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.

 

6. Kouba V.(1994). - EPIZOO: a computer software 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.

 

10. Putt, S.N.H., Shaw, A.P.M., Woods, A.J., 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

------------------------------------------------------------