining and others
losing infectivity or virulence.
c) Among airborne
transmission factors are droplet nuclei (usually the small residues
which result from evaporation of fluid from droplets emitted by infected hosts)
and dust (small particles of widely varying size which may arise from
soil, bedding, contaminated floors, etc.).
d) Soil and dust of
horizontal surfaces (floors) when indoors can contain etiological agents which
can use the soil for reservoir (e.g. mycotic agents); most bacterial pathogens
which are soil-borne are spore formers.
10.8 Transmission by
biological products
The alive vaccines with virulent etiological
agents (insufficiently mitigated) and serum containing etiological agents (due
to insufficient sterility control) as well as virulent strains cultures with
these agents can facilitate their transmission to susceptible hosts.
11. NATURAL ENVIRONMENTAL FACTORS
=================================
11.1 Introduction
a) Environmental
factors as one of disease determinants act on life, development,
characteristics of animals and etiological agents as well as on the
interactions between them. All together participate in an integral complex
- ecosystem.
b) Environmental
factors influence significantly animal population health/disease and
epizootiological situation. Disease
occurrence is related to the environment of the
species concerned.
c) Natural
environmental factors are inorganic (physical and chemical) and organic
(biological). They can be divided in atmospheric, geospheric, hydrospheric,
biospheric and nutritional.
d) The relationship of
living organisms and their environment is studied by ecology.
11.2 Environment influences
Environment acts on almost all factors which characterize epizootiological
situation and define its development. The influence can be positive or
negative.
11.2.1 Influences on animal population
a) Environment
represents the conditions of life of all animals. From epizootiological point
of view the most important are those that influence animal health and
resistance.
b) Environmental
factors which are within physiological (normal) limits for the
animals support population health and resistance against etiological
agents and thus block the origin and/or development of diseases. Environmental
factors values outside of these limits have opposite impact. Some
of these factors can act as disease agents in their own right.
c) Infinite
variability of environmental factors is reflected in infinite variability
of animal health/disease and epizootiological situation.
d) The impact of
abnormal interaction between the damaging environment and animals can provoke
different reactions such as stress (alarm) reaction or adaption
to new conditions. Sudden major changes of these factors have negative effect.
e) Movement of an
animal species into a new environment may result in disease if there is a
pathogen present which has a well‑balanced
relationship with species already
present in that particular environment
(e.g. cattle from Europe suffers in tropical Africa by trypanosomiasis more
than Bos indicus species).
11.2.2 Influences on etiological agents
Environment represents the conditions of
life of all etiological agents being outside of animal or vector body.
Extreme conditions reduce their ability to survive, reproduce, adapt and act as
pathogens. Favourable conditions have opposite effect.
11.2.3 Influences on
vectors of etiological agents
Environment represents the conditions of
life of all vectors of etiological agents. Extreme conditions reduce their
ability to survive, reproduce, spread, adapt and act as pathogens vectors.
Favourable conditions have opposite effect.
11.2.4 Influences on epizootiological situation
a) Environmental
factors favourable for animals and unfavorable for etiological agents
and their vectors create conditions for maintenance of good and improvement of
bad epizootiological situation.
b) Environmental
factors unfavourable for animals and favourable for etiological agents
and their vectors create conditions for worsening of epizootiological
situation.
c) Multifactorial
environmental influences multiply the variability of epizootic processes. These
influences on one hand support and on the other hand can slow, break
or to make impossible the origin and course of epizootic processes.
d) Environmental
factors influence on the existence and characteristics of etiological agents
sources and way of transmission, on the occurrence, frequency, course and
spread of animal diseases, their distribution in space and time (e.g.
periodicity, seasonality) as well on their consequences.
11.3 Characteristics
of environment actions
a) Environmental
factors can act separately or in complex. Generally they act in integral
complexes where some have primary and some secondary role.
b) Impact on animal
health/disease and epizootiological situation depends on the intensity,
stability/variability and space and time of environmental factors action.
11.4 Atmospheric factors
a) Atmospheric factors comprise the components
of weather to which animals are exposed such as rainfall, temperature, solar
radiation, humidity and wind (direction and speed). Abnormal (extreme) values of these factors
may affect the health of the host rendering it more susceptible to disease.
They can also affect the survival of
etiological agents as well as of the vectors.
b) Air content such as oxygen, chemical
and physical pollution substances, dust and air microflora in extreme
(abnormal) values have negative influence on animal health resistance and in
some cases can provoke themselves (as etiological agents) a disease.
b) Air temperature. Each animal species
and vectors can live only in particular zone of temperature depending also on
body thermoregulation (e.g. homeothermic animals such as mammals and birds,
poikilothermic animals such as fish, reptiles, etc.). Animal species
distribution according to thermic geographical zones is reflected also in
geographical occurrence of specific animal species diseases (e.g. tropical
diseases).
c) Air humidity. Excessive humidity and
in particular if combined with extreme temperature (heat, frost) has not only
negative effect on animal resistance but also can cause itself a disease up to
fatal cases. This factor influences also geographical distribution of animals,
vectors and many etiological agents species.
d) Air movement in form of wind (or
draught in stables) as factor influencing animal health depends on its speed
and direction. It facilitates spread of etiological agents of respiratory
diseases and can transport flying vectors to distant places propagating
different animal diseases.
e) Atmospheric pressure decreasing due to
reduced oxygen content (e.g. low pressure in high mountains) has influence on
"vertical distribution" of animals, vectors and etiological agents
species and on their occurrence density. Sudden decrease of air pressure can
provoke a fall in resistance and give the change to "sleeping" etiological
agents in animal body.
f) Precipitations,
solar radiation and light in extreme values have similar effect on animals
and vectors as other atmospheric factors.
g) Climate
represents the complex of atmospheric factors acting on land fauna and flora.
It can be differentiated in:
- macroclimate or weather of grand
territories,
- mesoclimate of minor territories
- microclimate of local size (refers to
restricted area where the host, agents, vectors or intermediate host actually
live - stable, biotope, etc.)
Climate of inferior stratum where the
majority of terrestrial animals live is of particular importance.
Climate influences in a decisive way the
life of all terrestrial animals, their species structure, distribution and
development under natural conditions what is reflected in animal population
health and epizootiological situation. Besides ubiquitous diseases distributed
in all the world (e.g. salmonelloses), the other ones can be distinguished as
tropical, subtropical, polar, of mild zone, desert, etc.
Microclimate is of particular importance in
intensive animal production premises with high animal concentration. Any major
deviation can cause important production losses.
11.5 Geospheric
factors
a) Geomorphological
relief has the influence on the animal distribution and grade of their
territorial isolation due to natural barriers such as mountains (deep
valleys, large rivers, etc.). On the other hand the plains (lowlands)
facilitate spread of diseases.
b) Soils by
interacting with climate determine vegetation and the environment in which
animals live. The main effect of vegetation is on nutrition. Soils therefore
act indirectly as determinant of disease by causing starvation if there is a
little or no vegetation or nutritional imbalance. Soils have also effect on the
ability of many etiological agents to survive in the environment.
Soils constitute main substrate and natural
shelter for terrestrial animals, vectors and microflora including some
etiological agents able to survive in these conditions (B. tetani, B.
anthracis). Soil types are based on chemical composition, content of
organic substances, physical structure, temperature, humidity and soils fauna
(including microflora).
11.6 Hydrospheric factors
a) Hydrospheric abiotic
factors representing aquatic environment surrounding animals such as
fish, aquatic mammals, a lot of species of inferior classes including some
vectors and etiological agents species. Water represents also the temporal
habitat for aquatic birds and amphibian animals.
b) The influence of
these environmental factors on animals and etiological agents depends on water
content (salt, pollution), microflora, temperature, movement,
hydrostatic pressure, etc.
c) Water distribution,
reservoirs and currents determine not only aquatic animals distribution but
also create natural barriers for terrestrial animals as well as conditions for
different vectors and etiological agents requiring for their living this environment (e.g. mosquitoes, Leptospira
spp.).
11.7 Biospheric
factors
Biospheric factors (biologic climate)
are represented by flora and fauna influencing the forms of interactions
of animal populations and etiological agents species. All these belong to
biospheric factors as their integral component. These factors have a key role
in the origin, development and result of animal disease and in particular
epizootiological situation.
11.7.1 Biosphere
Biosphere, also called as "zone of
live" represents the space where animals and plants live. Global
biosphere is composed by marine and terrestrial biomasses.
11.7.2 Ecosystem
a) Ecosystem is a complex composed of biotic
communities and their abiotic environment. The plant and animal life of a
region are considered in relation to the environmental factors that influence
it; it is the fundamental unit in ecology, comprising the living organisms and
the non-living elements that interact in a defined region (e.g. pasture, lake,
desert, forest, river, marsh, etc.).
b) Ecosystem function is based on food chain
among different components. Plants represent the first , herbivores the second
and carnivora the third trophic level. These relations can be expressed in so
called "production pyramid", "biomass pyramid" and
"number pyramid".
c) Ecoton is the zone where two or more
different ecosystems are mixed creating particular conditions for more species
of animals, vectors and etiological agents than in individual ecosystems alone
(border principle). Therefore the epizootiological situation is there more complex.
d) Ecosystems can be differentiated
according to their size, from microecosystem up to macroecosystem:
- habitat is a
complex of environmental conditions locally delimited occupied by an organism
or specific community or population (e.g.,shelter, nest, stable, etc.);
- biotope is a
biogeographic area with more or less uniform life conditions and populated by a
characteristic type of biocenosis (e.g., lake, forest, pastures, etc.);
- biom is a
grand biogeographic area with a complex of communities characterized by
particular climate (e.g. desert, taiga, tropical forest, etc.);
- biogeographical
region represents a continent or subcontinent with relatively uniform
geography, flora and fauna (Australian, Antarctic, Neartic, Agrotropical,
etc.); these regions can be subdivided in biogeographical subregions, provinces
and districts.
11.7.3 Biocoenosis
a) Biocenosis or biotic
community is the alive (biotic) part of the ecosystem, i.e. plant and animal
populations. Every biocenosis has different composition and structure
according to animal and plant species. Many animal diseases are result of a
complex interplay between animal and plant species.
Structure of animal species has a
grand importance for epizootiological situation in a given ecosystem (e.g. wild
fauna structure has decisive role in multihostal diseases with natural
nidality).
b) Each biocenosis can
be subdivided not only horizontally in subcommunities but also vertically
according to different strata (e.g. subterranean, soil, grass, bush and tree
levels).
11.7.4 Biotic
interactions
a) In natural
biological communities no one species lives in an isolated form. Their
interactions can be:
- competitive, or beneficial or neutral
- temporal or continuous
- of vital importance or of minor importance
- intraspecies or interspecies.
b) Intraspecies
interactions competing for food tend to avoid overcrowding of the area with
limited food resources. Cooperative interactions contribute to form families,
herds and communities, to reproduction, protect new born animals, etc. These
relations facilitate the spread of transmissible diseases within a given animal
species.
c) Interspecies
interactions can be based on symbiosis (mutualism, comensalism) or antagonism
(depredation, competition, obligatory or facultative parasitism, antibiosis,
etc.). These relations facilitate the spread of transmissible diseases among
different animal species.
11.7.5 Nutritional factors
a) Nutritional factors
are of decisive ecological importance for any animal population, vectors
and etiological agents. The life depends on nutritive substances and water
cycles as well as on energy.
b) Availability of
physiological quality and quantity of protein, carbohydrates, fats, salts,
mineral elements and vitamins is of vital importance not only for animal surviving
but also for their resistance to diseases.
c) Distribution and
concentration of animal populations are in close relation to availability
necessary nutritional factors having decisive impact on spatial distribution
of diseases.
d) Drinking water
is a basic need of any animal influencing not only their physiological
functions but also their resistance against diseases.
12.
INTERACTION ANIMAL-ETIOLOGICAL AGENT-ENVIRONMENT
====================================================
12.1 Introduction
a) Interaction animal - etiological agent -
environment ( e t i o l o g i c a l t r i a d ) creates the basis of animal
disease process. This multifactorial
process can be infectious when caused by biological agents or non-infectious
when caused by abiotic agents. Defense reaction of animal is aimed to
maintain or reach again physiological balance, i.e. health.
b) Infected animal (as mentioned in
chapter 9) is an animal who harbours infectious etiological agents and who has
either manifest disease (sick animal) or unapparent infection. Infectious
animal is one from whom the infectious etiological agents can be naturally
acquired.
c) Infection is the
entry and development or multiplication of infectious etiological agents in the
body of animals or man. Infestation
is the lodgement, development and reproduction of arthropods on the surface of
the body .
d) Infectious
disease is a disease caused by pathogenic microorganisms (viruses,
bacteria, fungi, parasites, etc.) and may be transferred from one host to
another or may arise from the host's own indigenous microflora. An infectious
disease is not necessarily a contagious disease.
e) Contagious disease
is a disease capable of being transmitted from one individual to another. Also called a communicable disease.
f) Infectiousness
is a characteristics of the disease that concerns the
relative ease with which it is transmitted to other hosts.
g) Non-infectious
disease is a disease caused by other than infectious agent.
h) Dose‑response
characterizes the relationship between the agent dose and the host reaction.
12.2 Postulates of
infection process
Following postulates (Evan's based on
Henle-Koch model) should be met before a causative relationship can be accepted
between a particular bacterial parasite or disease agent and the disease in
question:
a) Prevalence of the
disease should be significantly higher in those exposed to the hypothesized
cause than in controls not so exposed.
b) Exposure to the
hypothesized cause should be more frequent among those with the disease than in
controls without the disease - when all other risk factors are held constant.
c) Incidence of the
disease should be significantly higher in those exposed to the hypothesized
cause than in those not so exposed, as shown by prospective studies.
d) The disease should
follow exposure to the hypothesized causative agent with a distribution in
incubation periods on a bell shaped curve.
e) A spectrum of host
responses should follow exposure to the hypothesized agent along a logical
biological gradient from mild to severe.
f) A measurable host
response following exposure to the hypothesized cause should have a high probability
of appearing in those lacking this before exposure (e.g. antibody), or should
increase in magnitude if present before exposure. This response pattern should
occur infrequently in animals or persons not so exposed.
g) Experimental
reproduction of the disease should occur more frequently in animals or man
appropriately exposed to the hypothesized cause than in those not so exposed.
h) Elimination or
modification of the hypothesized cause should decrease the incidence of the
disease (i.e. attenuation of virus).
i) Prevention or
modification of the host's response on exposure to the hypothesized cause
should decrease or eliminate the disease (.i.e. immunization, drugs
applications, etc.)
j) All of the
relationships and findings should make biological and epidemiological
sense.
It is not necessary
that the causative agent meets all the criteria. However, the more criteria
that are met, the more likely the agent is the putative one.
12.2.1 Etiological triad
a) For causation of an
infection disease is necessary that etiological agents penetrate in
susceptible host body under given environment conditions. These three
factors determinating infection diseases are called "etiological
triad".
b) Etiological
(infectious) agents must enter the susceptible organism in such a quantity
(number, infectious doses) and with such properties that enable to provoke a
specific disease. Following characteristics are of major importance:
- host range (the wider the host range, the
more capable the organism is of spreading and developing disease in animals)
- ability to survive
in the environment
- infectivity (the
ability of the etiological agent to infect and multiply; often related to dose
and pathogenity)
- pathogenity (the
ability to produce pathologic changes or disease)
- virulence (the
degree of pathogenity and the ability to cause disease despite host defenses; a
truly successful pathogen would not be so virulent that it would cause the
death of its host and thus, itself).
- other factors (see
chapter 8).
b) Individual host factors
can be either
intrinsic (endogenous), i.e.
predetermined such as species, age, breed or sex, or extrinsic (exogenous,
environmental), i.e. can be manipulated
such as immunization
status, feed, contacts, use or
behaviour. (See chapter 2).
c) Environment
factors such as climate, bedding, housing, hygiene etc. (see chapters 11 and
16) influence the interaction between etiological agents and host in positive
or negative sense.
- Predisposing factors are those that
prepare, sensitize, condition, or otherwise create a situation such as a level
of immunity or state of susceptibility so that the host tends to react in a
specific fashion to a disease agent;
- enabling
factors are those that facilitate the manifestation of the disease, ill-health
or conversely those that facilitate recovery from disease, maintenance or
enhancement of health status;
- precipitating
factors are those that associate with the definite onset of a disease, illness,
behavioural response, or course of action;
- reinforcing
factors are those tending to perpetuate or aggravate the presence of a disease,
disability, pattern of behaviour or course of action. They may tend to be
repetitive.
12.2.2 Portals of
entry of etiological agents
a) Portal of entry is the place
through which etiological agents enters the macroorganism. Each of the species
of these agents is characterized by a specific portal(s) of entry.
b) Digestive tract
is for most etiological agents the most frequent portal of entry, in particular
for alimentary - gastrointestinal infections (salmonellosis, infectious
gastroenteritis, etc.).
c) Respiratory tract
is the main portal of entry for respiratory pathogens of aerogenic infections
(influenza, infectious laryngo-tracheitis, etc.).
d) Urogenital
(reproductive) tract is the main portal of entry for venereal diseases
(trichomoniasis of cattle, dourine, etc.).
e) Mammary gland
(teat canal) is the portal of entry for different agents causing infectious
mastitis (e.g. Streptococcus agalactiae).
f) Skin is the
portal of entry of a series of etiological agents transmitted by bite (rabies),
sting (tick-born diseases), wound (tetanus) or by
simple contact (mycotic skin diseases).
g) Other portal of
entry are umbilicus (umbilical infections) and conjunctiva
(ocular infections).
h) However in many agents the portal of
entry may vary (Bacillus anthracis, Brucella abortus).
i) Accidental entry
may occur through human interventions
such as use of
blood contaminated needles
or other equipment
to spread blood‑borne
pathogens (Anaplasma spp. infections).
12.3 Course of
infection process
The course of infection process depends on
all three components of etiological triad. It has infinite variability. There
are not two identical cases in absolute terms. Not always the course has all
stages. The course can vary from very mild up to very severe (fatal) one.
12.3.1 Stages of
infection process
a) Incubation period
is the time interval between entry of etiological agents and appearance of the
first sign or symptom of the disease in question. The duration, depending
mainly on quantity and pathogenity of agents and host resistance, is very
variable also in the same disease. Usually short incubation period is understood
up to 7 days (e.g. in foot-and-mouth disease), medium length period one up to
three weeks (e.g. piroplasmosis) and long period can be of months eventually
years (e.g. brucellosis, tuberculosis).
b) Prodromal period
is manifested by unspecific signs of infection process, like increase of body
temperature, pulse and respiration rates, inappetence, etc. This stage usually
corresponds with the penetration of etiological agents into blood stream
disseminating them throughout the body (viraemia, bacteriaemia, bacillaemia,
pyaemia, toxaemia, parasitaemia). Septicaemia means that the agents multiply in
blood.
c) Manifestation
period is characterized by specific clinical signs typical for a given
infection. It corresponds to the period of localization of the agents and their
toxins in particular tissues and organs.
d) Final period
reflects the results of a given infection process:
Convalescence means animal recovery
and getting rid of the agents completely or not becoming asymptomatic carriers
with the risk of disease recurrence (repeated flare-up). This happens mainly in
animals with mild course of disease.
Fatal result means the dead of
affected animal and usually also of etiological agents inside of dead body.
This happens mainly in animals with severe course of disease.
12.3.2 Forms of infection process according its duration
a) According to the
duration of infection process following forms can be differentiated:
- peracute when
within a very short time, suddenly or within several hours up to few days after
first clinical signs, the animal dies (e.g. peracute form of foot-and-mouth
disease);
- acute when
clinical signs last from few days up to two weeks (e.g. acute form of
foot-and-mouth disease);
- subacute when
clinical signs last from about two weeks up to one month (e.g. subacute form of
rinderpest);
- subchronic
when clinical signs last few months (e.g. subchronic form of respiratory
infection);
- chronic when
clinical signs last many months and years (e.g. chronic form of tuberculosis).
b) In many infection
processes can be found all above mentioned forms of duration depending on
particular etiological triad interaction.
c) Latent period
is the delay between exposure to disease-causing agents and the appearance of
manifestations of the disease and the duration of unapparent form.
12.4 Forms of
infection process according its manifestation
a) Apparent form
is clinical manifestation of the process which can be either typical
(specific for the infection) or atypical (different from the typical
form). Unfortunately, atypical forms very often represent the majority of
cases.
b) Abortive form
is characterized by clinical signs hardly to be noticed and disappearing
relatively soon.
c) Unapparent (latent,
subclinical, asymptomatic) form is the presence of infection process in a host
without occurrence of recognizable clinical signs or symptoms. Unapparent infections
are only identifiable by laboratory means. For disease propagation the most
dangerous are animals with unapparent form harbouring
etiological agents which can be eliminated or can cause a flare-up and spread
of the disease if animal resistance is reduced.
b) In many infection
processes can be found all above mentioned forms depending on particular
etiological triad interaction.
c) Syndrome is a
symptom complex in which the symptoms and/or signs coexist more frequently than
would be expected by chance on the assumption of independence.
d) "Iceberg
effect". In many, mainly chronic diseases, only a relatively small fraction of
diseased animals can be detected clinically.
12.5 Classification
of animal infections
There are many criteria for animal
infections classification which are being used separately or in combination of
different views.
12.5.1 Classification
according to hosts
Classification can be according to:
- zoological class,
order, family, genus and species (e.g. cattle infections)
- animal category
(e.g. calves infections)
- number of
susceptible species (e.g. monohostal, polyhostal infections)
- transmissibility to
man (zoonotic infections)
- transmissibility to
new born animals (e.g. congenital infections, hereditary infections).
12.5.2 Classification according to etiological agents
Classification can be according to:
- biological criteria
(e.g. viral infections, mycotic infections, helminthic infections)
- origin of
etiological agents (exogenous infections, endogenous infections - agents
latently surviving inside the animal body; indigenous infections, exotic
infections)
- contagiousness
(contagious infections, non-contagious infections)
- quantity of agent
species (monoetiological infections, polyetiological or multicausal infections)
- agents participation
order (primary infections, secondary infections)
- interaction with the macroorganism such as reinfections (repeated agents
penetration), superinfections (additional agents penetration), subinfections
(limited number of agents - infection doses), parainfections (caused due to
other agent species supporting influence), autoinfections (caused by conditionally pathogenic microflora of animal
body), intercurrent infections (when animal already suffers from other
pre-existing disease), recurrent infections (recidives - disease reappearance
due to the same etiological agents after their unapparent survival in
animal body).
12.5.3 Classification
according to transmission ways
Classification can be according to:
- mechanism of
transmission (e.g. aerogenic infections)
- portal of entry
(e.g. umbilical infections)
- localization of
agents (e.g. pulmonary infections).
12.5.4 Classification
according to other aspects
Classification can be according to:
- course of infection
process (e.g. latent infections)
- course of epizootic
process (e.g. enzootic infections)
- space factors (e.g.
autochthonous infections, tropical infections)
- consequences/importance
(e.g. infections of obligatory
notification, internationally reportable infections)
- role of man (e.g. iatrogenic
infections related to veterinarian activity; nosocomial
infections acquired in veterinary
ambulance or hospital).
12.6 Number of
animal infections
a) The number of
infection diseases is enormous. Practical veterinary medicine deals only with
selected diseases of major biological, economic, public health and social
importance. Internationally more than 140 specific diseases are monitored. If
we take all known etiological agents species we can estimate their number in
thousands, not considering their serological types and subtypes. The complexity
of infection diseases number is multiplied by enormous number of animal species
which can suffer from different etiological agents (number of known species is
estimated 4500 in mammals and 8600 in birds). This contributes to infinite
variability of infection processes.
b) For practical
reasons only the species of agents and animals of major importance for human
society and biosphere are the objects of professional service activities.
12.7 Non-infection
disease process
a) Non-infection
disease process is based on the interaction of abiotic etiological agents (see
chapter 8), animals and environment forming etiological triad.
b) General principles
described above for infection disease process are almost all applicable
also for non-infection disease process.
c) The course, forms
and duration of non-infection disease process are similar as in infection
disease process.
d) Classification
can be according to etiological agents, animal species, mechanism of
penetration, localization (skin diseases, cardiovascular diseases),
physiological functions (production diseases, reproduction diseases, metabolic diseases), etc.
13. EPIZOOTIC PROCESS
=====================
13.1 Introduction
a) Epizootic process
is a biological, dynamic and multifactorial phenomenon based on a complex and
continuous interaction among animal p o p u l a t i o n, etiological
agents and environment (epizootiological triad).
It is closely related to infectious process
(chapter 12) based on similar interaction cause by biological parasitism,
however at individual animal level only and not at population level as in case
of epizootic process.
b) According to etiological
agents these processes can be divided in infectious epizootic process
caused by biological agents and non-infectious epizootic process caused
by abiotic agents.
(This chapter describes mainly the aspects
of infectious epizootic process serving as the model for non-infectious one
concisely described at the end of this chapter).
13.2 Contents of
epizootic process
Infectious epizootic process is based on the
circulation of etiological agents in animal population creating so
called "epizootiological chain".
13.2.1 Animal
population participation
a) Animal population
creates the ground on which epizootic process develops.
b) Animal population in
confrontation with etiological agents reacts in defending and/or adapting way
winning or losing. It tends to maintain or regain biological inter-species
balance.
c) Animal population
acts against the population of etiological agents as an integral and
organic complex (collective
defense and/or adaptation) and not as a simple sum of individual isolated
animals.
d) Epizootic process is
influenced by animal population characteristics described in chapter 2. The
population size, species and categories structures, horizontal and vertical
movement and resistance / susceptibility levels belong among the main
influences. The dynamics of host populations is reflected significantly in the dynamics
of epizootic process.
e) According to
participation of animal species the epizootic process can be divided in monohostal
(including only one animal species) and polyhostal (including more than
one animal species).
13.2.2 Etiological agents participation
a) Etiological agents
search in animal hosts adequate conditions for their life, nutrition and
reproduction using parasitic forms.
b) In order to keep
their species existence the etiological agents need nutrition substances in
continuing form. This is made possible through the circulation of the
agents in respective animal populations using different ways of transmission to
new host offering desirable conditions. Therefore continuity of epizootic
process depends on horizontal and/or vertical circulation of etiological
agents.
c) Particular epizootic
process can be mono-etiological including only one species of
etiological agents or poly-etiological including more than one species
(up to etiological complex).
13.2.3 Environment
participation
Environmental (ecological)
factors (see chapters 11 and 16) create conditions for the life and
activities of animal populations as well as of etiological agents when outside
of animal body and thus influences epizootic process. It
represents also the medium in which etiological agents are transmitted
from one host to the other one.
13.2.4 Epizootiological
chain
a) Repeating and continuing
process of etiological agents transmissions between the susceptible hosts
creates so called "epizootiological chain". Basic link (component) of
epizootiological chain is the transmission from one to other individual host.
b) Etiological agents
circulating from one host can find adequate conditions in or on new susceptible
animal and prolong the process as continuing epizootiological chain or not and
thus interrupting this chain.
c) Epizootiological
chain can have different form such as:
- linear
(isolated and parallel)
- divergent
(double, multiple or star-shaped branches)
- convergent
(double, multiple or star-shaped branches)
- combined.
Anazootic is a process where all
cases have one common source of etiological agents.
d) According to the manifestation
epizootiological chain can be:
- apparent
(hosts with clinical symptoms) which can be divided in typical and atypical
forms;
- unapparent (hosts without any
clinical symptoms) which can be divided
in subclinical (internal changes without clinical manifestation) and latent (simple
carriers) forms.
e) According host species
participation epizootiological chain can be:
- homogeneous when only one
host species is involved (e.g. horse-horse-horse in glanders);
- heterogeneous when more
than one host species is involved (e.g. sheep-cattle-goat
in foot-and-mouth disease, horse-Glossina fly-cattle in trypanosomiasis caused
by T.vivax).
f) Infinite combination
exists in large epizootiological chains of polyhostal infections.
13.3 Course of
epizootic process
a) Any epizootic
process, similarly as any other biological phenomenon, has its origin,
development and end. This course is influenced by all components of
epizootiological triad and their characteristics.
b) In spite of extraordinary
variability of epizootic courses, determinate regularity can be
found in specific diseases under natural conditions.
c) Continuing epizootic
processes under natural conditions develop in epizootic waves
reflecting changes of triad components in space and time.
d) Epizootic stages and
curves can be expressed in number (or proportion from total animals) of
specific disease cases. The transition between individual stages is usually
gradual without clear cut limits.
13.3.1 Stages of continuous epizootic process
a) Interepizootic
stage is the period between postepizootic stage of epizootic wave and outset of
specific epizootic process activation - preepizootic stage. Survived animal
population from previous epizootic wave maintain elevated immunity and
therefore no specific disease cases or only sporadic ones, mainly with atypical
and chronic forms, can be detected. This stage is also called as latent,
"sleeping", occult, of epizootic stability or tranquillity.
b) Preepizootic
stage is the period between interepizootic and ascending stages. Population
specific immunity gained during previous wave is reduced and number of
susceptible animals is increased also due new born and introduced animals.
Etiological agents increase in number and their pathogenity gets stronger.
Sporadic specific disease cases mainly with atypical chronic manifest or
subclinical forms can be detected. This stage is also called as awakening
period.
c) Ascending
stage is the period between preepizootic and culmination stages reflecting the
most intensive part of epizootic process wave. Population specific immunity is
nearing to minimum and number, pathogenity and propagation of etiological
agents is nearing to maximum. Number of affected animals is increasing. At the
beginning of this stage disease cases are more of atypical and abortive forms
and later on are close to typical or typical not only clinically but also
pathomorphologically. At the end clinical cases have the tendency to acute and
peracute forms.
d) Culmination stage is the period
between ascending and descending stages reflecting relatively full biological
development of epizootic process in a given wave. Population immunity is at
minimal level and number, pathogenity and propagation of etiological agents are
at maximal levels. At the beginning the cases are typical con tendency to acute
form and later atypical cases increase gradually with tendency to subacute form
and afterwards to milder course. Population immunity is getting slowly stronger
thanks to animals surviving the disease (natural immunization) and this
increasing barrier causes the reduction or slowing down of etiological agents activities and propagation.
e) Descending
stage is the period between culmination and postepizootic stages. Proportion of
naturally immunized animals is increasing while number of susceptible animals
is decreasing gradually towards the minimum. Number, pathogenity and
propagation of etiological agents is tending gradually
towards the minimum. Number of diseased animals is gradually decreasing as well
and the cases are getting more atypical, abortive up to subchronic and chronic
forms meanwhile the proportion of latent carriers is increasing.
f) Postepizootic
stage is the period between descending and interepizootic stages. Population
immunity is very high, number, pathogenity and
propagation of etiological agents fall to very low values up to relatively
minimal ones. Number of diseased animals is reduced to sporadic (exceptional)
cases. Latent forms in which
asymptomatic carriers prevail can be detected only using indirect
diagnostic methods (serological, allergic, etc.).
13.3.2 Stages of new
epizootic process
In new continuing epizootic process the above described stages under
natural conditions are complemented by incubation and prodromal stages.
a) Incubation
stage is the period between the onset of particular epizootic process and
prodromal stage. Incubation stage in a given place occupies the period from the
first moment of introduction (penetration) of first specific etiological agents
in a given population, herds or flocks. The number of affected animals slowly
increases without clinical manifestation.
Etiological agents, thanks missing population immunity, have the chance
to reproduce and propagate increasing their pathogenity when passing through
individual susceptible animals.
b) Prodromal
stage is the period between incubation and ascending stages. The first diseased
animals manifest general clinical and pathomorphological changes without
specific characteristics. The number of these animals is increasing as well as
the number, pathogenity and propagation of specific etiological agents.
13.3.3 Stages of terminating epizootic process
In terminating epizootic processes the above described stages
under natural conditions are complemented by extinction stage following
descending stage. It is the period in which epizootic process dies and become
extinct. Susceptible animals gradually disappear, number, pathogenity and
propagation of etiological agents go gradually down up to disappear in the given
space and time.
13.4 Forms of
epizootic process
a) Apparent or
manifest forms can be divided in:
- typical when
clinical and epizootiological characteristics correspond with specific features
- atypical in
opposite cases
- abortive when
clinical and epizootiological characteristics are only mildly expressed
(difficult to determine).
b) Unapparent (subclinical or
asymptomatic) forms are without any clinical manifestation,
however interaction between animal population and etiological agents takes place.
Morphological and/or physiological changes can be typical corresponding with
specific picture or atypical not corresponding with known specific picture.
c) Latent form
when etiological agents are inside the animals-carries without pathogenic
action.
d) Epizootic process
forms are usually mixed i.e. composed of more than one of the above
mentioned forms.
e) According the
severity epizootic process can be malignant with serious consequences or
non-malignant with mild consequences.
13.5 Range, intensity and grades of epizootic process
13.5.1 Population
range of epizootic process
Population range (width) of epizootic
process is expressed by absolute number and proportion of affected animals
(epizootiologically non-healthy) e x i s t i n g in one or more biological species in given
place and time. This criterion can be expressed in prevalence of
specifically affected animals. These values can vary from exceptional
occurrence up to ubiquitous values.
13.5.2 Population intensity of epizootic process
Population intensity of epizootic process is
expressed by absolute number and proportion of n e w
affected animals (epizootiologically non-healthy) of exposed
population(s) at risk in given place and period. This criterion can be
expressed in incidence of specifically affected animals. In local and
short duration outbreaks attack rate can be used. Infection rate is the
incidence of manifest plus unapparent infections, which can be
identified, e.g. by serological testing.
For decreasing intensity of epizootic
process indicator of extinction of specifically affected animals can be
used. (See indicators in chapter 6).
13.5.3 Grades of
epizootic process
a) Grades of epizootic process (pattern of
disease levels) is the outcome of combined values of
range (width), intensity, propagation, tendency as well as space and time
factors of particular disease. Every disease has determinate tendency regarding
the grade of epizootic process.
The bellow division is only artificial
framework for epizootic process classification. There are also transition
grades such as hypo-, meso- and hyper- grades (e.g. hypo-enzootic). Almost in
all infectious disease all mentioned grades can be found according to different
conditions. These grades have important role in defining control strategy and
measures.
b) Sporadic cases
occur irregularly, haphazardly from time to time, and infrequently (e.g.
anthrax).
c) Enzootic
(endemic in human epidemiology). The specific disease or infectious etiological
agents are continuously present in a given population or geographical area
during years, decades, eventually centuries, etc. It may also refer to the
usual prevalence of a given disease within such area or group.
Hyper-enzootic disease is constantly present
at a high incidence and/or prevalence rate with a persistent intense
transmission.
d) Epizootic
(epidemic in human epidemiology) is the occurrence in a population or region of
cases of a disease clearly in excess of normal expectancy, i.e. in relation to
usual frequency of the disease in the same area, among the specific population,
at the same season of the year. The number of cases indicating the presence of
an epizootic varies according to the agent, size, and type of population
exposed, previous experience or lack of exposure to the disease, and time and
place of occurrence.
Epizootic
includes also a level of disease that occurs suddenly in excess of the normal
enzootic level in a population, herd or geographical area as well as new
outbreaks in previously free territory.
Point epizootic is the process in
which the disease requires sudden fast and efficient mode of transmission,
that does not spread further, and is due to a common exposure. (E.g.
mycotoxic infection due to contaminated feed centrally distributed to grand
number of animals).
Propagated epizootic requires a less
efficient mode of transmission and shows a pattern of spread from one animal to
another. This disease usually involves vectors or carriers.
e) Panzootic
(pandemic in human epidemiology) is a process occurring over a very wide area
and usually affecting a large proportion of the population. Panzootic wave can
cross countries and continent . Panzootic process can
be rapid propagating over large territories in a very short time (e.g. panzootic
waves of foot-and-mouth disease) or slow propagating over large territories
during long period (e.g. myiasis due to screwworm fly).
f) Mixed grades
reflect the situation when and where in the same space and time can one grade
be combined with the other one (e.g. enzoo-epizootic when in enzootic area an
epizootic appears).
13.6 Space aspects and epizootic process
a) Location. It
is obvious that epizootic processes are located in areas (nosoareas) with susceptible
species of animals.
b) Some epizootic
processes are located in particular regions (e.g. Venezuelan equine
encephalomyelitis in South America) due to specific environmental conditions or
other particular influences such as geographic isolation (islands).
c) Every epizootic
process has its concrete localization and delimitation which can be
identified using natural (geoecological) or artificial criteria (topographical,
social/economical or political/administrative).
d) Spacial range
(width) of epizootic process expresses the absolute size and proportion
from the total territory using surface measure units (e.g. km2) or
administrative units. Maps are usual means for evaluation of territorial
occurrence of diseases.
e) Spacial intensity
of epizootic process expresses concentration (density) and dispersion of
affected animals in a given space or territory. The evaluation is based on the
relation of the number of affected animals per surface measure unit. In many
epizootic processes there is a tendency to space clustering of affected
animals and foci.
f) Geographical
distribution of epizootic processes can be universal (ubiquitous) occurring
in the whole world (e.g. toxoplasmosis) or in limited territories at different
levels (e.g. glanders in Central Asia).
g) According to
different criteria the geographical distribution can be subdivided in so called
"epizootiological zones" such as:
- according to
epizootic process course (e.g. zone of culmination stage)
- according to
epizootic process form (e.g. zone of manifest forms)
- according to epizootic
process grade (e.g. zone of sporadic cases, enzootic zone)
- according to
epizootic process type (e.g. zone of sylvatic rabies).
h) Epizootic processes
are in permanent dynamic (mobility, changes) and only in relative
stability not only as far as their characteristics are concerned but also
regarding space aspects. Their horizontal movements are characterized by
direction, speed , distance and frequency.
i) The spread of a
disease can be towards different relatively separated places - dissemination
form or step by step covering major and major area - propagative form.
13.7 Time aspects
and epizootic process
a) For time
delimitation of epizootic process critical points are origin and final
moments of this process as well as origin and final moments of different
stages and waves.
b) From the global
point of view epizootic processes are uninterrupted (continuous
epizootiological chain) up to the moment of their global extinction. Duration
of these processes can be of hundreds and thousands years. In spite of the
interruption of overwhelming majority of epizootiological chains (effective
eradication programmes in different territories) converted in "blind
branches", the particular processes continue (up today no one known animal
disease has been globally eradicated). On the other hand it can be supposed
that many diseases disappeared naturally in the past.
c) From the local
point of view epizootic process duration is very different depending on many
factors. In small foci immediately liquidated the process can be of up to
several days and in grand foci can last many days (when liquidated), many
weeks, months, years (e.g. enzootic) and centuries (e.g. in diseases with
natural nidality).
d) In a given place or
territory specific epizootic process can occur only once (exceptionally)
or repeatedly (newly introduced) or in continuous form (including
the recurrence based on new manifestation of previously latent course).
e) Periodicity
of epizootic process is based on regular waves in disease occurrence - number
of affected animals and foci. It has different forms such as:
- seasonality
with regular variation during the year that conforms to a regular seasonal
pattern (e.g. typical for vectorial diseases with natural nidality);
- cyclicity with regular variation
during periods longer than one year (e.g. dependency on population immunity
levels variation and vectors biology; 7-10 years cycles of foot-and-mouth
disease in cattle under natural conditions).
f) Cases may occur in
time clusters, a pattern typical of outbreaks or epizootic. A useful means to represent the temporal
distribution pattern of disease events is to construct an epizootic curve
which can illustrate both the magnitude of the problem, i.e. the number of new
cases occurring, and the rapidity with which the epizootic progresses.
13.8 Dynamics of epizootic process
The dynamics of epizootic processes is
caused and influenced by a large complex
of changing factors
such as characteristics of animal populations, etiological agents, vectors populations and environmental conditions as well as
preventive and control measures. Infinite variability of the
characteristics of these factors and their interactions conduce to infinite
dynamics features of epizootic processes.
13.9 Classification of epizootic processes
Classification of epizootic processes can be
divided according to similar criteria as described in chapter 12. Other
criteria are according to epizootic process course (stages), according to space
and time aspects as described above.
13.10 Epizootic process of non-infectious diseases
a) Epizootic process of
non-infectious disease is based on interaction between a b i o t i c
etiological agents (factors) under given environmental conditions
(non-infectious etiological triad) which is reflected in the content,
course, forms, intensity, grades, dynamics as well as in space and time
characteristics.
b) Majority of general
principles applied in infectious processes are applicable on
non-infectious processes.
c) It is obvious that
these processes are relatively simpler due to the fact that abiotic etiological
agents are unable to reproduce and continue through epizootiological chains as
in case of infectious diseases.
d) Genetic diseases
transmission to other generation represent a form of genetic
chain. Short nutritional chain is known in some diseases caused by feed
residues which can pass to man through food of animal origin.
14. ANIMAL POPULATION DISEASE NIDALITY
======================================
14.1 Introduction
a) Nidality is the
characteristic of etiological agents to occur in distinct nidi associated with
particular geographic, climatic and ecological conditions.
b) Animal population
disease nidality reflects the heterogeneity in territorial distribution of
diseases based on places with concentrated sources of etiological agents -
foci.
c) Focus (nidus) of
animal disease (infection) is a place (area) with affected animals and/or other
concentrated sources of specific etiological agents. The term is usually
applied to a relatively small area in which
occurrence and spread of an infectious disease take place.
d) Very often the term
"outbreak" is used as the synonym of "focus".
Outbreak is an identified occurrence of disease involving one or more animals.
It means an occurrence of the disease in an agriculture establishment, breeding
establishment or premises, including all buildings and all adjoining premises
where animals are present. For each disease the definition should be adjusted
to its nature and given conditions.
e) Etiological
agents can be propagated from the foci
to other places (areas) and thus facilitating continuation of epizootic
process.
f) Territorial
occurrence of animal diseases is the subject of so called "landscape
epizootiology".
14.2 Foci according
etiological triad components
a) Foci according
participating etiological agents can be
divided in:
- foci of specific disease (species of agents)
- foci of specific disease types or subtypes
- monoetiological (formed by only one agent
species)
- polyetiological (formed by more than one
agent species).
b) Foci of specific
disease can be divided according animal population participation in:
- foci of the disease in particular animal
species
- foci of the disease in particular animal
category
- monohostal (only one animal species involved)
- polyhostal (more than one animal species
involved)
- foci with affected animals
- foci without affected animals (e.g. in
observation period)
- foci without any susceptible species animal
(depopulated)
- non-natural (man-made in domestic animals)
- natural
- combined (natural mixed with non-natural).
14.3 Foci according to their form
Foci of specific disease can be divided
according their forms in:
- apparent or manifest with clinical
manifestation
- abortive manifestation without
specific characteristics
- latent without clinical manifestation
- typical corresponding to usual specific
characteristics
- atypical not corresponding to specific
characteristics
- active with epizootic process
spreading
- non active (e.g. in interepizootic
stage)
- autochthonous originated without man's
intervention
- anthropurgic originated with man's
intervention.
14.4 Foci and space aspects
a) Foci of specific
disease can be divided according space factors in:
- grand (can be divided in subfoci, e.g.
ranch size in farm size)
- small (if very close they can be
united in one of major size)
- with demarcated borders (clear cut
limits separating from perifocal area)
- with diffused borders (not clear
limits)
- elemental (with more intensive
epizootic process inside of major focus, e.g. in natural nidi).
b) Geographic
distribution is of an extraordinary importance. The same values of nidality
rates and/or characteristics of particular foci have different epizootiological
importance according their localization in relation to susceptible species
distribution, exposition degree, ecological conditions, epizootiological
situation, etc.
14.5 Foci and time aspects
a) Time delimitation of
foci, mainly the moments of their origin and fin, indicates critical
moments for diagnostic and control activities.
b) Foci of specific
disease can be divided according time factors in:
- primary, i.e. the first focus in a
given area or territory; this criterion is relative (e.g. the same focus can be
a primary one for a particular region but not at national level)
- secondary following the primary one
- of short duration (e.g. in peracute
form of a disease)
- of long duration (e.g. in chronic
diseases)
- new (fresh - e.g. primary focus)
- old (residual - e.g. enzootic).
c) Frequency of new
foci is expressed by incidence indicator.
d) Stages of
foci are similar as the stages of epizootic process (see chapter 13).
14.6 Natural nidality
14.6.1 Concept of natural nidality
a) Natural nidus
(focus) is the area of a particular biocoenosis in which etiological agents
circulate independently on man.
b) Natural nidus is
determinated biologically (influenced by the flora and fauna of host and
vectors), geographically and ecologically (e.g. by the climate causing
seasonality).
c) Bioceonotic
relations among participating species are mainly of trophic (alimentary)
character. These interrelations ensure uninterrupted transmission of
etiological agents from wild animals-carriers to other susceptible animals
(receptors) through vectors, eventually by other ways.
d) The animal-receptor
converts in potential donor of etiological agents starting a new cycle of
transmission and thus keeping their circulation and continuation of
epizootiological chains through repeating cycles.
e) Also domestic
animals, eventually human beings can be infected after entering in the area of
natural focus and become receptors. Usually this branch of epizootic chain is
blind and does not continue.
f) Natural nidi are
characterized by typical enzootic grade of the process.
14.6.2 Circulation of etiological agents
a) Vectorial
circulation of etiological agents is based on the transmission by vectors,
mainly blood sucking arthropods, from animal-donors being in stage when
these agents are spread in blood or in accessible tissue (e.g. role of ticks in
tularemia). Vectorial transmission through vectors - non arthropods is
based on their active penetration in or
being ingested by animal-receptor.
b) Avectorial
circulation of etiological agents is based on direct transmission from one host
to other host without any vectors (e.g. leptospirosis).
c) Mixed
circulation of etiological agents combines vectorial and avectorial
transmission (e.g. Q-fever).
d) Circulation of
etiological agents among wild animals only constitutes so called wild cycle
(sylvatic cycle) unlike domestic cycle among domestic and synanthropic
animals (e.g. sylvatic and domestic cycles in many diseases with natural
nidality such as rabies).
e) Some birds
constitute reservoirs of different etiological agents of diseases with natural
nidality (e.g. pheasants in equine encephalitis).
14.6.3 Biotic
structure of natural foci
a) Biotic structure of
natural foci is composed by following components:
- etiological agents (pathoergonts)
- animals-reservoirs (donors)
- vectors (transmitters) in case of
vectorial circulation
- animals-receptors
- environment factors.
b) If more species
participate in the circulation of etiological agents then biotic structure is
richer and more complex.
c) Biotic structure is
the phenomenon which keeps natural foci very resistant against different
disturbances including control measures.
14.6.4 Territorial structure of natural foci
a) Natural foci
territorial structure is composed from focal nucleus (elemental focus - focal
"heard"), marginal area (around focal nucleus) and dispersion area
(around marginal area).
b) In focal nucleus
the epizootic process is the most active, consistent and lasting during
existence of a given natural focus. Etiological agents spread from this area
towards other focal areas often in form of waves. In this area the density of
focal populations (hosts, interhosts and vectors) is high and epizootic process
has hyper-enzootic character.
c) In marginal area
the epizootic process has more hypo-enzootic character being refreshed by new
waves of etiological agents propagation from focal nucleus.
d) In dispersion
area the epizootic process is manifested mainly by sporadic cases of
atypical course.
e) The borders between
different components of natural foci are not clear cut and fixed due to their
dynamics in space and time.
14.6.5 Biotopes
changes and natural foci
a) Natural foci and
their characteristics are closely related to biotopes and biocoenoses and their
changes. Natural foci exist not only in purely natural biotopes but also in biotopes
modified by human activities.
b) Human activities can
activate some natural foci, change their characteristics, create unconsciously
new ones or contribute to their disappearance. Disturbances of original
biocoenoses can push the animals and vectors to search for new areas or to
adapt themselves to the new conditions.
c) According to grade
of land cultivation following biotopes can be distinguished:
- natural (with original flora and fauna)
- poorly cultivated (isolated culturcoenoses
within the natural ones)
- moderately cultivated (natural and cultivated
areas mosaic)
- highly cultivated (cultivated area is
dominant)
- totally cultivated.
14.6.6 Wild animals and natural foci
a) All animal
species can participate in different natural foci of infectious disease:
mammals, birds, reptiles, amphibians and fishes.
b) Wild animals
according to their relation to man can be divided in exoanthropic living
freely and far from human activity and in synanthropic living closely to
man. The latter can be subdivided in permanent synanthropic (e.g. Mus
musculus) and semipermanent synanthropic (e.g. Apodemus
sylvaticus) living or penetrating in areas with domestic animals and human
population.
c) Migratory and
semipermanent animals have important role in creating new natural foci in
distant territories.
14.6.7 Disease with
natural nidality
Almost all known infectious diseases can
occur in form of natural foci. Examples:
- viroses (Rift Valley fever, rabies)
- bacterioses (pasteurelloses, tularemia)
- chlamydioses and rickettsioses (Q-fever,
cowdriosis)
- mycoses (aspergillosis, actinomycosis)
- protozooses (babesiosis, trypanosomiasis)
- helminthiases (trichinellosis, fascioliasis)
14.7 Indicators of animal disease nidality (focality)
a) Nidality (focality)
in quantitative terms expresses the grade in which a given territory is
affected by specific infectious disease foci. The first step is to define
clearly the limits and size of foci (individual diseases require different
definitions) in order to be able to apply respective measure units.
b) As measure units
can be used surface measure units
(e.g. km2) in natural foci or focal measure units such as production
units (stables, farms, ranches), villages, districts, etc. in other foci.
c) For territorial
distribution maps including not only "normal" foci but also
artificial foci (e.g. sanitary slaughterhouses, laboratories working with
infectious etiological agents, etc.) are used.
d) Nidality
(focality) is the basic indicator expressing the relation of the number
of foci to the total number of focal units; in natural nidality expressing
the proportion of affected territory size from the total territory.
e) In concrete cases of
specific diseases is necessary to respect time factors:
- moment prevalence rate of foci
- period prevalence rate of foci
- average prevalence rate of foci
- incidence rate of foci
- extinction rate of foci
Note: More information on the
indicators see in chapter 20.
f) Other indicators
used are the rates of affected farms, epizootiological zone rates (e.g. rate of
affected zone), territorial density of foci, etc.
14.8 Nidality of
non-infectious diseases
a) Also non-infectious
diseases can occur in distinct nidi associated with particular ecological
conditions reflecting the heterogeneity in territorial distribution of these
diseases (e.g. plant toxicoses, alimentary deficiencies).
b) In spite of the fact
that the nidality mechanism is completely different in comparison with
infectious diseases, many aspects and methods described above are applicable on
non-infectious diseases as well.
Note:
Landscape epizootiology (syn.:
epizootiological ecology, horizontal epizootiology, epizootiological geography,
etc.) is founded on the concept that if the nidality of diseases is based on ecological
factors then a study of ecosystems enables predictions to be made about the
occurrence of disease and facilitates the development of appropriate strategy.
15. DISEASES COMMON TO MAN AND ANIMALS
======================================
15.1 Introduction
a) Man and animals have shared the same
environment throughout their evolution. It is consequently natural that they
share many pathogens. Because of this closeness, some parasitic organisms have
developed life cycles which may include animals as well as man; others have
adapted themselves so as to accept man (or, viceversa, animals) as an
occasional host, not essential for the survival of the parasite species.
b) Those diseases and
infections that are naturally transmitted between vertebrate animals and man
are defined as zoonoses.
c) There are two groups
of communicable diseases common to man and animals:
aa) Zoonoses
represent infections or infectious diseases transmissible under natural
conditions from vertebrate animals to man where animals play an essential role
in maintaining the infection in nature and man is only an accidental host.
bb) Diseases where both
man and animals contract the infection from the same sources, such as
soil, water, invertebrate animals and plants; as a rule, however, animals do
not play an essential role in the life cycle of these etiological agents, but
may contribute in varying degrees to the distribution and actual transmission
of infections.
d) Due the fact that Homo
sapiens belongs biologically to
animal kingdom (mammals class) many general principles of epizootiology
are applicable on human epidemiology and vice versa. This is the reason
why epizootic and epidemic processes are so similar and in zoonoses are
commune. However, social/community and creative consciousness aspects
significantly differentiate human beings from animals with all
consequences for communicable diseases processes.
e) Majority of the
known infectious diseases in human population has historical or actual relation
to animal diseases.
f) When zoonotic
disease are occurring at a low incidence, the occurrence of human disease my
often be the first indication that infection is being transmitted (e.g.
chlamydiosis - psittacosis in slaughterhouse employees from infected turkeys).
15.2 Human
population and its basic characteristics
For epidemic processes of diseases common to
man and animals following characteristics of human population are of major
importance:
- absolute numbers of
inhabitants (demography)
- geographical
distribution
- density and
concentration
- relations between
number of inhabitants and animals
- category (age, sex,
etc.) structures
- living standard
- economic and social
activities
- culture life
- horizontal movement
- reproduction (much
slower than in animals)
- disease resistance
and susceptibility, etc.
15.3 Exposition of human population
a) Exposition to common
etiological agents can be direct or
indirect. It is obvious that rural population is more exposed to diseases
transmissible from affected domestic animals than urban population. Particular
zoonotic risk is there where humans and domestic animals live under the same
roof in daily contact between them.
Inhabitants are at zoonotic
risk when consuming crude food of animal origin.
b) Wild synanthropic
animals (e.g. wild rodents such as rat and mouse) living in the same areas
as humans and having access to stored food represent other type of risk.
c) Professional exposition
during human activities in risky environment such as livestock husbandry,
slaughterhouses, diagnostic and production laboratories, etc. causes many cases
of occupational zoonoses among farmers, workers, veterinarians, etc.
d) Recreational
exposition of tourists, hunters etc. in territory with natural nidality
diseases represents the risk of
acquiring local infections.
15.4 Sources of
common etiological agents
a) Primary sources
are diseased (affected) animals from which etiological agents can be
transmitted to man directly and/or indirectly (e.g. B. melitensis). In
some zoonoses also man can become primary source threatening humans and
susceptible animals (e.g. Taenia saginata). In majority of zoonoses man
has the role as final or accidental host.
b) Vectors as
intermediatory sources and transmitters play similar role in human as in animal
populations (e.g. ticks in Q-fever).
c) Food of animal
origin if infected and/or contaminated represents secondary source of many
zoonotic etiological agents causing alimentary infections in humans (e.g. milk
in brucellosis, meat in salmonelosis).
d) Soil, water
and other contaminated substances and objects can serve as sources of zoonotic
etiological agents threatening human beings (e.g. B.tetani).
e) Biological
products containing live zoonotic etiological agents such as laboratory
strains (e.g. B.mallei), live vaccines (e.g. anti-brucellosis vaccine B
19), etc. have caused specific diseases in humans.
15.5 Transmission of
common etiological agents
a) Forms of
transmission of etiological agents common to man and animals are similar
as described in chapter 10. This is valid not only for the transmission from
animals to man but also from man to animals. Interhuman transmission of
zoonoses is usually limited to exceptional cases, if any. However, interhuman
transmission of cutaneous zoonotic mycoses can be quite frequent.
b) Particular form of
transmission is in case when man participates in life cycle of some
zoonotic parasites(e.g.transmission of Taenia saginata to cattle through
ingesting eggs from faeces of man affected by adult forms).
15.6 Infections
common to animals and man
15.6.1 Classification of zoonoses
Classification of zoonoses has different
criteria. As example following subdivision is used:
a) Direct zoonoses: Infections perpetuated in a host and transmitted to
humans either directly by a bite (e.g.rabies) or indirectly by contamination of
human food (e.g. Salmonella
contamination of meat).
b) Cyclozoonoses:
More than one vertebrate species are required for survival (e.g. hydatid
disease due to Echinococcus granulosis depends on both a canid and
ruminant for survival).
c) Metazoonoses:
These zoonoses require both a vertebrate and an invertebrate host for survival.
Humans are infected when bitten by
vector species (e.g. viral
encephalitis survives in various avian species and transmission is
achieved by mosquitoes).
d) Saprozoonoses: These
infections require a non‑living site such as soil or water
to persist or multiply. Both parasitic (e.g. visceral larval migrans)
and mycotic (e.g. blastomycosis)
zoonoses fall into this group.
15.6.2 Examples of diseases common to man and
animals:
a) Viroses:
Argentine haemorrhagic fever, Bolivian haemorrhagic fever, bovine papular
stomatitis, California encephalitis, cat-scratch disease, Colorado tick fever,
contagious ecthyma, cowpox, Crimean-Congo haemorrhagic fever, eastern equine
encephalitis, encephalomyocarditis, fevers caused by group of C arboviruses,
foot-and-mouth disease, goatpox, haemorrhagic fever with renal syndrome,
hepatitis transmitted by non-human primates, herpes simplex, Herpesvirus
simiae, Ilheus fever, influenza, Japanese B encephalitis, jungle yellow
fever, Kyasanur forest disease, Lassa fever, louping ill, lymphocytic
choriomeningitis, Marburg disease, Mayaro fever, Murray Valley encephalitis,
Newcastle disease, Omsk haemorrhagic fever, Pawassan encephalitis,
pseudocowpox, rabies, Rift Valley fever, rotaviral enteritis, Russian
spring-summer encephalitis, St.Louis encephalitis, Sindbis fever, smallpox of
non-human primates, swine vesicular disease, vaccinia virus infection,
Venezuelan equine encephalitis, vesicular stomatitis, Wesselsbron disease,
western equine encephalitis and West Nile fever.
b) Bacterioses:
Erysipelas (human erysipeloid), animal tuberculosis, anthrax, botulism,
brucellosis, campylobacteriosis, clostridial food poisoning, clostridial wound
infections, colibacillosis, glanders, leptospirosis, listeriosis, melioidosis,
pasteurellosis, plague, rat-bite fever, salmonellosis, shigellosis,
staphylococcal food poisoning, streptococcosis, tetanus, tick-borne relapsing
fever, tularemia and yersiniosis.
c) Mycoses:
actinomycosis, adiaspiromycosis, aspergillosis, blastomycosis, candidiasis,
coccidioidomycosis, cryptococcosis, dermatophylosis, dermatophytosis,
histoplasmosis, maduromycosis, nocardiosis, phycomycosis, rhinosporidiosis and
sporotrichosis.
d) Chlamydioses and
rickettsioses: Asian ixodo-rickettsiosis, boutonneuse fever, flea-borne typhus
fever, fowl chlamydiosis, Q-fever, Qeensland tick typhus, rickettsial pox,
Rocky Mountain spotted fever and scrub typhus.
e) Protozooses:
African trypanosomiasis, American trypanosomiasis, babesiosis, cutaneous
leishmaniasis, malaria in non-human primates, pneumocystis pneumonia,
toxoplasmosis and visceral leishmaniasis.
f) Metazooses:
aa) Trematodiases:
clonorchiasis, dicroceliasis, echinostomiasis, fascioliasis, fasciolopsiasis,
gastrodiscoidiasis, heterophyiasis, opistorchiasis, paragonimiasis and
schistosomiasis.
bb) Cestodiases:
Bertielliasis, coenurosis, diphyllobothriasis, dipylidiasis, hydatidosis,
hymenolepiasis, inermicapsiferiasis, mesocestoidiasis, raillietiniasis,
sparganosis and taeniasis.
cc) Nematodiases:
Angiostrongyliasis, anisakiasis, ancylostomiasis, ascariasis, capillariasis,
cutaneous larva migrans, dioctophymosis, dracontiasis, esophagostomiasis,
gnathostomiasis, lagochilascariasis, strongyloidiasis, syngamosis, thelaziasis,
trichinosis, trichostrongyliasis, toxocariasis and zoonotic filariasis.
dd) Pentastomids and
arthropods: myiases (including screwworm fly Cochlyomyia hominivorax),
pentastomidiasis, tungiasis and zoonotic scabies.
Note: More information in human
epidemiology textbooks.
16. ECONOMIC AND SOCIAL FACTORS INFLUENCING
EPIZOOTIC PROCESS
=============================================================
16.1 Introduction
a) Epizootic process
is influenced not only by natural environment but also by economic and social
factors. All these factors act individually or in complex, positively or
negatively, directly or indirectly.
b) Economic and
social factors analogically as natural environmental phenomena can influence
animal population, etiological agents and their transmission. Therefore
these factors play very important role in epizootiological situation
development.
16.2 Influences of
economic factors
16.2.1 Influences of
economic level
a) Total national and
per capita crude product reflected in social level and in availability of material
and financial resources for animal health programmes influences epizootic
processes indirectly.
b) Better conditions
for the origin and development of epizootic processes are in territories with
low economic level or in economic crisis where due to shortage of economic
resources epizootiological situation cannot be controlled effectively or at
all and vice versa.
c) Animal health
programmes of poor countries have serious shortage of funds, professional
manpower, diagnostic laboratories, transport, equipment, drugs and vaccines
combined with underdeveloped communication, infrastructure, investment and
suffering by high inflation. Under these conditions it is very difficult to
carry out effective animal health actions. Opposite situation is in rich
developed countries.
d) International
technical assistance supporting national programmes is important, however it
can only help for relatively short period but cannot fully replace national
funding, manpower and infrastructure.
16.2.2 Influence of
urbanization and industrialization
a) Urbanization
aggregating human populations in cities creates new conditions for epizootic
processes due to changed ecosystems and urban and suburban biocoenoses. It
facilitates isolation of domestic animals from wild animals as well as
separation between circulation of etiological agents in urban and wild
(sylvatic) cycles (e.g. rabies).
Other impact of urbanization is isolation of
domestic animals kept individually slowing eventual disease propagation.
However, stray animals in cities are becoming serious problem in controlling
diseases, zoonoses in particular.
b) Industrialization
forms areas without or almost without domestic animals. However synanthropic
animals are able to adapt and survive.
c) Urbanization and
industrialization change significantly landscape, climatic, hydrologic and
biospheric conditions with usually negative impact on animal populations, their
health and resistance. Control of animal population diseases is very
distinct in comparison with rural areas.
16.2.3 Influences of
transport, trade and tourism
a) Transport in
general and in particular of animals and their products facilitates propagation
of etiological agents at local, regional, provincial, national and
international levels. Today transport means are able to reach any place in the
world in very short time which can intermediate rapid propagation of animal
diseases to very distant places. Increasing transport intensity increases the risk
to introduce exotic diseases into free territories. In principle, any
import of animals and their crude products represents epizootiological risk.
The most dangerous are animals - latent carriers (without clinical symptoms)
and their products certificated as healthy.
b) Trade routes and
market places imprint the framework for many infectious diseases and their
propagation (e.g. massive territorial spread of foot-and-mouth disease from
cattle market place where few latently diseased animals from one focus were introduced).
Intensification of trade and its vertical and horizontal networks increases
epizootiological risk. The traders do not always respect veterinary measures
and many try to cheat to save the profit.
Movement of animals is often associated
with disease outbreaks. Therefore
the history of recent
exposure through marketing
channels requires that purchasing policies be evaluated.
c) Increasing tourism
increases the risk of animal diseases propagation through wastes of crude food of animal origin
used for personal consume or of zoonotic diseases (parasites) where the man is
final host or interhost (e.g. invasion of cattle ingesting grass contaminated
by human faeces with eggs of Taenia saginata in tourist camping zone).
16.2.4 Influences of
rural development
Rural development based on transformation of
rural activities and conditions tending to improve agriculture production using
new technologies, irrigation, drainage, building dams, roads, communication
networks etc. changes land use, rural ecosystem and biocoenosis. This
has impact on epizootiological situation development (e.g. irrigation
facilitates origin and propagation of
mosquito-borne diseases).
16.2.5 Influences of economic activities
Economic activities not respecting animal
health reduce the resistance and thus contribute to worsening of animal
population health. Among the factors altering original ecosystems are pollution
of different types (physical, chemical and biological), destruction and
deterioration of environment, contamination of soil, water, feeds, etc. Use
of chemicals in agriculture causes the problems of residues in animal products.
16.3 Influences of
animal breeding and production
16.3.1 Grade of animal breeding and production development
a) Breeding and
production systems influence directly and indirectly epizootic processes and
epizootiological situation:
b) Primitive
natural animal breeding where etiological agents circulate freely among
domestic and wild animals due to lack of their mutual isolation mainly in vast
pasture territories is very favourable for epizootic processes development and
on the other hand is very difficult to control them (e.g. rinderpest in nomadic
cattle herds). Natural resistance of local breeds is relatively high.
Parasitoses can be found almost in all adult animals. Vectorial diseases are
relatively frequent.
c) Extensive
systems of raising livestock is characterized by animals being free within the
defined area of natural pastures. Animals are relatively dispersed and therefore
specific epizootic processes propagate slowly and within the area of pasture
limit (if there are not contacts with neighbouring domestic animals or wild
animals of susceptible species).
d) Intensive
breeding and production is based on major concentration of domestic animals
isolated from neighbouring herds and wild animals. Eventual epizootic processes
have easier conditions for local propagation within affected herds and more
difficult conditions for propagation to other livestock husbandry establishments.
Intensive raising include also grazing either crops or cultivated pastures.
e) Industrial type
of production characterized by high animal concentration, specialization and
yielding facilitates the protection against the introduction of etiological agents
from outside due to complex isolation. On the other hand in case of infectious
disease outbreak the propagation is relatively easy and speedy thanks also to
many passages of etiological agents conducing to higher pathogenity. Natural
resistance is low and stress factors due to artificial conditions can
contribute to the origin of endogenous diseases.
f) Pathogen free
breeding based on parturition and keeping of animals under artificial
conditions without etiological agents represent the most developed form of
animal industry. These animals are highly susceptible to different diseases and
require perfect protection.
g) The above
classification represent only an orientation structure. There are many mixed
and transition forms.
16.3.2 Organization
and structure of breeding and production
a) Organization and
structure imprint the framework for eventual epizootic processes and
epizootiological situation.
b) Horizontal
(decentralized) type of organization according to territories can
facilitate disease propagation among different establishments in a given
region.
c) Vertical
(centralized) type according to specialized breeding and production can
facilitate disease propagation between different territories, including distant
places linked in production and category specialized units chain (e.g.calves -
heifers - cows - feedlot units).
d) Organization
according to production sectors (private, cooperative, state, etc.)
creates other structure having also particular impact on epizootiological
situation.
e) Similar role has
internal organization and structure of individual establishments (farms,
ranches, etc.).
16.3.3 Animal breeding and production technology
a) There are many different
technologies. Each of them has different importance for animals, animal
health and disease and for etiological agents transmission. Therefore each
technology requires particular evaluation from epizootiological point of view
to protect animals and to apply necessary measures in case of an outbreak.
b) The availability of and
type of housing for animals
either complete or intermittent can have
very significant effects on disease occurrence either by increasing the
density of animals or as a result of air quality. The type of flooring and
bedding may influence the surviving of etiological agents eliminated by
affected animals. Contaminated facilities
for feeding, watering, and
maintaining hygiene can have similar role.
c) Physiological
deficiency and contamination of breeding and production mechanization
equipment can hurt or cause stress and provocation of endogenous diseases
or facilitate transmission of some etiological agents (e.g. mastitis through contaminated
machine milking).
d) Central preparation
and distribution of feed if contaminated can cause the spread of alimentary
diseases.
e) Genetic crossing
and selection, in particular of males for mating (artificial insemination),
if health aspects are underestimated and affected animals are used, can cause
dissemination of infectious diseases (e.g. venereal) to many females in the
given area.
f) Reproduction
system has the influence on vertical transmission of etiological agents to new
generation. New born animals breeding technology is decisive for later
development and health depending a lot on duration of contacts between the
mother and young animals. There are different technologies from semi-natural up
to artificial breeding (shortening and/or eliminating these pre-weaning
contacts).
g) Breeding and
production space and time cycles and rhythms reflect in eventual
epizootic processes cycles and rhythms. This is valid also for technology
synchronization influences.
16.3.4 Management
and hygiene influences
a) Many different
techniques are used to manage animals particularly under intensive production
systems. These include segregation of
species, "all in‑all out" policies whereby housing is completely cleaned and disinfected
between groups of animals, or continuous
flow systems where no attempt is made to segregate animals by age or state
of production. If infectious agents are introduced then in "all in-all
out" system the transmission to following "batch" is avoided
which is not the case in continuous flow system.
b) Similar difference
in the consequences can also be between "closed"
(self-sufficient) and "open" (supplied by animals from
outside) herds or flocks.
c) Methods of feed
and water delivery, as well as the sources, storage and pattern of
feeding play a role in eventual
transmission of infectious agents penetrating through oral portal of entry.
16.3.5 Processing of
products of animal origin
Facilities processing products of animal
origin from places of different epizootiological situation must be understood
as artificial foci of etiological agents. Therefore their wastes and
contaminated transport means can represent transmission factors in spreading of
many diseases (e.g. foot-and-mouth disease). Rendering plants and sanitary
slaughterhouses are of particular importance in this respect.
16.4 Influences of
social factors
a) Social factors have indirect
influence on epizootic processes and epizootiological situation. It is obvious
that high standard of living, cultural level, stable political
conditions and peace provide much better social environment for prevention
and control of animal populations health and diseases and vice versa.
b) It is obvious that
the hungry and poor people ("have-nots") need first to survive and
animal disease control is usually out of their actual interest.
c) Similar importance
have public and political support, animal health legislation, economic
support through differentiated prices and subsidizes, standard of veterinary
services, extension services, etc.
d) Animal health
programmes and measures are directly targeted against animal population
diseases (specific epizootic processes) to block, reduce, eliminate and
eradicate them.
17. CONSEQUENCES OF
ANIMAL POPULATION HEALTH AND DISEASES
=========================================================
17.1 Introduction
a) Good health of
animals is the main prerequisite for their effective production and optimal use
and for human health protection against diseases transmissible from animals.
Animal diseases have opposite impact.
b) Consequences can be
of biological, economical, public health and social character.
c) Benefit of animal
population health is much more important than the losses caused by animal
morbidity and mortality. Unfortunately, majority of this kind of analyses is
traditionally based on negative aspects. This debilitates our argumentation
when looking for public, economic and political support for animal health
programmes.
d) Analysis of animal
health and disease consequences complements the analysis of epizootiological
situation.
17.2 Characteristics
of consequences
Classification of consequences
characteristics of animal health and disease has many criteria such as:
a) according to animal
species and categories (e.g. losses in new born lambs)
b) according to causality
(e.g. losses due to goat brucellosis)
c) according to environmental
factors (e.g. losses due to bad ventilation causing respiratory infection
in calves)
d) according to health/disease
forms (e.g. losses due to nervous form of Newcastle disease in poultry)
e) according to quantitative
aspects (e.g. benefit in milk yield of mastitis free cows)
f) according to qualitative
aspects (e.g benefit in meat quality of cattle free of diseases - better
prices and no restrictions in distribution and consumption)
g) according to direct/indirect
impact (e.g. direct positive impact of health and negative of disease in
natality, indirect positive/negative impact in body weight growing)
h) according to space
(e.g. losses due to animal disease in ranch X.)
i) according to time
(e.g. benefit of animal health during year 2007)
j) according to visibility
(e.g. visible or observed losses due to dead animals, non-visible losses due to
sanitary restriction of animal movement and trade)
k) according to measurability
- ability to be quantified (e.g. measurable benefit in liters of milk,
un-quantifiable benefits of protection of territory against introduction of
exotic disease)
l) according to other
criteria such as evaluation of total or of a part of
benefit/losses, relativity , etc.
Example: FAO
estimated in 1962 that average total losses caused by animal diseases in
developed countries were about 15 percent and in developing about 35 percent;
stamping out as benefit for the society but as loss for the farmer; what is
loss for importing country can be benefit for exporting country.
17.3 Categories of health/disease consequences
17.3.1 Biological
consequences
Biological consequences reflect in changes
of epizootiological situation (e.g. changes of number and proportion of healthy
and diseased animals, disease free and affected herds), structure and
characteristics of the biosphere (e.g. disappearance due a disease of an animal
species having role in alimentary chain), density and structure of animal
populations, etc.
Example: Myxomatosis of rabbits introduced
into Europe in the middle of 20th century from Australia to France
devastated domestic and wild rabbits in
European continent during several months; wild rabbit population has not yet
been recovered.
17.3.2 Economic consequences
Positive economic consequences of
animal health and negative of animal disease are multiform being reflected
in:
a) Number of animals
(total, per surface unit, per space volume unit, species proportions, category
proportions, slaughtered animals rate, culled rate, culled rate due to
diseases, natural mortality, etc.). Diseases determine critical limit for the
concentration of animals in industrial type of production.
Examples: Complete depopulation due to
African swine fever mortality and sanitary slaughter in Dominican republic and
Haiti in 1980-1982 reduced swine population from 1,5 million to zero.
b) Genetic value
(proportion of genetically valuable animals, rate of high yielding animals,
culled rate due to production reasons, etc.);
Example: In Netherland in 1980 from total
number of culled cattle only 30 % were due to biological (production) reason
because 70 % must be slaughtered prematurely due to disease reasons.
c) Development
(natality rate, survival-to-weaning rate, survival rate, breeding animals
survival rate, fattening animals survival rate, age at weaning, age for
feedlot, age of sexual maturity, age at first parturition, age for slaughter,
new born weight, weight at weaning, weight at feedlot beginning, weight at a
given age, weight at sexual maturity, weight at slaughter, weight gain per time
unit, period for achieving a given weight, duration of active/productive life,
etc.);
Example: Aujeszky disease delayed feeding
period in evaluated intensive type swine feedlots by about 20 days.
d) Reproduction
performance (fertility, number of services per conception, duration of service
period, reproduction interval, number of
new born animals per female in reproductive age during one reproduction cycle and
during the whole life, replacement/restocking rate, etc.);
Example: In Hungary, 1981, bovine
tuberculosis caused sterility in 5 % of affected cows; bovine anaplasmosis in
Argentina evaluated herds (1980) prolonged sex (conception) maturity by 81 days.
e) production
performance (total production during a given period, during a production cycle,
per day, per year; average production per animal during a given period, during
a production cycle, per day, per year, per the whole life; production per time unit,
per space unit and per input unit such
as feed unit, monetary unit, man power unit, etc.);
Examples: In former Czechoslovakia in
1972 cattle affected by fascioliasis
produced in average by 20 % less milk than healthy cows and lost 25 kg of body
weight; according to FAO in 1980 following number of animals died in the whole
world: 64 millions of cattle (12,7 millions MT), 128 millions of pigs (9
millions MT), 100 millions of sheep (1.5 millions MT) and 43 millions of goats
(500 thousands MT).
f) quality of
products (yield of slaughter animals - dressed carcass, grade of biological
quality, grade of sanitary innocuity, grade of epizootiological innocuity,
grade of utility - ability to be processed and used without restriction, price
differences, etc.);
Example: Prices of animal product from
healthy herds are usually higher in comparison with diseased herds (if not
prohibited to trade).
g) cost of veterinary measures
(total, per animal, proportion from total cost of animal production, per year, per
production unit, cost of animal health programmes, etc.);
Example: Cost of successful programme of
eradication of New World screwworm from North Africa in 1989-1991 cost 80
million US dollars; in former Czechoslovakia in 1971 state veterinary service
budget was about 2 % of total value of domestic animals and their product.
h) economic impact of
epizootiological restrictive measures on national and international trade;
Example: During last three decades in the
20th century Southern American countries were prohibited to export beef to USA
and Europe due to foot-and-mouth disease occurrence.
i) cost/benefit
of animal production relating total input to total output in monetary terms
(see chapter 34).
17.3.3 Public health
and consequences
a) Positive
public health consequences of zoonoses free health and negative of
zoonotic diseases are multiform being reflected in: duration of
human life, duration of productive life, grade of human health,
grade of human welfare, grade of risk for human health, working
ability, cost of drugs, vaccines, treatment, sanitation, etc..
b) While the effect of
zoonoses on human production or output in terms of lost income and the cost of
treatment can be quantified, the value of mortality and human suffering
are completely different categories which cannot be evaluated in monetary
terms.
Examples: About fifty zoonoses can cause
death in humans; vampire bat-transmitted rabies caused several hundreds deaths
in Latin American countries.
17.4 Social consequences
Positive social consequences of animal
health and negative of animal diseases are multiform being reflected in:
standard of living (income and consumption), cultural level and political
activities.
Example: Former Czechoslovakia in 1965
closed the border with Hungary to protect its territory against
foot-and-mouth diseases nearing wave
which had political and social consequences (e.g. affecting more than one
million of tourists, not speaking about interrupted trade).
17.5 Estimation of economic consequences
17.5.1 Introduction
a) Analysis of
economic consequences of animal population health and disease in based mainly
on mutual comparison (differences) in quantity and quality between the
productivity (utility) of all animals (real utility), of healthy animals
(optimal/potential utility) and of diseased animals.
b) Analysis can include
all aspects of economic consequences or only some of them (see above). It is
recommendable to analyze first total and than partial consequences avoiding eventual
double counting if the losses are caused by a complex of factors. It is
important to keep in mind what the ceiling is on such losses.
Example: Total economic losses due to
foot-and-mouth disease in Southern America in 1970 were estimated to be of 378
millions US dollars; partial losses due to mortality and emergency slaughter
were of 102 millions, trade restriction of 170 millions, etc..
c) Identification of
disease is usually made on a clinical basis. Chronic debilitating diseases as
parasitic infections, nutritional and fertility disorders, mastitis, and
diseases of new born and young animals are caused by a wide variety of
etiological agents. These complexes include etiological agents difficult or
impossible to identify as the sole, or even the major cause of morbidity and
mortality. It is precisely these chronic disorders that cause the greatest
economic losses throughout the world.
d) Economic analysis is
divided basically in calculation of benefit due to animal health and losses
due to animal disease.
17.5.2 Principles
for economic consequences calculation
a) The analysis
processes concrete available data (direct calculation) or estimated
data (indirect calculation). For both types clear cut definitions of
measure units as well as of parameters and indicators to be used. Relative data
must be combined with absolute data in order to be comparable with situation in
other places and time. Therefore, any analysis must have also clear definition
of space and time.
b) Direct losses
can be calculated as the quantity of lost animals or their products
multiplied by their prices. It is preferable to calculate first the loss
in production measure units (easily comparable) and then to convert it in
monetary terms (variable due to different money rates and inflation) as
economic cost of disease.
c) Losses in animal
products such as milk, eggs, etc. can be calculated as the multiple of
the number of diseased animals by average loss (known or according to
literature data).
d) Estimates of
consequences in animal production can be calculated in following ways:
aa) Multiple of the
difference between average production of healthy and diseased animals by the
average number of healthy animals (benefit) or by the average number of
diseased animals (loss).
bb) Multiple of the
difference between the average production of total population and of diseased
animals by the average total of animals (benefit).
cc) Multiple of the
difference between the average production of healthy animals and of total
population by the average total of animals (loss).
e) Estimates of
consequences in input for animal production can be calculated in following
ways:
aa) Multiple of the
difference between average input for production of diseased and healthy animals
by the average number of healthy animals (benefit) or by the average number of
diseased animals (loss).
bb) Multiple of the
difference between the average input for production of diseased animals and of
total population by the average total of animals (benefit).
cc) Multiple of the
difference between the average input for production of total population and of
healthy animals by the average total of animals (loss).
18. INVESTIGATION OF
EPIZOOTIOLOGICAL SITUATION
===============================================
18.1 Introduction
a) Investigation of
epizootiological situation consists of activities to detect health/disease
reality in animal populations and their environment. The epizootiological
diagnosis is the objective and needed for epizootiological analysis and
measures.
b) Prominent among the
objectives are to detect the presence or to confirm the absence of
disease-causing agents and factors.
c) There is significant
difference between epizootiological and clinical investigations. In case
of disease in one animal for the clinician is sufficient to investigate this
animal taking into consideration influencing factors in very limited range to
decide clinical (symptomatological) diagnosis. However, in the same case the
epizootiologist must investigate all animals in the place, try to define etiological
and epizootiological diagnosis (very often with the help of laboratory
investigations) analysing many influencing factors inclusive in the
neighbourhood and in the past, searching for the origin, sources and ways of
introduction and propagation of respective etiological agents. However,
investigation of individual animals is usually the basis for epizootiological
investigation and diagnosis at population level.
d) The investigation
has to find answers to the questions: "what, where, when, how and why ?"
18.2 Principles of investigation
a) The focus of the investigation is determined
by the specific purpose to be achieved, i.e. the investigation should be
target-oriented, sound and useful.
b) The investigation process leading to
epizootiological diagnosis detects and measures first the features (phenomena
or symptoms) of animal population health and/or diseases and different factors
acting upon them. Next the findings and their importance are evaluated and then
the final diagnosis made.
c) Investigation should
use exact and reliable diagnostic methods to identify the situation.
These should give an undistorted, true picture of the reality. Therefore,
highly specific and sensitive methods as required by the particular case should
be used.
d) Investigation
combines the qualitative and quantitative aspects of the
epizootiological situation. The results are easier to compare and understand
when the biological values of the diagnostic findings are also expressed
numerically.
e) To achieve
comparable results standardized - uniform diagnostic methods are
required. This is particularly true for the identification od specific disease
(etiological diagnosis) using indirect methods only.
f) Investigation, in
line with its purpose and requirements, can cover the whole population
(territory) or only a part over a sufficiently long period.
g) Investigation should
lead to the swiftest possible epizootiological diagnosis, in accordance
with the given objectives, particularly where the epizootiological situation is
highly changeable.
h) Investigation
utilizes different diagnostic methods. These vary in accordance with the object
and specific purpose. More weight is given to decisive methods which
look for the origin of specific disease(s). It is preferable to use a
combination of available diagnostic methods.
i) Investigation should
be flexible and adaptable to dynamic changes of the epizootiological
situation.
j) Atypical or barely
expressed symptoms and features deserve special attention as they are difficult
to detect by the current diagnostic methods. Borderline areas between
health and disease, negativity and positivity should receive similar attention.
k) In every case of
new or suspected outbreak, one must first think about the worst, i.e.
the most dangerous communicable diseases, and decide at the outset if such
disease is involved or suspected. Only when the possibility of such a
disease has been ruled out, beyond all suspicion, investigation moves on to
detect what other cause might be involved.(See chapter 28).
l) Investigation for confirming
disease free status of the populations, herds, flocks or groups requires
well adjusted procedures which are not always the same as those for identifying
the disease (where is often sufficient to isolate particular etiological agent
in only one animal).For confirming disease-free status is necessary to
investigate all animals or a representative sample using different methods.
m) Investigation
methods should be feasible, relatively simple, technically realistic, rapid,
easy-to-use and harmless. Unfortunately, very often the simpler and cheaper
a method the less likely it is to produce specific, sensitive and exact
results.
n) Vertical structure
of diagnostic system has several steps between basic field level and highly
specialized laboratories up to international reference centers for very
specific investigations.
o) A network of
diagnostic laboratories is often supplemented by mobile laboratories working in
the field and processing fresh specimens.
18.3 Types of investigation
a) The types of
investigation differ in accordance with their purpose, availability of
biological and technical procedures, organizational conditions and economic
resources.
b) How specific,
complex and systematic the investigation is, as well as when and where it takes
place, it depends on the analytical needs of the specific
epizootiological situation. A detailed, broad-scope, in-depth investigation is needed
to make a full and correct epizootiological diagnosis, covering the components
and characteristics of both health and disease in a given animal population.
c) Different types of
investigations are used separately or in combination in various situations
in accordance with different purposes.
It is preferable to use the surest
and most important types, i.e. etiological investigation versus no-etiological
investigation, complex versus partial, systematic versus operational, active
versus passive, etc.
It is preferable to combine various
types as appropriate: general with specific, systematic with operational, local
with territorial, one-time with long-term, related to preventive measures with
those related to recovery measures, etc.
d) The basic
requirement for a solid analysis of the epizootiological situation is an etiological
diagnosis which reports on causes: disease agents and factors. Etiological
investigations are oriented towards direct confirmation of the presence or
absence of disease agents.
Direct etiological investigation is
based primarily on microbiological, parasitological, toxicological methods and
biological tests.
Indirect etiological investigation
traces the presence or absence of disease agents through diagnostic methods
focussing on specific impact (consequences) of these agents.
Epizootiological and clinical investigations
often lead to etiological diagnosis of the diseases with typical
epizootiological characteristics and obvious typical clinical symptoms.
Epizootiological and morphological
investigations often produce an etiological diagnosis in diseases with typical
epizootiological characteristics and typical morphological changes (postmortem
investigation).
Serological, allergic and biochemical investigations
etc. often indirectly produce etiological diagnosis.
e) General (non
specific) investigation covers animal populations, their products,
environment, etc. from the general, no-specific etiological standpoint of
animal health and disease, i.e. covering full etiological spectrum. The point
in this case is to find out whether the object of the investigation is healthy
or not.
f) Specific
investigation covers the same above-mentioned objects from the standpoint of
specific animal health and disease, i.e. from the standpoint of a single type
of disease agent (mono-etiological investigation) or of a particular group
(complex) of etiological agents and factors (poly-etiological investigations).
g) Systematic
investigation is planned and oriented towards specific objectives in contrast to occasional, non-systematic operational
one. The first type is closely related to active investigations particularly
connected with planned epizootiological measures.
h) Investigation
differs in form, content, scope, place and time, selection of methods, etc.
also in accordance with epizootiological measures preparation and
monitoring the situation and results. They can be oriented to:
- active creation of
animal population health
- preventive/protection
measures
- epizootiological
monitoring
- epizootiological
surveillance
- disease-reducing
measures
- disease-eliminating
measures
- disease-eradicating
measures, etc.
i) When the
investigation does not cover all animals or other objects then it can be
subdivided into representative one using sampling methods and non-representative
one when sampling is not applied or the sample is too small.
j) Investigation
according to space differs as to
coverage which may be local, farm, ranch, village, municipal, regional, provincial,
etc. Investigation can also be broken down by location: selected sites and ares
(affected, threatened, disease-free, etc.), selected breeding/production units,
by critical epizootiological sites (outbreaks, quarantine, markets, border
stations, vectors areas, critical places of epizootic process, etc.)
k) Investigation
according to time can be divided into: momentary, covering a period
(short, medium, long), interrupted or continuous, one-time or repeated, regular
(periodic, seasonal, cyclical) or irregular, etc. Key moments and periods of
the epizootiological situation, development and measures always have the
priority. Momentary investigation or in relatively very short period
identifying the profile (cross cut) situation is often combined with longitudinal
ones covering a longer period in the same animals (cohort).
18.4 Field survey
and screening test
Field survey and screening test represent
diagnostic activities for defining general or specific epizootiological
situation pinpointing its components (e.g. health, at risk and diseased
animals, sources, disease-free zones, outbreak limits, etc.).
18.4.1 Field survey
a) Ad hoc survey
is used to detect diseased or suspect animals, sources, ways of transmission,
particularly in a new suspected or confirmed outbreak, to certificate
disease-free animals, their products and neighbourhood zone for export and
sale, etc.
b) Planned survey
is used for analysing epizootiological situation as a basis for strategy,
planning, executing, monitoring and evaluation of relevant measures, etc. This
is highly demanding, expensive, laborious kind of investigation, as a rule, and
for this reason is often done on representative samples.
c) Passive survey
is that diagnostic activity which occurs accidentally in the normal course of
veterinary work or cases where the veterinary service has been called in by
stock-owner, etc. These are usually casual, isolated findings which help in a
limited way to determine the occurrence or the absence (this is not reliable)
of a specific disease in a given place.
d) Prevalence survey
is used to determine the frequency and distribution of etiological agents by
measuring the occurrence of antibody (produced to the agents) in the serum of
the animal.
e) Longitudinal
survey is that which lasts a longer period (years) investigating
periodically selected health phenomenon in determined animals population, herd,
flock or group.
18.4.2 Screening test
a) Screening test is
the presumptive identification of unrecognized disease by application of
simple tests to sort out apparently healthy animals which may have the disease
from those that probably do not have the disease. Therefore, screening test
(e.g. using serological, haematological, etc. methods) is applied to
apparently healthy animals in search of disease with the objective of its early
detection and therefore, early measures.
b) Generally, screening tests are applied to a large
number of animals and are often followed by other tests on those animals
that are found to be positive. Most screening tests are aimed at high risk
groups.
c) Screening is usually
concerned with chronic diseases and aims to detect disease not yet under
professional care. Mass screening means the screening of all population.
Multiple or multiphasic screening involves the use of a variety
of screening tests on the same occasion. Selective screening means the
testing of certain group of animals.
18.5 Epizootiological
investigation methods
a) Epizootiological
investigation (diagnostic) methods consists of: observation of
investigated objects (animals, animal products, terrain, sites, feeds,
environmental factors, etc.), anamnesis, complex diagnostic methods
carried out directly in the field (clinical, morphological, biological
tests, etc.) and laboratories, available relevant documentation on
the results of associated investigations in particular conducing to
etiological and epizootiological diagnoses.
b) Epizootiological
investigation usually selects the animals, animal products and other
examination subjects as well as the kind, sites and time for taking and sending
samples for laboratory examinations, and orient these
investigations.
c) Epizootiological
investigation methods interpret, combine, compare and relate to the actual
field situation and, lastly, amalgamate (synthesize) all findings of all
diagnostic methods used.
d) The main
objective of epizootiological investigation is to make an epizootiological
diagnosis by detecting and identifying the causes, occurrence, spatial and
temporal frequencies, dynamic, trends and conditions of animal population
health and disease.
e) Epizootiological
investigation is primarily oriented towards the key components of collective
health and epizootic processes at critical
(decisive) time and places. It is necessary to determine qualitative and
quantitative properties of these components providing the basis for
epizootiological indicators.
The diagnostic systems and procedures differ
according to epizootiological situation and its variability, purpose, potential, degree of urgency (from
preventive up to emergency investigations), etc.
f) Detected facts
should be recorded. The finding should be noted down with care immediately
(memory is always subject to lapses).
g) Epizootiological
methods are based on the fact that each process of specific collective health
and each epizootic process has its specific epizootiological properties
reflecting the specificity of the interacting population-etiological agents and
the environment.
Specificity is not reflected in clinical,
morphological and physiological similarity alone, but also in epizootiological
similarity (values of morbidity, lethality, focality, trends, ways of
propagation, species susceptibility, etc.). Each epizootiological investigation
method has limitations as to the scope of detecting disease in different
situations.
h) Epizootiological
investigation methods not only identify the situation, they also and primarily
attempt to find the causes of a given situation and influencing factors for a
correct orientation of subsequent measures - investigation for action!
i) Very important is to
have an epizootiological hypothesis based on initial data. This is
gradually confirmed, altered or substituted by a new or more likely hypothesis.
"Blind" epizootiological investigation should be avoided. All
investigations into concrete cases must be selective depending on the
hypothesis and epizootiological logic (sense).
Method of "gradual elimination" of
different hypotheses is often used to find the most likely result.
j) It is essential
that epizootiological investigation be carried out directly in the field,
on specific sites.
k) Easily determinable
and testable data and concrete finding are not always available, particularly
on past events or in distant sites. For this such methods as estimations,
associations, deductions, correlations, probability and hypotheses are also
used.
l) For diseases common
to both animals and man epizootiological investigation should be coordinated
with epidemiological investigation of the human population.
m) It must be
considered that laboratory investigation results can be obtained when field
situation has already been changed in comparison with the moment of collecting
the samples.
18.6 Diagnostic test quality
a) Accuracy is the degree to which the
measurement, or an estimate based on measurements, represents the true value of
the attribute that is being measured. The accuracy of a test (proportion of
those animals correctly identified by the test) can be measured and expressed
by its ability to correctly classify the animals according to their
health/disease status. These measures are termed sensitivity and specificity.
b) Sensitivity
is the probability of a test to identify correctly those animals that are
infected.
c) Specificity
is the probability of a test to identify correctly those animals that are not
infected.
d) True positive
rate is the proportion of truly diseased animals that the test identifies
as positive (predictive value of true positive results). A positive result can
mean an infected, incubating, or recovering animal.
e) False positive
rate is the proportion of the truly non-diseased animals that the test
identifies as positive (predictive value of false positive results).
A positive result does not necessarily mean
that the animal has been recently infected with the etiological agents in
question. A positive result may also indicate a prior vaccination to the agents
or passive antibody transfer (i.e. through the colostrum). A test may also be
positive due to laboratory or sample selection/handling errors. Occasionally,
antibodies to another agent that has infected the animal will cross-reacts in
tests used to determine exposure to another agent.
f) True negative
rate is the proportion of truly non-diseased animals that the test
identifies as negative (predictive value of true negative results).
g) False negative
rate is the proportion of the truly diseased animals that the test
identifies as negative (predictive value of false negative results). False negative
animals are dangerous representing one of major transmitters of infectious
diseases due to their transfer as healthy animals.
A test may be negative when the animal is
actually infected. An animal may have been recently enough infected at the time
of the test that it has not had enough time to develop an antibody response (it
takes usually two weeks before an IgM response is developed). In addition, a
test may not be finely tuned enough to detect small quantities of antibody to
etiological agents. A negative result could also be due to laboratory or sample
selection/handling errors.
h) The sensitivity of a
test is directly related to the amount of false positive and conversely, the
specificity is related to the amount of false negatives. Specificity allows
more confidence in a positive test. Sensitivity allows more confidence in a
negative test.
i) Iceberg
phenomenon is the portion of disease that remains unrecorded or undetected
despite of diagnostic endeavours and population disease surveillance procedure
called "submerged portion of iceberg". Detected or diagnosed disease
is the "tip of iceberg". The submerged portion comprises disease not
attended, attended but not accurately diagnosed and diagnosed but not reported.
j) Other important
characteristic of diagnostic methods quality is repeatability or
reproducibility (capacity of a measuring procedure to produce the same result
on each occasion in a series of procedures conducted under identical
conditions).
k) Diagnostic method
general efficiency rate ("diagnosability") represent the multiple
of test specificity and sensitivity.
18.7 Sampling in animal population investigation
a) Sampling theory is a
study of relationships existing between a population and samples drawn from the
population permitting to estimate certain characteristics of the whole
population.
b) Sampling represent
the basic tool in investigation of major animal populations facilitating
to identify animal health and disease situation without testing all animals
(which is extremely costly and demanding on manpower and other input resources
and capacities).
b) Sampling uses the elements
as the basic objects of interest (animals), sampling units
(non-overlapping collections of elements which make up the population) and frame
(list of sampling units in the population).
c) Probability
sampling is a random access to every individual. Every individual in the
population has a known chance of being sampled. Inference of the sample
is applied to the rest of the population. The degree of bias depends on how the
sample was taken and this will determine if the sample truly represents the
rest of the population.
d) Non-probability
(non-random) sampling is done on the basis of convenience or haphazard and
the sample is usually not representative of the population under investigation.
The problem with this type of sampling comes when the results are incorrectly
applied to the entire population. This type of sampling may work and may
actually be necessary at the beginning of an investigation because it may
answer an initial questions about the situation.
e) Types of
Probability Sampling
aa) Simple random
sampling where every sampling unit has an equal chance of being selected in
the sample. This type uses random number tables or software (assign each
individual sampling unit a number).
bb) Simple
stratified sampling where the population is divided into strata (subgroups)
according to certain criteria that is important to the investigation. Then a
random sample is performed among each strata.
cc) Proportional
stratified sampling takes into account the problem of strata of unequal
size. The sample among strata is obtained with regard to the contribution of
the strata to the size of the total population.
dd) Cluster sampling
where the unit of sampling is a group of individuals rather than a single
individual. Every animal in sampled units must be surveyed.
ee) Multistage
sampling is when more than one of the above methods is incorporated into
the investigation design.
f) Sample size.
The more units are sampled, the more accurate the results are. The more common
the disease or characteristic are, the fewer is needed to sample. For the
calculations of minimal number of sampling units in concrete cases relevant
statistical methods to be used (see textbooks on statistics).
19. EPIZOOTIOLOGICAL INFORMATION SYSTEM
=======================================
19.1 Introduction
a) The purpose of
epizootiological information system is to record, report, collate, process,
store, transfer, diffuse and utilize data relevant to epizootiological
situation. The system provides a basis for knowing the situation,
understanding the various epizootiological phenomena and their frequency and
dynamics and for decision-making on animal population health strategy
and programmes/measures.
b) This system should transform
raw epizootiological data into organized and meaningful information.
c) Information system
components should form an integrated, functional unit designed to
provide information needed for epizootiological analyses and actions, i.e. it
should be tailored to concrete local requirements and conditions.
d) Standardized
methods and definitions should be used. The data collected should be
reliable, sound and above all useful (essential) in the light of the
objectives.
e) Different
epizootiological situation and requirements for decision-making need different
data and adapted information system and sub-systems which should be flexible
and dynamic with necessary grade of stability.
f) In every country
exists a particular official information system linked with an international
system and adapted to the local situation, needs and conditions.
19.2 Objectives of the
information system
The objectives are to supply essential data
for:
- early warning and
alarming system
- analysis of
the epizootiological situation
- analysis of specific
epizootic processes
- analysis of external
factors influencing these processes
- guiding and
facilitating future diagnostic activities
- epizootiological surveillance
- application of emergency
measures
- epizootiological control
of animals and their products movement
- issuing certificates
and declaration of disease free status
- identifying epizootiological
strategy, priorities and tactics
- application of
effective epizootiological measures
- planning of animal
population health programmes
- monitoring the
progress and goals achievement of these programmes
- evaluating the
effectiveness of epizootiological actions
- early detection of
constraints of programmes and measures
- effectively
administrating, directing, controlling and coordinating epizootiological
activities, etc.
19.3 Information system data
a) Data for the
information system should be above all:
- identified in
advance in line with the purpose
- set out in a sound
comprehensible standardized form
- reliable,
sufficiently exact, official, specific and where possible numerical
- clearly defined in space
and time
- primarily in absolute
values as a basis for calculation of comparable indicators
- very carefully
selected to record, report and process only a reasonable number of data.
b) The system has to
cover the information on relevant characteristics of animal populations,
etiological agents, their sources and
way of transmission, environmental (ecological factors), infectious processes,
epizootic processes, economic and social factors, consequences of population
health and disease as well as on different epizootiological activities
(diagnostic, vaccinations etc.).
19.4 Data sources
a) The main data
sources are the first-hand records of the veterinary services. There are
different kinds of primary records such as card catalogues, cards,
files, records books, statistical forms, etc. The data are kept and reported in
the form of tables, maps, graphs, numerical, textual, photos, tapes, diskettes,
etc.
b) Veterinary
services registers are the most useful data sources. These are mainly
first-hand records or protocols from:
- field veterinarians
- veterinarians in food
industry
- border veterinarians
- veterinary clinics
- diagnostic
laboratories
- production
laboratories
- rendering plants
- veterinary services
at all levels reporting notifiable diseases
- veterinary
educational, scientific institutions, etc.
c) Non-veterinary
services, agencies and organizations provide relevant data related to
epizootiological situation:
- breeders and
producers (state, cooperative, private)
- breeders' organizations
- agencies monitoring
wild fauna
- organizers of stock
shows, fairs, markets and transport
- owners of processing
plants for animal origin products
- insurance agencies
- services dealing with
environmental aspects
- human epidemiological
(public health) services
- meteorological
services
- publications of
national statistics and census agencies, etc.
19.5 Reporting
a) Reports on
epizootiological data are to be standardized as to content, form, place and
frequency to make data comparable and easier to process and assuring the uniformity.
b) Reports contents
should correspond to the purpose and requirements respecting particular
conditions and possibilities. Every data reported should have a specific
importance and usefulness, i.e. it must be both necessary and useful.
c) Reporting priority
have so called "notifiable diseases" which should be specified
by national law or regulation including also the diseases according to
international commitments (e.g. the diseases of the OIE list).
Information on the occurrence or suspicion
of notifiable diseases originate from:
- report on disease
suspicion sent by the animal owners or other persons responsible according to
the national legislation;
- findings made in
routine veterinary work and inspection at the farm level;
- at official
veterinary inspection particularly slaughter animals and meat inspection,
import inspection, inspection of animal markets and other gatherings;
- notice received by
compulsory notification;
- epizootiological
preventive or control investigations.
There should be appropriate and effectively
enforced provision to ensure that every reasonably suspected case of a
notifiable disease is reported by the general public to the official veterinary
service, without delay. Effective measures should be applied to ensure that
such obligation is complied with by veterinarians, livestock owners and
attendants, butchers, knackers, and other persons which by profession, trade or
regular occupation are directly concerned with animals or carcasses thereof. To
that effect, it should be provided for that every case of serious disease or
death in an animal, or alteration in a carcass, should be considered as being
suspected of a notifiable disease, unless the symptoms can reasonably be attributed
to another disease, which is not notifiable.
d) The forms of reports
should correspond to their contents. Following informative tools - forms
can be distinguished:
- telegrams, telex,
fax, radio messages and courier services are used mainly in new cases of
suspected or confirmed, highly dangerous, or exotic emergency diseases, or for
as yet unknown disease of exceptional importance (spreading rapidly, causing
high losses, etc.) in sites or areas previously free of such diseases, i.e.
when the report must be sent immediately
- special printed forms
(e.g. questionnaires for interview or mailed)
- cards containing
first-hand data to be computer-processed
- protocols (standard
form or free verbal text)
- text (letters)
describing and commenting the situation
- epizootiological
maps, tables, graphs (bar, line, scatter, area, pie, mixed, etc. types),
photographs and recorder tapes
- telephone
communications
- computer diskettes
and tapes, CD-ROM
- computer modems,
e-mail, on-line networks, etc.
The most simple forms are
the interviews.
Almost all the above-mentioned forms can be
replaced in the future by on-line computer network linked with geographic
information systems and eventually with satellite imagery.
e) The report
channels go from field services through different management levels up to
national veterinary headquarters. Data processed by the central organization is
sent (feed-back) to inferior level reporting units and to relevant
agencies and authorities.
f) According to
epizootiological importance the reports to be sent
- immediately in
new case of very dangerous disease and then continuously following its
development (e.g. every day or in very short intervals)
- on regular basis
in the case of notifiable diseases, on finding in slaughterhouses,
laboratories, etc., on the progress of epizootiological actions, etc. (e.g.
weekly, monthly, quarterly, six-monthly or annually)
- ad hoc in
cases of particular interest, field surveys, diagnostic findings of major
importance, etc.
- as compulsory
- mainly on notifiable diseases as defined by national governments
- on voluntary or
supplementary basis.
19.6 Data processing
a) At each level of
organizational structure of veterinary services data of epizootiological
importance are collected through the channels of communication
(source-route-receivers) and processed.
The data/reports are processed i.e. reviewed,
summarized, interpreted and analyzed in different ways: by hand, machine,
electrically or electronically.
The data are arranged, grouped, classified
and compiled in accordance with different purposes for which they were
collected.
b) The information
derived from the processed data is presented to users in appropriate
forms for the specific purpose. Epizootiological bulletins or reports are
published regularly at different intervals or irregularly in case of a danger
of delay. These information is extremely important for all components of
veterinary services providing them by the orientation necessary for their
future diagnostic activities and epizootiological measures. Some information
are addressed to public, first to animal owners.
c) Animal disease information
archives are an important source to tap for retrospective epizootiological
analyses of variations and long-term trends, comparisons and prognoses. The
most advanced data source are disease data stored in the data banks of
computers memory.
19.7 International systems
a) International
epizootiological information systems based on the exchange of national data:
- are responsible for
issuing warning on threatening dangerous diseases alerting close
surveillance and preparatory measures for eventual emergencies
- assist with market
analyses for decisions on whether to import animals or animal products
- assist in national
and international control of the major diseases
- are responsible for
international coordination including the preparation of international
standards, definitions and codes assuring the uniformity and compatibility of
all countries information systems.
b) By international
agreements, animal disease reports are sent to veterinary services of
neighbouring or cooperating countries and to multilateral, regional and world
agencies such as International Office of Epizootics - OIE (Paris), Food
and Agriculture Organization of the United Nations - FAO (Rome) and
World Health Organization - WHO (Geneva). These international
organizations publish bulletins, periodicals and yearbooks (e.g. actually OIE
World Animal Health; FAO-OIE-WHO Animal Health Yearbook - up to 1996,) with
data received from member countries.
20. ANALYSIS OF
EPIZOOTIOLOGICAL SITUATION
==========================================
20.1 Introduction
a) Analysis of
epizootiological situation (epizootiological analysis), based upon the
interpretation and dialectic study of the results of epizootiological
investigations, determines, evaluates and explains the characteristics and
causes of collective
animal health and epizootic processes.
b) Analysis of
available data allows
to deduce real epizootiological picture or that close to objective
reality taking into consideration probability grade, epizootiological laws,
scientific knowledge and previous experience.
c) Analysis initially
seeks and studies influencing
factors which determine the origin, distribution, course, behaviour and
extinction of respective health/disease processes in animal populations.
Analysis helps to explain the differences in the health and disease status between
different animal populations, herds and flocks and between different places and
time.
d) Analysis facilitates
the determination of the importance of different epizootiological phenomena as
well as of the causal association between them. Knowing the natural or
artificial causes, the hypothesis on the consequences can be formulated
and checked. Knowing the consequences, the hypothesis on the causes and
other determinants of the given epizootiological situation can be defined.
Casual relations are the most
important of the whole complex of interrelations between different components
of the complex system - epizootiological situation and its development.
Epizootiological analysis is
supported by the hypotheses testing methods that investigate the association
between given health or disease related events and possible causation factors.
e) Descriptive data
obtained by the investigation of epizootiological situation serve as the source
of epizootiological hypotheses which are to be proved using explicative
analytical methods, and eventually experimental methods, followed by concluding
synthesizing methods.
The analysis should arrive at one or more
hypotheses as to: the kind of epizootic process, the source of etiological
agents, the possible mode of spread, etc. It should be checked that the
hypothesis fits all the facts, i.e., that it is compatible with all
observations; if it does not fit, the hypothesis should be revised.
f) The complexity,
variability and dynamics of collective health process, epizootic process,
influencing factors as well as different objectives and depth of the analysis
call for a wide variation of analytic methods well adapted to the
problem to be studied and evaluated.
g) Analysis procedure
should be systematic and well programmed to include all important
components and to be as most effective as possible.
h) Analysis should
answer not only such questions as: what, where, in which species and
category, when and how, but also why.
i) It is necessary to
consider the fact that two same cases and two
same epizootiological situations in absolute terms do not exist.
j) Generally, comparative
methods are used which permit the evaluation of changes, differences and
correlations between different phenomena, situations, places and time. The
comparison of a specific case with those known in the past as well as
accumulated experience and knowledge is of particular importance.
k) Analysis results
help also to orientate subsequent investigation of epizootiological
situation (selection of adequate places and time, animals to be investigated
and sampled, types of specimens, types of investigation methods, diagnostic
results interpretation, etc.).
l) Synthesis of the
analysis results represents the basis for decision on epizootiological
prognosis, strategy and measures as well as for their optimization, planning,
execution, management, adjustment, control and evaluation.
20.2 Principles of
epizootiological analysis
a) For each specific
analysis the objectives, objects, space and time should be defined in
order to obtain valid conclusions with respect to the problems under study and
the measures to be taken.
b) Well defined objectives
ensure the correct orientation of analytical procedures toward the phenomena
which are decisive for the solution of specific animal population
health/disease problems.
c) Each analysis should
have well defined and delimitated objects giving priority to those of
major animal health importance. There is a need for defining which animal
species and categories, etiological agents/factors, specific health and/or
disease, epizootic process, forms and stages, space and time, etc. should be
the major objects.
d) All analyses should
study and evaluate each phenomenon from the point of view of space and time
which are closely interrelated and in which the movement, changes and
development are reflected. The space analysis determines the space dimension
and relations with other places (e.g. neighbourhood). The time analysis
determines the duration and relations with the past, present and future.
e) The phenomena should
be analyzed not only from the point of view of absolute values but also
of relative values.
f) Analysis should
study and evaluate the phenomena not only from the qualitative
(characteristics) but also quantitative (dimension) points of view.
Quantitative changes reaching critical values lead to qualitative changes.
g) Analysis of animal
population health phenomena should study and evaluate not only their forms
but also their contents (substance) which are mutually interrelated and
interdependent.
h) It is desirable that
the analysis is based on a "system approach", i.e. the
particular components to be analyzed as integral parts of a system. The
phenomena should be analyzed as an integral component of the whole
complex system (epizootiological situation). Individual components form a
particular structure of the system and are mutually interrelated. In analysing
a disease situation individual affected animals should always be considered as
integral components of a complex - affected herd, flock or population.
i) Animal health
phenomena should be analyzed dynamically, i.e. from the point of view of
absolute changes, movement and development with relative stability.
20.3 Type of epizootiological analysis
a) Types (methods,
contents, form, range) of epizootiological analysis vary according to different
aspects such as objectives, objects, specificity, space, time, complexity, etc.
Each analysis and its procedure should be well adjusted to its purpose, conditions
and feasibility respecting importance grade of the given problem.
b) According to the
objectives, following analyses can be distinguished: preparative analysis,
preventive analysis (related to animal health protection), recovery analysis
(related to recovery, elimination and eradication measures), surveillance
analysis, evaluating analysis (related to the effect of measures), certifying
analysis (related to veterinary certificates), scientific analysis (related to
experimental works), etc.
c) According to the
objects, following analyses can be distinguished: analysis of all animal
populations, of the populations of a group of species, of the population of one
species; of all the categories (perfil analysis), of one group (cohort
analysis); of a healthy population, of a diseased population; of a suspected
population, of population at risk, etc.; of etiological agents, of their
sources and ways of transmission; of the environmental factors; of collective
health process; of epizootic process; of influencing factors; of individual
cases (case study); etc.
d) According to
etiological aspects, following analyses can be distinguished: etiological
analysis (from the point of view of determined etiological agents) or
non-etiological analysis which does not respect the causes but only the
external manifest forms and consequences.
e) According to the
specificity, following analyses can be distinguished: general analysis (non
specific - including all etiological agents/factors aspects); specific
monocausal analysis (according to one species of etiological agents);
polycausal analysis (according to a particular group of etiological agents species and etiological factors).
f) According to
space aspect, following analyses can be distinguished: local analysis (of a
herd, flock, farm, etc.), geographical - territorial analysis (of a lowland
area, desert area, island, etc.); political - territorial analysis (district,
regional, provincial, national, etc.); sectorial analysis (of private,
cooperative, state sectors, etc.); rural zone analysis, urban zone analysis;
analysis of disease free zone; analysis of intrafocal area; analysis of
perifocal area, etc.
g) According to time
aspects, following analyses can be distinguished: analysis at a given
moment (transversal), of a period (longitudinal); retrospective analysis (of
the past - historical - ex-post); actual situation analysis; continuous
analysis; periodic analysis (regular - daily, weekly, monthly, yearly,
seasonal, cyclical, etc.); no periodic analysis (irregular - operational; ad
hoc; case study, etc.).
h) According to the
complexity aspects, following analyses can be distinguished: complex
analysis including all the phenomena of a given situation and using all
available data; incomplete analysis dealing only with a part of the components
of a given situation; deep analysis evaluating the details; superficial
analysis evaluating only orientation factors.
i) According to the
other aspects, following analyses can be distinguished: analysis of the
herds according to their size, concentration, specialization, technology, etc.;
analysis using different forms such as network analysis, "critical
path" analysis, etc.
20.4 Evaluation of
epizootiological phenomena
a) In analysing animal
population health situation there should be always the effort to evaluate it in
the most objective and precise form as possible. This requires first
that the given phenomena (variable) are well defined and delimitated and where
possible, in a standard and measured form using respective numerically
expressed measurement units. Mathematical expression facilitates the
application of statistical methods needed for evaluation of mass phenomena and
the use of relative indicators (rates, ratios, indexes). Statistics is the tool
by which data are reduced to manageable form and tested for significance.
b) Generally, the
analysis is supported by a great number of different data and information which
can be properly utilized only when using statistical, in particular
bio-statistical methods. In this connection the analysis utilizes the
indicators in the form of respective symbols and formulae.
20.4.1 Standardization and measurement
a) The fundamental
condition for good analysis is its uniformity, i.e. standardization of
the definition of the phenomena and used indicators. The definitions are
usually based upon experimental results or so called "normal values"
- averages obtained by evaluating a great number of data. If international or
national standards are available then they should be used.
b) In order to be able
to use statistical methods, there is a need to define the given
epizootiological phenomena units as statistical units.
c) The data used can
be: quantitative - expressed numerically and able to be measured; qualitative
- indicating
only if the given variable is present or absent, eventually quality grading (in
proportions); continuous which also includes values between whole numbers
(integers); discrete which include only the values expressed in whole numbers.
d) Different scales
are used for analysing data: ordinal scales, interval scales (with changeable
or fixed initiation, with regular or logarithmic intervals),
graduation scales (parallel or cumulative).
e) Following units
of measurement are used: biological (animal, disease case, herd, outbreak,
etc.); measure units of length, surface, volume, mass, time, velocity (length
by time unit), density (quantity by space unit), etc.
20.4.2 Frequency
distribution of animal health phenomena
a) The distribution of
animal health phenomena varies greatly. Frequency distribution
of absolute quantitative values are often grouped in different
intervals, classes or categories so that they can be better processed.
Frequency distributions of qualitative values are based on the
proportion (percentage) of a given variable within a given complex (population
sample) of statistical units. Another type is represented by cumulative
frequency distributions.
b) Frequency
distributions can be symmetrical or bell-shaped (Gauss curve) or asymmetrical
which is the case in the majority of epizootiological phenomena. The
distribution can be modal, bimodal, binomial, etc.
c) The expressions such
as "usual, habitual, current, regular, normal, etc." are not
sufficiently determinant and comparable. This is the reason why more precise
definitions are preferred using statistical methods when and where possible.
These methods facilitate the synthesis of frequency distributions using
a few values of statistical parameters of their central tendency and their
dispersion.
The averages and measures of the central
tendency represent the center of frequency distribution. For this purpose
arithmetic mean, median, mode, geometric mean, etc. are used.
In order to characterize properly a complex
(population or sample) of quantitative values it is necessary to know also dispersion
or variation around the central value. For this purpose range (dimension)
of variation, average deviation, variance, standard deviation, coefficient of
variation, etc. are used.
d) Evaluating frequency
distribution curve following statistical criteria are used: confidence
intervals, confidence limits, confidence level, confidence coefficient, etc. By
convention 95%-99% confidence limits are usually chosen.
e) The results obtained
by statistical processing of available data should be tested for statistical
significance - level of significance (acceptable maximum of probability
to risk an error). In animal population health analysis, 0.05 (5 %) and 0.01 (1
%) levels of statistical significance are usually used.
f) In the ensemble of
phenomena which are defined through two or more different quantitative values
there is the need to study relations - dependency between those variables using
regression and correlation methods. Each acceptable association between
animal population health variables calculated as
statistically significant should have clear epizootiological (biological)
sense.
g) The tests of the difference
between two or more means and two or more proportions are very often used.
h) For the estimation
of unknown values interpolation methods have proved to be useful.
Note: More information see in statistics textbooks.
20.4.3 Space aspects
a) Each animal
population health phenomenon is related to a given space (place) and therefore
also its location, density (concentration) and space dispersion (regular,
irregular, in cluster, etc.) should be evaluated.
b) Space location
and delimitation are expressed in terms of natural, political -
administrative, economic and other limits. In vast territories geographical
coordinates are used for this purpose.
c) Space size or
dimension is expressed in surface units, eventually in volume units (in case of
aquatic phenomena).
d) For evaluation of
phenomena density the relation of the number of units to the space size
is used.
e) Each analysis should
give priority to the places of major epizootiological importance, i.e.
those which are decisive for the given situation and for respective measures (critical
places).
f) The maps
(spot, transparent overlay, etc.), area charts or cartograms with
epizootiological phenomena location represent fundamental documents for space
analysis.
20.4.4 Time aspects
a) Each animal
population health phenomenon is related to a given time and therefore its time
delimitation should always be evaluated giving the priority to those
moments and periods which are of major importance for the given situation and
measures (critical moments/periods).
b) Chronological
distribution evaluation is based on the study of time series (stable, evolutive
or periodical, such as seasonality, cyclical variation, etc.). Current and
chain indexes are used for comparative studies of time series.
c) It must be always
considered that all phenomena change and develop during time. In animal
populations, herds and flocks the replacement of old by new generations
is of particular importance as diseased, vaccinated, investigated, etc. animals
are gradually replaced by healthy, non vaccinated, non investigated animals,
etc.
d) The beginning and
end as well as duration of each phenomenon should be evaluated using
calendar units or relative units such as "animal/days",
"animal/years" or "outbreak/days", etc. The average
duration as well as periods with and periods without a given phenomenon
(diseased animals, specific foci, etc.) are also important time criteria.
20.4.5 Indicators for epizootiological analysis
a) Epizootiological
analysis uses not only simple absolute data, parameters, determining the
frequency and magnitude of the phenomena as the result of respective
investigations and measurements, but also relative data in the form of indicators
(rates, ratios, indexes). When presenting relative data respective absolute
data should be always mentioned.
b) Rates or
proportions express the relation of phenomenon frequency as a part of a total
of respective statistical measurement units. The values are presented in the
form of simple proportions, of %, %o or by every 10000 or 100000 measurement
units.
c) It is always to take
into consideration the relativity of the indicator values. The same
absolute value gives different values in relation to different size of
population (e.g. 100 affected animals signify only 1 % within a population of
10000 animals while within a population of 1000 animals this represents 10 %).
d) It is always
desirable to mention investigation grade, i.e. the proportion of
investigated animals or herds multiplied by the value of general efficiency
rates of the given investigation methods.
20.4.6 Prevalence,
incidence and extinction
a) Each
epizootiological phenomenon has its beginning, duration and end. In
relation to a given period it can:
- begin and terminate
within the same period
- begin in and continue
after this period
- exist before the
beginning of the period and terminate within the period
- exist before and
continue after the period.
This four possibilities form the basis for
differentiation of prevalence, incidence and extinction rates.
b) The phenomena which exist
at a given moment are projected in point prevalence rate, those which
exist during the period are projected in period prevalence rate and the
average of these phenomena existing during the period is projected in average
prevalence rate.
Point prevalence facilitates the evaluation
of the perfil ("transversal cut") of a given situation at a given
moment without providing information on the dynamics of the situation. It is
often used for evaluating the phenomena of longer duration, e.g. in chronic
diseases. The period prevalence and average prevalence include all the
phenomena existing during a given period without taking into account their
beginning and end.
c) The new
phenomena in a given period are projected in incidence rate. This rate
must respect time factors, i.e. in relation to the number of animals existing
in a given period or in average or at a given moment (e.g. initial).
The incidence expresses the changes (new
units) during a given period and therefore it is very important indicator for
evaluating epizootiological dynamics and grade of stability or instability having major use in acute
diseases.
d) The extinct
phenomena in a given period are projected in extinction rate. This rate
must respect time factors, i.e. in relation to the number of animals existing
in a given period or in average or at a given moment (e.g. initial).
The extinction (negative incidence) also
expresses changes during a given period due to disappearance of a given
phenomenon (e.g. elimination, recovery, transfer, slaughter, etc. of diseased
animals) and has major use in monitoring and evaluating the measures aimed to
achieve animal health recovery (reduction, elimination or eradication of animal
diseases, etc.).
e) Prevalence
rates are based on the number of existing, incidence rate on the new
and extinction rate on disappeared phenomena during a given
period in a given place.
(Attention: Prevalence, incidence and
proportion of positive tests are often confused !).
See indicators formulae in
annex.
20.5 Animal population disease
dynamics
For evaluation of animal disease dynamics following criteria or indicators can be used:
- chronological
time series of disease frequencies;
‑ comparative indexes (current
and chain) correlating given values with the initial or previous
ones;
‑ average of changing numbers
of diseased animals during
individual sub-periods of a total evaluated period;
‑ seasonality of
animal diseases as monthly
proportions in the total year value;
‑ surviving rates
of diseased animals based on multiplication of surviving probability rates in
individual sub-periods;
‑ birth rate as ratio of new born per female of reproductive
age;
‑ changing number
of diseased animals in reproduction cycle;
‑ changing number
of diseased animals in production cycle;
‑ tendency indicators of
animal disease based on the
balance between new and extinct cases during a given period in relation to their number at the beginning;
‑ cyclic tendency disease development based on the
difference between the minimum and maximum numbers of diseased animals and
duration between two peaks of the curve;
‑ ascending
tendency disease development based on
the difference between the initial
minimum and final maximum
number of diseased animals
and duration of period
for reaching the maximum;
‑ descending
tendency disease development based on the difference between the
initial maximum and final
minimum number of
diseased animals and duration
of period for
reaching the minimum;
‑ diseased animals
territorial movement criteria including direction, velocity (distance per time measure unit),
distance (in length measure units) and
duration (in time measure units); etc.
20.6 Analysis of
animal population health status
a) The purpose
of the analysis of animal population health status is to determine its
epizootiological characteristics using the whole analysis systems of
population collective health and morbidity/mortality parameters and indicators
respecting animal population basic characteristics.
b) Epizootiological
risk is the probability that an event will occur, e.g. that animals will
become diseased or die within a given period of time or age. The qualitative or
quantitative estimation of the likelihood of adverse effects that may result
from exposure to specific health hazard or from the absence of beneficial
influences is to be evaluated. Epizootiological risk is usually assessed using
such indicators as relative risk grade, odds ratio, attributable risk grade,
proportion of attributable risk, etc.(See Annex).
c) Analysis of general
and specific resistance against disease is of particular importance.
Following indicators can be used: immunized animals
rate (vaccination coverage of population) which could be combined with
herd coverage or territory coverage by vaccination; relation
vaccinations/animals; vaccination repetition rate. The immuno-effectiveness of
vaccine used should be always taken into consideration.
d) In analysing population
collective health situation and process, the objects, such as etiological
aspects, contents, forms etc. of a given collective health must be well
defined. Healthy animals rate (proportion of
healthy animals in the whole herd or population) represents the basic indicator
for this purpose. Animal population viability analysis uses the
indicators such as population viability index, natality and fertility and
survival rates.(See chapter 5 and Annex).
To be able to declare and maintain the herd
and/or population or territory as specific disease free, particular
analysis is required to prove epizootiological wholesomeness.
e) In analysing animal
population morbidity the diseased animals rate
is used as a basic indicator. The group of mortality rate indicators
(proportion of dead animals) can be subdivided into the group of natural and
artificial mortality. In specific disease the indicators of lethality
(proportion of dead diseased animals) reflect the severity of the disease. (See
chapter 6 and Annex).
f) Epizootiological
structure of a given animal population is a very important component of the
analysis. The basic structure is represented by the population composition and
proportions of diseased, indeterminate and healthy animals (exposed and not
exposed, resistant and susceptible, investigated and not investigated, etc.).
(See chapter 7).
20.7 Analysis of other components of epizootiological situation
a) Etiological
analysis is aimed first at confirming the presence or absence of
etiological agents (factors) in a given population or in a given place
(territory). If their presence is confirmed then the analysis studies their
properties of epizootiological importance, sources and ways of transmission.
(See chapters 8,9 and 10).
b) Environment
factors analysis evaluates their impact on epizootiological situation and
its development using specific criteria and measurement units for the
assessment of atmospheric, geospheric, hydrospheric and biospheric influencing
factors. (See chapter 11).
c) In analysing
specific infection process main attention is given to its origin, course,
stage, duration and forms including its severity (see chapter 12). Animals -
carriers without clinical symptoms are of particular importance.
d) Analysis of the
epizootic process is based on the study and evaluation of the interaction
among the animal populations, etiological agents and environment (epizootic
triad).
In specific epizootic processes the analysis
is aimed at the study and evaluation of the epizootiological chain of the
circulation of etiological agents (more complex in polyhostal processes),
course and stages, forms, range and intensity, grade (exceptional, sporadic,
enzootic, epizootic, panzootic), etc. Diseased animals
rate in the form of attack rate is used in acute disease outbreaks. Further
aspects of analysis are: epizootic process localization, territorial
delimitation, distribution, movement and development, time delimitation,
duration, etc. (See chapter 13).
e) In the case of zoonoses
the analysis of epizootic and epidemic processes in animal and human
populations requires very close cooperation between animal and public health
services. The above-mentioned principles for epizootiological analysis are
similar to those used for epidemiological analysis. (See chapter 15).
f) Territorial
analysis includes the study and evaluation of the foci (outbreaks) and
epizootiological regions (zones).
aa) The foci
characteristics of major importance to be analyzed are: focus etiology, focus
host spectrum, form, size, manifestation grade, activity, localization, time
delimitation, stages, etc. The natural foci study is also oriented
towards the circulation of etiological agents in nature, biotic structure,
vectors and reservoirs activity, etc. The perifocal situation should
always be evaluated together with intrafocal analysis.
bb) Nidality
(focality) grade is measured using indicators such as nidality (focality)
rate, affected herds rate, etc. (See chapter 14).
cc) Epizootiological regionalization
structure is based on determination of disease-free zones, affected zones and
threatened zones. For the measurement of these zone
the indicators of the epizootiological zone rates can be used.
20.8 Analysis of
economic and social factors
a) It is preferable to
combine epizootiological analysis with the analysis of economic and social factors
influencing animal population health and measures. Among the most important
factors belong: rural development and in particular economic activities in
livestock husbandry, local economic level, urbanization, industrialization,
transport, trade, tourism, etc. Living standard, cultural level, political
conditions and other social factors cannot be underestimated. (See chapter 16).
b) This analysis also
includes the study and evaluation of biological, economic, public health and
social consequences of population health (benefit) and mass diseases
(losses). (See chapter 17).
c) Epizootiological measures
should be always taken into account. In order to evaluate the extent of these
measures following indicators can be used: treated animals rate, treated
diseased animals rate, territorial coverage by
measures, population coverage by measures, herds coverage by measures, period
coverage by measures, etc.
20.9 Forms of analysis presentation
The forms used for the presentation of the
results of epizootiological analysis are similar to those described in chapter
19 dealing with information system.
21. EPIZOOTIOLOGICAL
MONITORING AND SURVEILLANCE
================================================
21.1 Introduction
a) Epizootiological
situation and its development as well as the influencing factors are subjects
of monitoring and surveillance in order to obtain and maintain the knowledge
about the situation and to apply corresponding actions in time.
b) Epizootiological
monitoring and surveillance represent particular systems based on
investigation, information and analysis activities described in chapters
18, 19 and 20. Both types are interrelated.
21.2 Epizootiological
monitoring
a) Monitoring means to
collect, process and use of ad hoc information and investigation results
obtained during the current veterinary services activities to assess the
specific health and diseases status.
b) Monitoring consists
in operative investigations to decide immediately about epizootiological
measures, usually at local level or to prepare further steps for the analysis
and eventual animal health programme.
c) Monitoring involves
regular (routine) or irregular veterinary inspections in the field and
slaughterhouses, laboratory investigations, veterinary control in the
disease foci, in quarantines, etc.
d) Monitoring not
always is followed by concrete actions if the disease is not of actual
importance or has not necessary priority in epizootiological strategy.
e) Initial
monitoring represents the first phase of epizootiological activities before
starting active screening and animal health programmes. It consists in
obtaining the first information on specific disease occurrence in the territory
if the disease exists or not and in positive case what is its approximate
distribution and consequences. Initial
monitoring has different forms where the first investigations and ad hoc
findings are complemented by the first active "sondes" to achieve
better knowledge about the "new" problem.
f) In the majority
of known diseases initial monitoring represents still the only specific activity
due to the fact that there are not yet the conditions for starting complex and
effective programmes for their reduction, elimination and eradication.
g) Follow-up
monitoring means the observation over a period of a group, or initially
defined population whose appropriate characteristics have been assessed to
observe changes in health status or health-related variables.
21.3 Epizootiological surveillance
21.3.1 Principles
a) Epizootiological
surveillance is a continuous process of observing, investigating, analysing,
evaluating and examining carefully and constantly all development and pertinent
alterations of the epizootiological situation and factors influencing it. The
purpose is to predict and detect in time (i.e. at the onset) dangerous
epizootic threatening to worsen a given situation, to determine the state of
epizootiological emergency, and to immediately alert the services and
organizations involved.
b) Surveillance
particularly attempts to predict and detect, measure
and evaluate changes or trends for the worse in the behaviour of problem
diseases, detect and assess unexpected risks and determine critical moments
and places.
c) Surveillance mainly
scrutinizes quantitative changes for worse of selected phenomena which have
reached a critical point, producing a qualitative change for the worse,
i.e. deteriorating of epizootiological situation.
In essence, it is a scrutinizing of certain
events that is used to detect a change in animal health/disease trend or
distribution to modify investigative or control measures and to identify
promptly any emerging disease syndromes which would require immediate response
from a veterinary or a public health services.
d) Surveillance makes
possible timely preparation, application and eventual modification of
epizootiological strategy, programmes and measures. Surveillance provides the
fundamental basis for epizootiological prognoses and continuous upgrading
of specific activities for a swift appropriate reaction to changes in the
animal population health situation.
e) Surveillance
provides information required to meet international reporting need and to support
declarations of disease status for trading purposes particularly if
exotic diseases involved.
f) Surveillance can be:
- general,
covering the full etiological spectrum
- specific,
covering particular etiological aspects, i.e. specific animal health or disease
processes.
As a rule, epizootiological surveillance
focuses on a limited number of carefully selected diseases such as the
exotic ones, the most dangerous ones and those under intensive reduction,
elimination or eradication programme, i.e. diseases having a major economic and
public health impact.
g) Calling attention
for the new situation and threatening risks (particularly the
appearance, suspicion and possible outbreak of an exotic disease) represents a
major step forward in prediction and prevention.
h) Early warning
system is a specific procedure to detect as early as possible any worsening
departure from usual or normally observed frequency of dangerous animal
health/disease phenomena. The warning system for health hazards for people, animals,
environment can include also chemical contaminants.
i) Surveillance is a continuous
process which depends on scientific developments and field experience. On
the other hand, it influences the theory and practice through new finding and
results as well as its demand for new methods and supply of new ideas.
21.3.2 Objects of epizootiological surveillance
a) The objects of
surveillance are all phenomena connected with collective health and
epizootic processes, as well as factors influencing these processes.
Of priority interest are alarming manifestations, potentially or
actually prejudicial to the future development of the epizootiological
situation which can lead to the spread of existing diseases or to the
origin (introduction) of new ones.
b) Surveillance of animal
populations mainly applied to changes in numbers and structure by
species and categories, in animal productivity, movements, trade, etc.
Surveillance focuses on changes which tend to increase the degree of exposure (risk)
to selected specific diseases and to decrease population resistance
(immunity levels) against these diseases.
Among the main objects of surveillance are
healthy populations, herds and flocks at epizootic risk, recently recovered
(risk of recurrence), affected and suspected, forms of disease, morbidity and
mortality trends, etc.
c) Etiological
agents and their characteristics of
selected infectious diseases and of newly detected are studied evaluating
changes and appearance of new types, subtypes and strains, undesirable major
changes of pathogenity, antigen structure, immunogenity, tenacity, drug
resistance, etc.
d) Transmission
factors of etiological agents are also scrutinized, in particular the movement
and import of domestic and wild animals and their products, human population
long distance migration as well as behaviour of vectors of selected diseases of
natural nidality. Factors which could lead to the introduction of a new
dangerous exotic disease have top priority. The surveillance of all aspects
facilitating the long-range spread of disease heading to eventual panzootic is
of particular importance.
e) Surveillance objects
involve also influencing environmental factors such as atmospheric,
hydrospheric, geospheric, biospheric and nutritional ones of which some major changes
could worsen the epizootiological situation. Their impact can be in lowering
animal population resistance, promoting overpopulation and the spread of
etiological agents vectors and reservoirs and
contributing to transmission mechanism.
f) Surveillance studies
and evaluates changes in epizootic processes which may lead to a
worsening of the epizootiological situation. This mainly involves changes in:
stages of epizootic processes (e.g. post-epizootic into inter-epizootic,
pre-epizootic into ascending stages, etc.), forms (from mild to severe), grades
(an enzootic threatening to become an epizootic, an epizootic to become a
panzootic), range of distribution (risk to be spread in disease-free areas),
etc.
g) Once a disease has
been eradicated, the recovered population (zone) is "watched" for a
long period until it is quite sure that specific etiological have been
completely exterminated (risk of recurrence).
h) Where selected
diseases can be transmitted from animals to man, data and findings of human
epidemiological surveillance are also used.
i) Economic and
social factors having potentially a major negative impact
on epizootic processes are objects of surveillance as well.
j) Deficiencies and
gaps in epizootiological measures against selected diseases belong among
surveillance objects similarly as in diagnostic efficiency and capability,
extension activity and specific research. In this context the availability, use
and side effect of vaccines, insecticides, drugs and other substances used are
continuously studied.
k) The biological,
economic, public health and social consequences of selected diseases are
studied and evaluated to detect and predict worsening undesirable adverse
effects.
21.3.3 Epizootiological
surveillance methods
a) Full spectrum of
epizootiological investigation, data gathering and processing, analysis and
monitoring (inspection) methods can be used for epizootiological
surveillance. (See chapters 18,19 and 20).
b) The methods should
be carefully selected and tailored to the purpose of surveillance, to
the special nature of selected diseases (specific epizootic processes) and to
particular situation at a given time and in a given place.
c) In addition to
examining and evaluating available data on the results of investigations,
analysis and monitoring, data from related branches such as public
health, livestock husbandry, processing industry, trade, etc. are also studied and
evaluated. Data which may indicate a possible worsening impact on epizootiological
situation receive a particular scrutiny.
d) Data on international
disease situation, particularly in neighbouring and exporting countries are
carefully examined and evaluated.
e) Data published in
the literature on new scientific discoveries and field experience pertinent
to selected diseases and unofficial information on their
occurrence are also examined and evaluated.
f) Available data are
sometimes supplemented by: field surveys, additional experiments,
comparative studies of microbial strains and serums of different provenance
and dates (microbes and serum banks), etc. Among important studies belong the
comparison between field strains and specific vaccine production strain to
confirm if they are compatible or not.
g) To study and evaluate
the risk of penetration or survival of specific disease agents, highly
susceptible and healthy animals so called "sentinel animals"
free of specific agents, are sometimes placed in sites which are suspect or at
risk. The purpose is to detect in time the eventual existence and/or frequency
of these agents as they are "caught" by the sentinel animals. These
animals are controlled and investigated in an attempt to isolate the suspect
agents and/or detect sera conversion in animal's blood (appearance or increase
of specific antibodies).
h) Surveillance can be
passive, i.e. rely on legal or voluntary reporting or active in which
aggressive efforts are made to collect information from all available sources.
21.3.4 Organization of epizootiological surveillance
a) The vertical and
horizontal organizational structure of the veterinary services provides
the basis for epizootiological surveillance organization including almost all
components of these services.
b) Surveillance is
organized at all levels, from field to national and international levels.
c) For processing and
evaluating surveillance data, use is often made of specialized reference
centres at national and international levels which have
specialists in respective diseases.
d) The results
(findings) of epizootiological surveillance are distributed in time to
agencies and officers participating in and responsible for selected animal
population health programmes.
e) Epizootiological
surveillance is multidisciplinary by nature and for this reason requires
very close cooperation among animal health specialists (epizootiologists) and
experts in other related fields (epidemiologists, microbiologists,
parasitologists, immunologists, ecologists, entomologists, biostatisticians,
etc.).
f) Good coordination
between epidemiological and epizootiological surveillance is essential for
diseases common to humans and animals. In fact, a joint surveillance programme
is the best.
22. EPIZOOTIOLOGICAL
THEORY, EXPERIMENTS AND STUDIES
====================================================
22.1 Introduction
a) Epizootiology,
similarly as other biological sciences, has its constant development
based not only on the analysis of natural events and practical experiences but
also on theoretical and experimental works.
b) Field or
laboratory practice is the best testing place for the confirmation
that the conclusions of the theory and experiments are correct and useful.
22.2 Theoretical methods in epizootiology
22.2.1 Development
of epizootiological theory
a) Epizootiological
theory consists in a complex of reasoning which explain and make
intelligible the epizootiological phenomena.(Theoretical epizootiology).
b) The development of
this theory is based mainly on generalization of discoveries produced by
scientific activities such as experiments under field, laboratory or simulation
conditions, practical (empiric) experiences and field observations. From these
results new laws, principles, standards, norms, classification, definitions and
methods are deduced and formulated.
c) The development of
epizootiological theory is based also on the application of some laws,
principles and methods of other related
medical, natural and technical and philosophical sciences.
d) Majority of
epizootiological phenomena has mass character where probability theory
permit to study their features using exact mathematical methods.
22.2.2 Epizootiological
hypothesis
a) Epizootiological
hypothesis represents the idea with the expectation that it will conduce to the
explication of epizootiological phenomena (historical, actual, future) or to
the discovery on new facts, relations, causes or consequences, using
accumulated knowledge and experiences as well as epizootiological logic.
b) Formulating the
hypothesis methods of difference, concordance, analogy, causal association
(cause - effect / consequences), etc. are used.
Causal association. A
"cause" is a thing or event which when changed itself makes something
else also change. "X causes Y" if a change imposed directly on X
results in a change in Y.
c) The null
hypothesis states that the result observed in a study, experiment, or test
are no different from what might have occurred as a result of the operation of
chance alone.
d) Each hypothesis
is to be confirmed or refused. In the latter case it should be replaced by
a new one. The confirmation can be based on natural observation or the
experiment. Generally, for the comparison the sets of experimental and control
units are used or past and actual phenomena values (comparative retrospective
study). Principles of sampling must always be respected (see chapter 18).
e) The quality
(probability) of the hypothesis expressed in numerical values (statistical
hypothesis) should be tested by statistical methods indicating also the
grade of error.
22.2.3 Epizootiological
prognosis
a) Epizootiological
prognosis is the prediction of the future development of particular
phenomenon based on available past and actual data on it, on its past tendency
as well as on accumulated knowledge and experience.
b) Prognosis is very
important for epizootiological strategy,
programmes and measures. It requires good knowledge of respective
processes and influencing factors.
c) The tendency can be
determined in form of graph projecting the past time series data in a
line or curve extending it into the future (prospective line or curve). It is
obvious that in case of a disease with the periodicity (seasonality or
cyclicity) the curve must be adequately adjusted.
d) If no major changes
are expected the tendency can continue in the same trend. If major
changes are expected then the tendency will be changed according to the changes
of phenomenon determinants (e.g. increase of resistance due to mass vaccination).
22.2.4 Epizootiological
modelling
a) Modelling of
epizootiological phenomena using abstractions of the reality through language
of symbols (verbal, physical, schematic or mathematical) facilitates to understand
epizootic processes as well as to forecast the future development.
b) Mathematical/statistical
models explain different aspects of the occurrence of a variety of
diseases. With some infectious diseases, models have been generated to
elucidate the reason for epizootic and/or to predict the behaviour of the
disease in reaction to given control measures. It is a representation of a
system, process or relationship in mathematical form in which equations are
used to simulate the behaviour of the system or process under study.
c) Simulation is
a model system, e.g. a mathematical model or an animal model, to approximate
the action of a real system, often used to study the properties of a real
system.
d) The models should be
logical, flexible and based on professional knowledge and experience. The field
or laboratory practice tests the reliability of the model results.
22.3 Experimental methods in epizootiology
22.3.1 General
principles
a) Epizootiological
experiments (experimental epizootiology) consist in artificially
inducing a characteristic which changes natural course of animal population
health/disease or influencing factors in order to discover, confirm or
demonstrate determined epizootiological phenomena, their causes, functions and
consequences. These experiments represent the strictest tests of causal
relations in epizootiology.
b) Generally, in
epizootiological experiment a population is selected for a planned trial of a
regimen whose effects are measured by comparing the outcome of the regimen in
the experimental group with the outcome of another regimen in a control
group. To avoid bias members of the experimental and control groups should
be comparable except in the regimen that if offered them. Allocation of
individuals to experimental or control groups is ideally by randomization
(randomized controlled trial).
22.3.2 Types of
epizootiological experiments
a) Epizootiological
experiments have different objectives, contents and forms. The basic form is the comparison between the tested
and compared "control" animals (or other sets of units). The
comparison can be simultaneous (with paired or standard units) or successive
(with past units - historically). Both types can be carried out between
different groups treated differently or between identical groups treated differently.
b) Epizootiological
experiments are different in field conditions (field trials) where virulent etiological agents cannot be used
and in laboratory with much better conditions for work with dangerous agents
and for more precise selection, monitoring and evaluation of experiments.
Laboratory and field experiments are usually interlinked.
c) Relatively often, before
introducing new epizootiological methods, vaccines or drugs in practice, pilot
tests are used (after confirming the effectivity and harmlessness first
under laboratory conditions and following tests are applied in a small groups
of animals under field conditions to confirmed the results).
d) Intervention
trial is a planned experiment designed to assess the efficacy of a
treatment in animals/herds by comparing the outcome observed under the test
treatment with that observed in a comparable group of animals/herds receiving a
control treatment. ( The term "treatment" is used as a broad
definition of any possible intervention such as treatment for a particular
disease or diseases, vaccination programs, and management/environmental
maneuvering).
e) Experimental
studies strives to make the treatment groups as similar as possible (e.g.,
breed, age, sex, nutrition, housing) with the only "difference"
between the groups being the treatment given.
By "controlling" everything the outcomes can be compared
(usually using simple statistics) with relative assurance that any difference
seen was in fact due to the treatment.
f) Bioassay
consists in quantitative evaluation of the potency of a substance by assessing
its effects on tissues, cells, live experimental animals, etc. Bioassay may be
a direct method of estimating relative potency: groups of subjects are assigned
to each of two (or more) preparations; the dose that is just sufficient to
produce a specified response is measured, and the estimate is the ratio of the
mean doses for the two (or more) groups. In this method the death of the
subject may be used as the "response". The indirect method (more
commonly used) requires study of the relationship between the magnitude of a
dose and the magnitude of a quantitative response produced by it (dose-response
assessment).
22.3.3 Evaluation of
epizootiological experiments
a) Generally, the
results of the experiments are evaluated using statistical methods. The
purpose is to confirm or reject epizootiological hypothesis, define central
values and the dispersions, associated values, correlation and regression
values, difference between experimental and control units, test of statistical
significance (probability) and its level.
b) The smaller the probability
(P) value, the less likely it is that the observed relative difference is just
due to random variation. P-values generally need to be smaller than 0.1 or 0.05
to be considered significant. (Not all statistically significant associations
are causal.)
c) Among statistical tests
often used in epizootiology belong: Student 't' test, F test, chi-square test
(contingency tables), etc.
d) Interpretation of
the results must have epizootiological sense (logic), i.e. must be biologically
meaningful.
22.4 Epizootiological studies
a) Different observational
study designs may be used to test hypotheses about causes of disease. A common component of all observational study
designs is the comparison of two or more groups. Studies may be broadly
classified as prospective or retrospective depending on whether the outcome of
interest has already occurred.
Observational studies are used the most in
epizootiological studies. This kind of studies allows nature to take its course
without intervention in the disease process. Studies may be done with
relationship to time, factor exposure or disease status.
In these studies, called also analytic
studies, individuals may be
classified according to absence or presence (or future development) of specific
disease and according to "attributes" that may influence disease
occurrence. Attributes may include age, race, sex, other disease(s), genetic,
biochemical and physiological characteristics, economic status, various aspects
of environment, etc.
b) A chief advantage
of observational studies is that they are directed towards the species of
concern in its natural environment. This greatly reduces the problems
associated with extrapolation of results from a particular study to the target
population. It also allows to test a much broader range of hypotheses than
would be possible under controlled experimental conditions.
c) A major disadvantage
of observational studies is the possibility of biases especially confounding
which may distort the true relationships between variables. Missing data are
also a common problem in prospective studies because animals are sold, die or
lose their identification.
d) Types of observational
studies
aa) Cross-sectional
study (syn.: disease frequency survey, prevalence study) where the sampling is
without regard to exposure or disease status. The prevalence of the disease in
question are measured and compared among those with and without the suspected
risk factor(s) of interest. Evidence for a causal associations is only
realistically produced for permanent factors such as breed and sex.
bb) Case-control
study where the sampling is on the basis of disease status. The animals,
litters, herds, or some other units with the disease of interest as the
"cases" and units without the disease as "controls" are
selected. Information about prior exposure history to "risk factors"
is gathered and compared among the groups. These studies are well suited to
rare diseases and many risk factors can be evaluated at the same time. They are
quick and easy to perform but subject to many potential biases.
cc) Cohort study
(follow-up study) where the sampling is on the basis of exposure status.
Representative animals (or litters, herds, etc.) exposed to a selected risk
factor are selected along with a comparison group which is not exposed to the
factor. The natural course of events is merely observed. After a suitable
period (longitudinal study) the incidence of disease of interest is compared
between the groups.
The groups or populations, selected on the
basis of whether they have been exposed to risk, received a specified
preventive or therapeutic procedure, or possess a certain characteristic, are
followed to assess the outcome of exposure, the procedure, or effect of the
characteristic, e.g. occurrence of disease.
dd) If all the relevant
events (exposure, disease, etc.) have taken place prior to the study then the
study is retrospective, otherwise the study is prospective.
Cohort and case-control studies can be both prospective and retrospective.
ee) Ecological
study in which the data are collected on the basis of a group rather than for
each animal individually. This kind of study is done when more than one factor
needs to be studied in their relationship to the disease.
e) Blind(ed) study
(Syn.:masked study) in which is intended to keep participants in a study from
knowing some facts or observations that may bias or influence their actions or
decisions regarding the study. These studies can be single-blind, double-blind
or triple-blind trials.
22.5 Bias
a) Bias is the distortion
of results by neglected factors, e.g. due to systematic error in the data.
It is not a matter of random variation or imprecision. Bias is caused by
messed-up study designs.
b) Selection bias
is a systematic error in the way that the samples of subjects were drawn from
their underlying populations, or in the way that subjects were assigned to
interventions (or other maneuvers).
c) Information bias
is a systematic error in the way that data were gathered or measured. All data
potentially are subject to imprecision (random variation). There may be a
systematic error (inaccuracy) in the measuring tool compared to the best possible
tool. There are also systematic errors that are applied differentially between
groups.
d) Diagnostic bias consists in errors when using
non-standard (non-uniform) methods and tools (objective errors), when different
investigators using the same methods
obtain different results in the same animals or samples (subjective
errors) and when methods with poor sensitivity and specificity are used.
e) There are other
types of bias such as:
- confounding
epizootiological indicators (e.g. prevalence, incidence and proportion of
positive results)
- making conclusions
without taking into consideration the dynamics of population health/disease
processes
- not respecting space
and time aspects
- making conclusions
not respecting the possibility of false negative and false positive results, of
possible influence of previous vaccination or treatments
- making premature
conclusions based on incomplete analysis
- lack of critical
evaluation of the results
- lack of
epizootiological logic when interpreting the results, etc.
Note: More information on
statistical methods see in respective textbooks).
23. EPIZOOTIOLOGICAL
STRATEGY AND MEASURES
==========================================
23.1 Epizootiological strategy
23.1.1 Introduction
a) The strategy for disease prevention,
control and eradication (further "strategy") represents the policy
which, respecting society needs, biological and methodological as well as economic
and organizational feasibilities, identifies main concepts, priorities and
objectives and determines systems - principal methods for achieving these
objectives in a most effective way.
b) The strategy, programmes and practical
measures should be based on an adequate analysis of epizootiological
situation as well as on prognosis considering influencing factors.
c) The decisions are made at different levels
of organization management structure. Usually, the strategic decisions are
carried out at international and national level (political - strategical). The
decisions about the concrete field measures are made at the lower - local
levels (operational - tactical).
d) The priority intention is to maintain and strengthen
benefit factors of population health and reduce till eliminate damaging
factors of diseases.
e) Protection of human health represents
the priority objective of society. One of the component
of public health strategy is the protection of man against diseases
transmissible from animals.
f) Before a specific strategy is decided, the
level of knowledge about the causes of the disease, its sources and
transmission ways, host range and the nature of the host/parasite relationship
as well as diagnostic feasibility should be considered.
23.1.2 Society needs - strategy basis
a) Strategy for disease
prevention, control and eradication represents an integral component of the
overall social and economic development strategy. The needs of human
society are to be fully respected and its requirements met effectively as a
contribution to social and economic development of the country.
b) The strategy should first create as
effectively as possible the animal health conditions, particularly for the
improvement of the production of food of animal origin in its quality
and quantity and the contribution to the protection of human health
against zoonoses and food-borne diseases.
c) The strategy should contribute to desirable
and uninterrupted development of animal breeding, production and reproduction
with minimum possible inputs.
d) Some difficulties are linked with interests
and priorities differences between particular social groups, between
society and individuals, between state and private sectors, between producers
and consumers, between national and local levels, etc.
23.1.3 Strategy concept
a) Strategies for animal disease prevention,
control and eradication apply
general social and economic development strategies in the field
of animal health and protection of human health.
b) The strategy is
based on the
epizootiological, epidemiological, economical, social and
political analyses and on the tendency studies and prognosis.
c) The strategy reflects general veterinary
philosophy prevailing in the given country. Preventive medicine concept is
prevailing in some countries, while the curative concept is prevailing in the
others.
d) Only healthy animals create conditions for
effective breeding and production and for the protection of human health
against the diseases transmissible from animals. Therefore, the preventive
concept is more effective and cheaper with better impact on production
improvement than curative treatment of sick animals ("fire brigade"
concept).
e) The problem is not to achieve the objectives
at any costs but to achieve them in a most effective way.
f) Preference should be given to the strategy
and application of those concepts and methods which have already proved
in practice to be feasible and effective under comparative conditions.
g) The positive results of the strategy and its
implementation programmes should be consolidated and maintained, i.e. to
convert them in lasting effect.
h) The strategy should be flexible in
case of significant changes in epizootiological situations and/or in
influencing factors to be able to adjust the programme and keep it functional.
23.1.4 Strategy priorities
a) In all the countries, there are so many
animal health problems that in none of them the solution of all problems is
realistic in the next future. Therefore, there is an imperative need to identify
the most urgent problems which solution is feasible under the given
conditions.
b) Identification of the priorities helps the concentration
of the available forces and resources on the main problems solution which
promises successful and beneficial results.
c) The strategy priority represents the cross-result
of the complex analysis and identification of biological, economic, public
health and social priorities after being corrected by the degree of feasibility
and expected benefits.
d) The procedure can start with preparing a list
of epizootiological problems in the order of importance and urgency.
Problems which solution is still unrealistic will be eliminated from the list.
Among the high priority problems should be included the most important ones
considering the specific diseases situation and grade of feasibility and
expected effectivity of the programmes.
One of the methods which has proved to be
useful consists in following procedure: First step is to prepare a list of
diseases to be
considered and then they
should be evaluated by
their biological, economic,
public health and social importance grades (e.g., scale from 0 to 10). The
obtained values to
be multiplied by importance coefficients for
different forms of
importance. The results (numbers of
points) is corrected (multiplied) by solution feasibility and inputs availability grades (e.g. scale could be also
from 0 to 10). The order of priority is
given by the number of points obtained.
e) Those diseases which are in final phase before
their eradication, although the losses caused by them are not more the
major problem, should be included among the priorities.
f) Particular attention is given to those
diseases which are officially notifiable due to their importance. The
situation of these diseases is usually better known than of the others giving
more reliable basis for the analysis and prognosis.
23.1.5 Strategy objectives
a) Every strategy should have its objectives to
be achieved. The clear cut objectives facilitate to maintain the orientation
"line" and to concentrate on programmes and activities aiming at the final
goal. The objectives expressing the situation which should be by the end of
the given period represent the basis for planning of particular measures.
b) The objectives should define what, where, when and what
level or quality should be achieved. Measurable parameters have the
preference. The objective should be demanding and mobilizing but feasible.
c) Examples of objectives to be achieved:
- specific
disease-free territory created, protected or extended;
- specifically
healthy herds, farms or population created, protected or extended;
- proportion
of specifically healthy animals detained or increased to a given level;
- fertility,
natality or/and weaning rate increased to a given level;
- general
or specific morbidity detained or reduced to a given level;
- specific
disease(s) eliminated or eradicated;
- specific
disease(s) focality detained or reduced to a given level or eliminated;
- specific epizootic
process detained, reduced to a given level, slowed down to a given level,
interrupted or eliminated, etc.;
- general
or specific disease mortality / lethality or other losses detained, reduced to
a given level or eliminated; etc..
d) In the case of healthy animals, diseased
animals and focality rates the use of point prevalence rate is
preferable.
e) The objectives of the strategy and
implementing programmes can also be expressed in public health terms (specific
human health protected, improved, specific zoonoses morbidity decreased or
eliminated, zoonoses mortality eliminated, etc.), economic
terms (e.g. particular productivity maintained, increased, improved),
ecological terms (e.g. healthy environment created, protected, expanded,
recovered),etc.
f) Global strategy aims at gradual
eradication of specific diseases of major importance in the whole world.
g) The strategy and implementing programmes
should have not only temporary but sustainable effect which requires
often demanding follow-up activities.
23.1.6 Strategy types
a) The strategy can be general covering
animal health and diseases in full etiological spectrum or specific
dealing with specific health and diseases only.
b) The strategy can cover all animal species
in a given country or territory or only one or few selected animal species.
c) The strategy can be a complex one
covering all aspects of the given problem or partial, dealing only with
specific aspects of the given problem.
d) The strategy can be oriented to particular objectives
such as:
- active
creation of animal population health;
- prevention
(protection) of animal population health;
- increased
rate of healthy animals;
- simple
control of epizootiological situation;
- reduction
of animal morbidity;
- elimination
of specific disease (point prevalence = 0);
- eradication
of specific disease (incidence = 0);
- reduction
of animal mortality;
- protection
of human population health;
- reduction
of human population zoonoses incidence;
- elimination
of human population zoonoses incidence; etc.
e) In zoonoses particular importance has
the common epizootiological and epidemiological strategy protecting, expanding
and recovering human population health through improved specific animal
population health.
f) The strategy can cover (determine) also the activities
such as:
- diagnostic
activities;
- prophylactic
activities;
- sanitation
activities;
- mass
treatment activities;
- epizootiological
surveillance; etc.
g) According to the space aspects the
strategy can be territorial covering whole countries or zones or particular
sectors, production branches, etc.
h) According to the time aspects the
strategy can be temporal, short-term, middle-term, long-term or perspective.
i) In some diseases doing nothing can be applied when
- the morbidity may be
absent or reduced by natural changes in host/parasite relationship without the
intervention of man (e.g. due to the lack of natural vectors, inadequate
climate, absence of susceptible species, etc.);
- the low importance
or lack of necessary conditions for desirable actions do not require or permit any control programme for the
moment.
Today, unfortunately the majority of the
many hundreds of transmissible and non-transmissible diseases (mainly chronic
and subclinical) are still waiting for specific actions which have been
postponed for the future after solving the diseases of major importance with
the chance for successful programmes.
23.2 Factors influencing epizootiological
strategy
23.2.1 Introduction
a) To select an optimal strategy it is necessary
to analyze, respect and eventually exploit the influencing factors such
as biological, ecological, economic, social, political, organizational, etc.
These factors, on the one hand, orient and create conditions for practical
programmes and measures and on the other hand limit their feasibility.
b) When deciding on the strategy, the
biological, public health, economic and social influences and consequences
of the animal health and disease must be taken into consideration.
c) The diversity of the specific
epizootiological situation and complexity of many influencing factors require concrete
decisions to be taken in any particular situation.
d) Comparison of expected effectiveness
of a given strategy with the expected (supposed) situation development when no
adequate actions are taken, is of particular
importance.
23.2.2 Biological and ecological factors
a) The biological factors and aspects have the
primary importance for the decision on disease prevention, control and
eradication strategy. In particular, the epizootiological situation as well
as biological and methodological feasibility must be considered.
Unfortunately the effective and reliable biological methods for achievement of
desirable strategy objectives are not always available.
b) Among biological and ecological strategy
influencing factors belong the following ones:
aa) animal population
general characteristics: (number of animals, population structure according
species and categories; animal productivity and behaviour; physiological and
genetic development; animal reproduction; population location, density and
concentration in farms, herds or flocks; population horizontal and vertical
movement; nutrition status; etc.); (see chapter 2);
bb) animal population
epizootiological characteristics:
general and specific
susceptibility and resistance grades; values of animal population health
(general and specific) grade and viability; animal population morbidity and
mortality (general and specific); animal population epizootiological structure
(healthy, diseased, suspected, threatened, exposed, etc.).; (see chapters 3-7);
cc) etiological
agents and their characteristics of epizootiological importance (such as
pathogenity, tenacity, transmissibility, life cycle, etc.); sources of
etiological agents and ways of their transmission; (see chapters 8-10);
dd) environmental
factors and their characteristics: atmospheric, geospheric, hydrospheric,
biospheric, nutritional, etc.; (see chapter 11);
ee) infectious
diseases that affect or can be transmitted by only one species of host are
relatively easier to control and eradicate that the diseases with a wide host
range; diseases due to exogenous agents can be identified and controlled easier
than diseases due to endogenous and ubiquitous agents (their overall
eradication is impracticable).(See chapter 12).
ff) epizootic
processes characteristics such as specificity, contents, susceptible
species, development stages, forms, range, intensity and grade, location,
distribution and propagation, time delimitation, duration, frequency,
variability, etc.; (see chapter 13);
gg) nidality
characteristics; diseases with natural nidality involving wild animals and/or
with arthropod vectors are particularly difficult to control and eradicate;
(see chapter 14);
hh) etiological agents transmissibility
to man and zoonoses epidemic process characteristics such as specificity,
development stages, forms, morbidity, location, distribution, time
delimitation, frequency, etc.; (see chapter 15).
c) Effective specific prevention, control and
eradication are possible only if a particular disease can be recognized and the
main epizootiological characteristics are known.
d) The possible ecological consequence of a
given strategy as well as
the desirable ecological balance in a given ecosystem
should always be considered, particularly in diseases with natural nidality.
23.2.3 Economic factors
a) Economic factors usually play a supporting,
stimulating or limiting role when the strategy is to be decided.
Among these factors belong such as economic
situation of the country or territory, of the breeders, producers,
consumers, of veterinary services, etc. There is the problem of economic
feasibility of the strategy.
Necessary economic resources are not always available.
b) Among the economic factors are those as
general economic level, grade of urbanization and industrialization,
transport and communication, trade, etc. Rural development, livestock
breeding and production development (organization, structure, concentration,
specialization, technology, management, etc.) should also be considered. (See
chapter 16).
c) Availability of necessary inputs such as
financial funds, transport, fuel, energy, investments, facilities for
diagnostic and production laboratories, veterinary drugs, equipment,
instruments, etc. are indispensable for animal health strategy and programmes.
This availability depends on the priority grade within the state and private
sector budgets.
d) Losses compensation, prices'
differences, livestock insurance complement economic factors complex, as well
as financial contribution of the enterprises, cooperatives and private sector
to the animal health programmes. Governments usually cover the cost and
consequences of obligatory measures of society interest such as mass
investigations and vaccinations, final sanitation in foci of dangerous
diseases, losses due to emergency slaughter, etc.
e) Some animal health programmes temporally limit
economic activities, reduce breeders and producers income (limitation or
prohibition of animal movement; limitation of animal products use and sales or
their confiscation, etc.). Drastic measures consequences require adequate
economic compensation.
f) The importance, role and position of
animal production in general economic development as well as in the export
or/and import of animals and their products should be always taken into
account.
g) If the strategy and programmes are to be
successful, necessary economic resources must be available or the strategy and
programmes must be adjusted to limited resources.
h) If a control or eradication campaign involves
the slaughter of many animals, sufficient replacement stock should be available
in livestock industry to minimize the disruption of the breeding and
production.
23.2.4 Social factors
a) Social factors influence the strategy and
programmes supporting, stimulating or limiting them. The main social
factors are standard of living, culture standard, political conditions as well
as conditions of peace and war.
b) In the countries and territories with very
low living standard where the poor hungry people have the priority in
surviving, the desirable interest in animal disease control strategy is usually
lacking.
c) In the countries and territories with low
percentage of alphabets and low culture standard, the strategy is much
more difficult to apply than in the countries and territories with opposite
characteristics.
d) The strategy can also be supported or limited
by traditions, habits and religious prejudices.
e) Availability and adequate standard of mass communication
(extension) means for informing, explaining, convincing and mobilizing the
farmers and other inhabitants, are of extraordinary importance.
f) The selected strategy is easier to apply
where political system is stable, where laws and government authorities
are respected, where the local power and political organizations actively
support animal health programmes.
g) Social differences in interest in the
strategy and programmes such as between the society and individual and group
interests, national and local interests, should always be taken into
consideration.
h) Support of the selected strategy depends a
lot of conviction
grade of concrete programmes feasibility. Examples of successful results under
similar conditions are the best arguments. Uncertainty about results is one of
the most difficult obstacles. Veterinary service itself should be convinced first.
i) Usually the strategy and implementing
programmes are easier to apply in state sector than in private sector where
economic profit aspects are decisive.
j) Views of veterinarians, public health
specialists, breeders, producers and society must be always considered. Their
opinion and the degree of their cooperation are influenced by their
understanding of the strategy and implementing measures.
k) Suitable legislation with provision for
compensation is indispensable.
l) Effective strategy and implementing
programmes can be carried out only under peace conditions.
23.2.5 Organization factors
a) Every veterinary strategy requires adequate
organization of animal health services in terms of its standard,
capacity, activity, structure, diagnostic facilities, manpower (standard,
number, distribution), funds, transport, etc. at all levels from field up to
national.
b) The strategy and implementing zoonoses
programme requires also adequate organization of public health services.
c) Organization of animal husbandry, food
processing as well as political and administrative system and structures must
be respected.
d) Animal health services cooperation and
coordination with public health services and other participating services and
organizations is essential for any successful strategy and programmes aimed at
animal disease prevention, control and eradication.
e) Strategy based on obligatory measures
supported by government infrastructure and legislation is easier to apply than
the campaign on voluntary basis requiring adequate incentives.
23.3 Epizootiological measures
a) Epizootiological
measures represent practical activities for the implementation of the
strategy and programmes aimed to reach respective objectives.
b) Measures should be
well targeted to their purposes and goals using the most convenient and
effective methods. Due to the fact that every case is different, the selection
of adequate measures in concrete cases based on previous analysis is decisive.
Routine and "mechanical" application should be avoided. Applied
measures should be indispensable, reasonable, logical, feasible and well
justified and defensible.
c) Where possible planned
and complex measures are preferable to ad hoc and isolated measures
(e.g. in first moments of a new disease outbreak when applying provisional
measures).
d) Preference is given
to the primary measures against the cause of animal disease, i.e.
against specific etiological agents, their sources and ways of transmission to
interrupt epizootiological chain and thus protect healthy animals. These
measures are decisive, however, they are usually
complemented by a series of supporting secondary measures.
e) Measures should be
defined in a very concrete form. Everybody responsible for
particular measures application should know what, how, how much/many, where,
when and why they should be carried out. The measures should be
fully understandable which is the preconditions for
the discipline in their applications.
f) Majority of
effective preventive and control measures consists in prohibiting, limiting,
restricting or regulating respective activities of animal owners, breeders,
producers, public etc. Therefore, the measures are to some extent conflicting
and "complicate" the normal life and production.
g) The methods to be
used should be based on previous practical experience where they proved to
be feasible and effective under the given field conditions. New methods
should be first applied in limited number of animals in a form of pilot test.
For
current daily field epizootiological work so called "experiments"
should be avoided. Any fiasco causes general mistrust in the professionals'
competence and in the given programme feasibility which is very difficult to
overcome in the future.
h) Measures can be obligatory or voluntary depending on local legislative and government decisions. For major programme at population levels to