PRINCIPLES AND METHODS
Prof.MVDr Václav K o u b a, DrSc.
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. *)
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
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
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
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.
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.
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.
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
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
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
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
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.
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).
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).
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.).
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.
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).
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.
Heritability of etiological agents is their ability to transfer specific genes (features) of the species upon new generations keeping the basic innate characteristics.
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.
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.
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.).
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
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
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
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.
Biosphere, also called as "zone of live" represents the space where animals and plants live. Global biosphere is composed by marine and terrestrial biomasses.
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.
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
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
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)
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
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
- 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)
- 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
- 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.
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
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.
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
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
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
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
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
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.
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
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
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
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
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.
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
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
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.<