COMPUTERIZED SYSTEM OF QUANTITATIVE INDICATORS FOR ANIMAL
POPULATION HEALTH AND DISEASE ANALYSES
V.Kouba
Summary: Quantitative indicators are indispensable for
the analyses of epizootiological situation as the basis for planning,
implementation and evaluation of animal health programmes as well as for risk
assessment and health status certification. An integral system of indicators
for animal population health and disease analyses was developed and tested. It
includes methods for the analyses of: animal population collective health,
morbidity, viability (survival), mortality, nidality, animal disease
territorial distribution, animal population health structures, epizootic
process dynamics, etc. All the indicators were converted into computer language
and incorporated in the latest version 4.0 of the EPIZOO software package for
practical use. Detailed information on all individual indicators, i.e. the
formulas, their components and calculation procedures were included in a new
subsidiary software package called EPIZMETH. The system containing more than
one hundred indicators provides an extensive multiform source from which it can
be selected those the best fitting for the given animal population health
analysis.
Introduction
Animal population health and disease
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.
With an increasing demand for applying more
exact and complex measurement of animal population health and disease
situation, the use of an integral system of quantitative indicators has become
very important. Unfortunately, a complex system of indicators covering major
range of different animal population health and disease situations as well as
its computerization have been missing.
The indicators are indispensable for the
analyses of epizootiological situation as the basis for planning,
implementation, monitoring, surveillance and evaluation of animal health
programmes and for risk assessment and health status certification as required
by trade.
The first international definition of
disease prevalence and incidence was elaborated by WHO Expert Committee on
Health Statistics in 1957 (14). In veterinary medicine OIE presented general
definition of incidence and prevalence in its International Animal Health Code.
More detailed elaboration of animal morbidity and mortality indicators were
published in several veterinary textbooks (2,4,5,8,10,11,12,13).
However, it does not exist international
standard system of this kind of indicators based on exact mathematical
formulas, facilitating uniform application and interpretation. The complexity and dynamics of population
health and disease called for a wide variation of analytic methods.
The task was to elaborate an original
integral system of quantitative indicators offering the basis for selection of
suitable criteria when solving particular animal population health problems.
The computerization of the indicators' system is the way how to ease its
practical use.
Material and methods
First step was the inventory of the
indicators as presented in the literature and select those which had already
proved as useful in practice and to include them in the system. Author's
publications (3,4,5) reflecting his long-term activities in improving and
expanding previous indicators' range served as the main source. The author was
urged by emerging needs to elaborate, test and use a more complex system when
he was working as veterinary officer responsible for solving different
practical local, national and international problems (at national level as the
Chief Epizootiologist and Technical Director of State Veterinary Service and at
international level as the expert-epizootiologist in developing countries, OIE
informatics expert, Senior Officer and
finally as the Chief, Animal Health Service, Food and Agriculture Organization
of the United Nations).
The intention in developing the system was
as follows:
- the methods to be general so they could be
applicable anywhere and any time for any animal health/disease related states
and events and for any animal species and category
- to reach the balance
between disease indicators with the
health indicators, i.e. combining the
negative (diseased, disease incidence, etc.) with the positive (healthy,
disease extinction, etc.) aspects
- to respect time and space
factors as well as the speed of animal reproduction process
- to use as the indicators'
components (numerators, denominators, etc.) the parameters which can be
available in veterinary practice
- the indicators to be expressed
mathematically to avoid misunderstanding when only verbal name is used; rates
and ratios to be main forms
Rates as a measure of the frequency of occurrence of a phenomenon using
fractions with numerator as the number of epi. units and denominator as the
number of basic units. Ratio as an expression of the relationship between a
numerator and a denominator when the two usually are separate and distinct
quantities, neither being included in the other.
- after consolidating and testing the
indicators' system a particular computer programme to be produced, i.e.
individual indicators' calculation procedures to be converted into a software
language.
The intention was the software to be
user-friendly and easy to operate (not need for manual or special training) and
the input to be based on a question-and-answer format and to ensure that the
output is obtained immediately in the form of text and, eventually, tables and
graphs.
Results
The integral indicators' system has
following components:
a) General indicators for animal
population health analysis
representing general structure of the prevalence, incidence and extinction
rates indicators reflecting the situation at a given moment and during a given
period (table 1).
Notes: The abbreviation
"epi." means here "epizootiological" or
"epidemiological"; "basic unit" means animal, herd, farm,
territory surface unit, etc. and "epi. unit" means basic unit with
particular epi. characteristic, e.g. diseased, healthy, suspect, etc.). In the
indicators ending by "initial total" or "existing total" or
"average total" the "total" means "total of
corresponding basic units"; "extinction" means disappearance of
a given phenomenon during a given period.
Each phenomena has its beginning, duration and end. It can begin and
terminate within the period; begin in and continue after this period; exist
before and terminate within the period; exist before and continue after the
period. These four possibilities form the basis for differentiation of
prevalence, incidence and extinction indicators.
b) Indicators of
presence/absence of animal health phenomena involving animal health
phenomenon presence/absence relation, average duration of presence of the
phenomenon, average duration of absence
of the phenomenon, time proportion of presence of the phenomenon, time
proportion of absence of the phenomenon, ratio of periods with/without the
phenomenon and ratio of periods without/with the phenomenon. Dr Astudillo's
indicator of disease persistence index (1) is included.
c) Indicators of animal population collective
health ("salubrity") representing the opposite to the indicators
of animal morbidity are subdivided in prevalence, incidence and extinction
rates of healthy animals (table 2). "Extinct healthy animals" mean
those becoming diseased, slaughtered, etc.. As addition several ratios were
included: ratio of healthy animal per diseased one, ratio of diseased animals per healthy one,
ratio of healthy animals per intrafocal one, ratio of intrafocal animals per
healthy one, etc.
d) Indicators of animal population morbidity
are subdivided in prevalence, incidence and extinction rates of diseased
animals. "Extinct diseased animals" mean animals recovered from
disease, slaughtered, dead, transferred, etc. Groups of indicators in form of
ratio of diseased animals to animals with other epi. characteristics and of
attack rates were added. (table 3). Mathematically expressed definitions of
diseased animals prevalence and incidence rates were included into FAO-OIE-WHO
Animal Health Yearbook (last page) from 1983 by the author, when he was its
Chief-Editor, with the aim to help to start with some kind of international
standardization.
e) Indicators of animal
population viability (survival) represent the opposite to the indicators of
animal mortality. (table 4)
f) Indicators of animal population mortality
(death rates) are subdivided in total mortality rates, natural mortality rates,
artificial mortality rates (slaughtered animals rates), specific disease
mortality rates and lethality rates (table 5). Total mortality rates include
the numbers of naturally death and slaughtered animals.
g) Indicators of animal disease nidality
(focality) are subdivided in
prevalence, incidence and extinction rates of foci, affected herds as well as
several density rates. (table 6).
h) Indicators of animal disease territorial
distribution are subdivided in prevalence, incidence and extinction rates
of affected zones (table 7).
i) Indicators of animal/human populations
zoonoses are represented by ratio of
diseased animals per one person, ratio
of diseased animals per one diseased person,
ratio of diseased persons per one animal and ratio of diseased persons
per one diseased animal.
j) Indicators of animal population
health structure (table 8)
facilitate to asses the
proportions according to different animal health and disease characteristics
within the total of evaluated phenomena.
k) Selected indicators of
epizootic process dynamics (table 9.) measure the changes in animal
population health development.
l) Other indicators are
related to some aspects of susceptible, resistant and investigated animals.
The indicators' system was included in a computerized
form into the new version 4.0 of EPIZOO software package (6,7) as its
integral component. To offer the user
detailed information on any of the indicators, i.e. purpose, formula,
components (required inputs) and procedures, a special subsidiary software package
called EPIZMETH was elaborated. All programmes get on one 1.4 standard diskette
without condensing. It can be run on any IBM compatible computer using MS-DOS
or any WINDOWS operating system. As independent software it can be operated not only from the hard drive after
installation but also directly from the
diskette drive, i.e. without installation on the hard disk (easily
"portable" software).
Discussion and Conclusion
The purpose of this work was to offer an
integral system of indicators which could serve as the source from which the
user can select appropriate indicators according to the given situation and
problem to be solved trying to characterize the situation as exact as possible.
The application of morbidity indicators used
in human medicine on animal morbidity conduced to many confusions. The
reproduction process in humans is much slower (number of inhabitants at the
beginning and the end of the year is almost the same) than in animal
populations (e.g. broilers can be replaced several time during one year).
Therefore in the presented system for the denominators different values at the
beginning, of the existing during the period and of the average were used.
Point prevalence evaluates the situation at
a given moment; period prevalence and average prevalence include all the
phenomena existing during the period without considering their beginning and
end. Incidence expresses new cases during a given period. Extinction (opposite
to incidence) expresses changes due to the disappearance of a given phenomena
(e.g. elimination, recovery, transfer, slaughter, etc. of diseased animals)
during the period.
A combination of negative criteria such as
disease and death with positive criteria such as health and life represents a
more objective and complex approach needed not only for analyses related to
reduction, elimination and eradication measures but also to preventive
programmes. Indicators for the analysis of collective health, of diseases free
zones, etc. are becoming more important than in the past due to the need for
certifying diseases free status of animals and their products for trade and
export. The diagnosis and confirmation of population, herds or flock collective
health is much more demanding than to prove the existence of a disease.
Not always it was possible to name the
indicators in the shortest way or to find a suitable name corresponding with
the contents as best as possible. However, mathematical expression facilitates
the understanding also between different languages.
Uniform use of any indicator depends on clear definition of their
parameters. All the indicators are only the tools to exploit the results of
field and laboratory investigations which are decisive for any analysis of
animal health and disease at population level.
The work represents the modest contribution to the development aiming to
provide more exact and multiform measurement tools for the analysis of animal
population health and disease and fills the gap as far as the availability of
specific software is concerned.
REFERENCES
1.
Astudillo, V.M., Hora, A. (1984). - El "grado de endemismo" como uno de los indicadores del sistema
ecologico de las enfermedades. Veterinaria, 3, 17, Porto Alegre, p. 11-22.
2. Cannon, R.M., Roe, R.T. (1982). - Livestock
disease surveys. A field manual for
veterinarians. Australian Government
Publishing Service, Canberra, 35 pp.
3. Kouba V. (1976). - Methodological principles
of a system of quantitative indicators
of the grade of animal population
salubrity and morbidity for epizootiological analysis (in Czech). University of Veterinary
Sciences, Brno, 272 pp.
4. Kouba V. (1987) - Epizootiologia general. 2nd edicion. Edicion Pueblo y
Educacion, Instituto del Libro, La Habana,
867 pp. (in Spanish).
5. Kouba V. (1994) - General Epizootiology.
University of Veterinary Medicine,
Kosice, 209 pp.
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package of methods for animal population
health analysis and programming.
Rev.sci.tech.Off.int.Epiz.,13(3), 637-650.
7. Kouba V.(1995). - EPIZOO: software for
veterinary epidemiology training and
problem-solving. Bulletin of the World Health Organization, 73 (1), 77-83.
8. Martin S.W., Meek A.H., Willeberg P. (1987).
- Veterinary epidemiology - principles
and methods. Ames, Iowa, Iowa State
University Press, 343 pp.
9. OIE (2001). - International Animal Health
Code. Tenth Edition. Paris, 473 pp.
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Tyler, L. and James, A.D. (1987). - Veterinary epidemiology and economics in Africa. International Livestock
Centre for Africa, Manual No. 3, 130 pp.
11. Schwabe C.W., Riemann H.P. & Franti C.F.
(1977). - Epidemiology in Veterinary
Practice. Lea & Philadelphia, 303
pp.
12. Toma B., Benet J.-J., Dufour B., Eloit M.,
Moutou F., Sanaa M. (1991). - Glossaire d'epidemiologie animale.
Collection Medecine Veterinaire,
Editions du Point Veterinaire, 363 pp.
13. Thrusfield M. (1986). - Veterinary
Epidemiology. Butterworths Co.
(Publishers) Ltd, London, 280 pp.
14. WHO (1957). - Expert committee on health
statistics, Fifth report, WHO Technical
Report Series, No. 133, Geneva
ANNEX
Table 1.
General indicators for animal
population health analysis
---------------------------------------------------------
point prevalence rate of epi.
units
period prevalence rate of epi.
units
average prevalence rate of epi.
units
incidence rate of epi. units to
initial total
incidence rate of epi. units to
existing total
incidence rate of epi. units to
average total
extinction rate of epi. units
to initial total
extinction rate of epi. units
to existing total
extinction rate of epi. units
to average total
---------------------------------------------------------
Table 2.
Indicators of animal population
collective health
-------------------------------------------------------
initial point prevalence rate
of healthy animals
period prevalence rate of
healthy animals
average prevalence rate of
healthy animals
incidence rate of healthy
animals to initial total
incidence rate of healthy
animals to existing total
incidence rate of healthy
animals to average total
extinction rate of healthy
animals to initial total
extinction rate of healthy
animals to existing total
extinction rate of healthy
animals to average total
-------------------------------------------------------
Table 3.
Indicators of animal population
morbidity
-------------------------------------------------------------
point prevalence rate of
diseased animals
point prevalence rate of
clinically diseased animals
point prevalence rate of
subclinically diseased animals
proportion of clinically diseased
animals
proportion of subclinically
diseased animals
ratio of clinically diseased
animals per subclinically one
ratio of subclinically diseased
animals per clinically one
period prevalence rate of
diseased animals
average prevalence rate of
diseased animals
incidence rate of diseased
animals to initial total
incidence rate of diseased
animals to existing total
incidence rate of diseased
animals to average total
extinction rate of diseased
animals to initial total
extinction rate of diseased
animals to existing total
extinction rate of diseased
animals to average total
ratio of diseased/healthy
animals
ratio of healthy diseased
animals
ratio of diseased/intrafocal
animals
ratio of intrafocal/diseased
animals
ratio of diseased/at risk
animals
ratio of at risk/diseased
animals
ratio of diseased/resistant
animals
ratio of resistant/diseased
animals
ratio of diseased/susceptible
animals
ratio of susceptible/diseased
animals
ratio of diseased/investigated
animals
ratio of investigated/diseased
animals
specific disease attack rate
specific disease initial attack
rate
specific disease postinitial
stage attack rate
proportion of initial stage
attack rate
proportion of postinitial stage
attack rate
ratio initial/postinitial stage
attack rates
proportional specific disease
morbidity rate
animal-time incidence rate
-------------------------------------------------------------
Table 4.
Indicators of animal population
viability (survival)
-----------------------------------------------------
animal population viability
index
animal population fertility
rate
animal population natality rate
animal population
survival-to-weaning rate
animal population survival rate
breeding animals survival rate
fattening animals survival rate
-----------------------------------------------------
Table 5.
Indicators of animal population
mortality
----------------------------------------------------------
animal population total
mortality rate to initial total
animal population total
mortality rate to existing total
animal population total
mortality rate to average total
animal population natural
mortality rate to initial total
animal population natural
mortality rate to existing total
animal population natural
mortality rate to average total
slaughtered animals rate to
initial total
slaughtered animals rate to
existing total
slaughtered animals rate to
average total
animal population crude fatality
(lethality) rate
specific disease total mortality
rate to initial total
specific disease total mortality
rate to existing total
specific disease total mortality
rate to average total
specific disease natural
mortality rate to initial total
specific disease natural
mortality rate to existing total
specific disease natural
mortality rate to average total
specifically diseased
slaugthered rate to initial total
specifically diseased
slaugthered rate to existing total
specifically diseased
slaugthered rate to average total
specific disease case fatality
(lethality) rate
specific disease proportional
fatality (lethality) rate
animal population neonatal
mortality rate
----------------------------------------------------------
Table 6.
Indicators of animal disease
nidality (focality)
-------------------------------------------------------
point prevalence rate of foci
period prevalence rate of foci
average prevalence rate of foci
incidence rate of foci to
existing total
extinction rate of foci to
existing total
average number of intrafocal
animals in a given period
average number of intrafocal
animals at a given moment
point prevalence rate of
diseased herds
average number of animals in
diseased herds
average density of foci per
territorial surface unit
average density of diseased
herds per surface unit
-------------------------------------------------------
Table 7.
Indicators of animal disease
territorial distribution
----------------------------------------------------------
point prevalence rate of
affected zone
period prevalence rate of affected
zones
average prevalence rate of
affected zones
incidence rate of affected
zones
extinction rate of affected
zones
average number of affected zone
animals per surface unit
----------------------------------------------------------
Table 8.
Indicators of animal population
health structure
---------------------------------------------------------
animal population
epizootiological structure
animal population disease space
structure
animal population diseases foci
types structure
territory epizootiological
structure
morbidity structure by
causes/forms
mortality structure by
causes/forms
nidality structure by
causes/forms
affected territory structure by
causes/forms
disease occurrence acc. animal
species and categories
disease occurrence acc.
breeding/production conditions
disease occurrence acc.
ecological conditions
proportions of disease dif.
forms/symptoms/findings
proportions of spec. etiol.
agents/antibodies' findings
---------------------------------------------------------
Table 9.
Selected indicators of epizootic
process dynamics
-----------------------------------------------------------
comparative indexes of
population health phenomena dynamics
average of changing numbers of
diseased animals and foci
seasonality of population
health/disease phenomena
tendency indicators of morbidity
and nidality
relations between new cases and
space/time/diseased/foci
population "vertical movement"
and chronic disease process
survival of diseased animals
according to replacement
diseased animals number
according to surviving rates
territorial propagation of
transmissible diseases
development of disease frequency
with cyclic tendency
development of disease with
ascending/descending tendency
chronological time series of
population health phenomena
------------------------------------------------------------