Epidemiological Field Studies of Animal Populations L. A. Selby, L. D. Edmonds and L. D. Hyde* ABSTRACT
Numerous survey designs have been developed for epidemiological field studies of human populations, most of which are also applicable to field studies of animal poulations. Each design has its own advantages and disadvantages. The final design selected for a particular study depends upon such factors as the overall purpose of the study, the geographic dimensions of the study area, the disease incidence or prevalence and species to be studied as well as the planned use for the data. Population dynamics including the distribution and density of the species to be studied are factors that should also be considered in the initial design of a study. A surveillance system, using mailed questionnaire data and a subsequent survey using direct interviews to validate the data in a statewide study of swine birth defects are used to illustrate some of the techniques that can be applied to domestic animal populations in a fairly large geographic area. The type of data collected, its use and its limitations are also considered.
maines. La plupart de ces plans peuvent aussi servir a des etudes similaires chez des populations animales. Chacun d'entre eux comporte cependant ses propres avantages et inconvenients. Le choix final d'un plan en vue d'une etude particuliere depend de facteurs tels que l'objectif global de l'etude, 1'etendue geographique impliquee, l'incidence ou la predominance des maladies, 1'espece qu'on desire etudier et l'usage auquel on destine les renseignements recueillis. La dynamique d'une population, y compris la distribution et la densit- de l'espce sur laquelle portera l'etude, constituent des facteurs dont il faut tenir compte lors de l'elaboration d'un plan d'6tude. Pour illustrer certaines des techniques qu'on peut appliquer 'a des populations d'animaux domestiques dans une region geographique relativement etendue, les auteurs utilisent l'exemple d'un programme de surveillance utilisant les donnees obtenues 'a l'aide d'un questionnaire postal et d'une enquete ulterieure, basee sur des entrevues destinees a verifier l'exactitude de ces donnes, lors d'une etude portant sur les anomalies congenitales observees chez les porcelets naissants sur tout le territoire de l'letat du Missouri. Ils commentent aussi le genre de donn4es recueillies, ainsi que leur utilisation et leurs limitations.
RASUME Des chercheurs ont deja developpe plusieurs plans d'6tude applicables 'a des enquetes epidemiologiques portant sur des populations hu-
*Department of Veterinary Microbiology, College of Veterinary Medicine, Department of Community Health and Medical Practice, School of Medicine (Selby), 31nvironmental Health Surveillance Center (Edmonds) and Environmental Trace Substances Center (Hyde), University of Missouri, Columbia, Missouri 65201. Dr. L. A. Selby is a former E.I.S. officer and L D. Edmonds is presently an E.I.S. officer with the Center for Disease Control, Atlanta, Georgia 30333. L D. Hyde's present address is American Industrial Hygiene Association, Akron, Ohio 44313. Submitted September 10, 1975.
Volume 40
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April, 1976
INTRODUCTION
An epidemiological study by definition is a population study. Because it is a population study rather than a study of an individual, methods have been developed that apply to population(s) dispersed over geographic areas of different sizes and dimensions. Each survey design has certain advantages and disadvantages. The final selection of the methods used depends upon such factors as the purpose of the study, 135
the geographic dimensions of the study area, the disease incidence or prevalence and the species to be studied. Of major importance is the consideration of how the data collected will be used. This report demonstrates how some techniques developed to study human diseases have been applied to the field study of animal diseases. A surveillance system developed to study swine congenital malformations illustrates the type of data one can collect in a field study of animal populations. The uses and limitations of these data are described. LITERATURE REVIEW
Most epidemiological studies are designed to evaluate or establish possible causal associations between a condition (i.e. disease) and various characteristics (e.g. etiological agent) (10). In addition to descriptive characteristics, the distribution of diseased individuals and the population-at-risk is of concern as well as variations in the distribution pattern. The spectrum of epidemiological study designs varies from the observational descriptive approach of "natural experiments" to the more complex experimental-controlled study (9-11). Each study design, (9, 10) or sampling method (7) has its own biases. The conceptual framework and genera] r:iethods for epidemiological field investigations of human populations have been published (4). A general discussion of the reliability of survey data and their limitaticns has been p)resented by a number of individuals (3, 6, 7, 16). Existing reporting systems on animal populations' morbidity and mortality (14) may be of limited value in epidemiological field studies because of variations in the quality of data, definition of terms and categorization of survey observations. Some investigators have been hindered by the lack of an accepted system of classifying diseases (16). Yet, a classification system devised for human populations (25) might be applicable to animal populations (19). Another problem in field studies has been the lack of methods to define animal populations at various times (16) or at variouLs localities. These problems have been minimized due to the rapid advances
136
in computer science. Such techniques as record linkage (1) and the ability to locate animal (s) geographically at different times (27) are techniques that can be applied to field studies of animal populations. Also, if there are problems in determining the population-at-risk, the relative risk technique developed by Mantel and Haenzel (11) is one possible solution that has been used (17). Designing a field survey of an animal population requires as much careful, concise planning, execution and evaluation of data as is required in experimental or laboratory studies. Existing data may be considered as a basis for designing a field study but before using them one must evaluate their completeness, accuracy and meaningfulness. Depending on the data to be collected appropriate survey instruments need to be designed and methods of analysis selected (7). In designing survey questionnaires one must write instructions for each question, pretest the qtuestionnaire and revise it before its extensive field use (2). As with laboratory studies, appropriate methods, equipment and trained personnel should be used in conducting the field survey. Data collected in a statewide study of congenital malformations in domestic swine will be presented to show how an epidemiological field study was designed to collect data from a fairly large study area where the population-at-risk and the prevalence of the disease of interest were unknown variables.
MATERIALS AND METHODS In the spring of 1966 a statewide multispecies study of birth defects was begun to evaluate the occurrence of defects in human beings and domestic swine. Two methods were established to determine the prevalence, distribution and type of malformations that occurred in domestic swine: a questionnaire survey and on site investigations. The questionnaire was mailed to each swine producer semiannually to collect data on the overall prevalence, distribution and general types of malformations observed. An evaluation of a pilot study that used this system (20) and an-
Can. J. comp. Med.
other study designed to evaluate the representativeness and accuracy of data collected have been reported (21). A computerized register of all known swine producers in Missouri was established and updated semiannually. University of Missouri Extension Specialists (county agents) with offices in each of Missouri's 114 counties served as the primary focus point for the mailed questionnaire study. Every six months (April and September) a questionnaire (12) and a cover letter written by the local Extension Agent were mailed to each producer. They were requested to report the number of pigs born and the number of birth defects by type for the previous six-month period. Questionnaires were returned to the Environmental Health Surveillance Center in Columbia via the local Extension office. At the Center each questionnaire was reviewed for completeness and questions in the comments section of the questionnaire were answered by telephone or letter. Jn addition, a 5%c' sample of producers reporting congenital defects in two or more pigs were interviewed at the farm by trained interviewers. Thus, the authenticity of the information on the original questionnaire was verified verbally. Because of limited time-cost factors it was decided to exclude herds where fewer than 50 pigs had been farrowed during the reporting period. Producers were asked to describe the defects observed, as well as to verify the total females bred and the total pigs born, stillborn and defective as reported on their mailed questionnaire. In addition, the breed of the dam, sire and malformed offspring was obtained. Statistical Programs for the Social Sciences (SPSS) (15) and Statistical Analysis System (SAS) (23), both com-
puterized systems of data analysis, were used in the data evaluation. In addition, the rates of congenitally malformed pigs were evaluated by the binomial test or analysis of variance (ANOV) (26). The Z value for each county rate was calculated.
Those counties with a rate that differed = 0.001 level) from the statewide mean rate were graphed. In instances where a nonparametric statistical test was considered more appropriate, e g. -2 test, it was applied to the data (24).
significantly (at the p
RESULTS Returns from the statewide mailed questionnaires survey in Missouri ranged from 34% to 40%7c for the six semiannual periods (October 1967-September 1970). Table I presents the total pigs born and the average litter size for the 13,036 producers reporting malformations and the 36,736 producers reporting no malformations. The number and rate per 1000 total births of stillborn and defective pigs is also presented. Using the two sample binomial test, the rate of stillbirths did not vary significantly (at the p = 0.05 level) between farms reporting malformations and farms reporting no malformations. The stillborn and malformed pig rates per 1000 pigs born were 59.2 and 6.7, respectively. Approximately one out of four farms reported that they had observed one or more defective pigs in their swine herds. Of the farms reporting one or more defective pigs 38.5% observed only one defective pig. The rate of births of congenitally malformed pigs per 1000 total
TABLE I. Total Pigs Born, Average Litter Size, Stillbirth Ratetand Malformation Rate for Missouri (Oct. 1967 - Sept. 1970)
Producers who Reported No Malformations Malformations Total
Farms ................................... 13,036 36,736 49,772 Total Pigs Bom . ... 2,843,030 3,794,346 . 6,637,374 Pigs Born Alive ........................... 2,664,659 3,579,532 6,244,191 Pigs Stillborn. 178,371 214,812 393,183 Stillbirth Rate Per 1000 Total Birthsa .' 62.7 56.6 59.2 Pigs with Malformations ................... 44,445 44,445 Malformation Rate Per 1000 Total Births .... 6.7 Average Litter Size ........................ 9.5 9.2 9.3 sNo significant difference between the rates for herds with and without malformations at the p = 0.05 level ...........................
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TABLE II. Number of Farms Reporting Birth of Malformed Pigs by Size of Herd Number of Malformed Pigs Observed
Size of Herd 1-99
100-199 200-499
500-998 456 130 133 96 70 89 67 53 47 23 54 118 23 9 3 4
999+ 48 15 17 12 8 13 9 7 11 5 11 46 16 3 0 4 225
Total Number of Farms
0.................... 23,921 36,736 8,240 4,071 1.................... 5,017 1,033 2,361 1,478 888 748 953 2.................... 2,739 431 525 559 3.................... 1,623 194 311 400 983 4.................... 123 188 278 691 5.................... 77 123 215 491 6.................... 33 54 97 244 7.................... 40 66 109 273 8.................... 25 29 49 131 9.................... 10 .................... 29 43 112 249 11- 20 .................... 35 186 71 456 21- 30 .................... 3 11 37 90 2 31- 40 .................... 2 11 27 41- 50 .................... 0 2 3 8 4 1 14 51-100 ...........1 Totals .................... 7,912 28,228 1,375 12,032 49,772 Rate of Defective 6.4 6.1 5.8 Pigs per 1000 Births' .. 7.3 8.9 6.3 'Calculated from grouped data for defective pigs presented in this table. This is the reason the overall rate of 6.3 is less than the rate of 6.7 in Table I
TABLE III. Comparison of the Number and Rate of Swine Congenital Malformations per 1000 Total Births by Body System and Type of Defect for Fall-Wintera Season versus Spring-Summerb Season Farrowings for Missouri (Oct. 1967 - Sept. 1970)
Body System
Fall-Winter Number Rate
Spring-Summer Number Rate
All Four Seasons Number Rate
Central Nervous 0.57 0.36 1,176 1,882 3,058 System 0.31 841 Head 0.26 1,019 863 0.26 335 0.10 Spine Special Sense 0.44 0.32 1,044 1,451 2,495 Organs 550 0.17 465 0.14 Eye 901 0.27 579 0.18 Ears Alimentary & 1.12 0.85 3,722 2,821 6,543 Respiratory... 685 0.21 513 0.16 Mouth 0.87 0.64 Rectum 2,903 2,124 134 0.04 184 0.05 Open Belly... 3.05 2.49 Rupture (hernia) 10,174 8,246 18,240 Genito-urinary 0.44 0.39 1,285 1,484 2,769 (sex organs).... 1.24 1.07 Bones and Joints 3,546 4,128 7,674 0.70 0.64 2,343 2,114 Legs 0.54 0.43 Tail 1,785 1,432 0.72 0.63 2,075 Other. 2,409 4,484 ... 0.36 985 0.30 Hair 1,201 722 0.22 498 0.15 Skin 468 0.14 592 0.18 Monster, etc. Total Malformations Re6.11 7.58 45,443 20,193 25,250 ported...... 3,306,428 Total Pigs Born. 6,637,374 3,330,946 October 1 through March 31 *Fall-Winter includes bSpring-Summer includes April 1 through September 30 Using total numbers by body system; x2 = 76.77 with 6 d.f. p < 0.001
0.46
........
1,860 1,198 0.38
........
........
........
........
........
138
0.28 0.18
1,015 1,480
1,198 5,027
0.16 0.22
0.99
318
4,457 3,217 2,186
2.77 0.42 1.16
0.68
1.220 1,078
0.18 0.76 0.05
0.67 0.49 0.33 0.18 0.16
6.85
Can. J. comp. Med.
births on a county basis was then compared. The county rates were then subdivided into extremes. Those counties with rates that differed significantly (at the p = 0.001 level) above or below the average statewide rate of 6.7 total defects were shaded and are presented in Fig. 1. Table II presents the number of farms reporting the birth of malformed pigs by herd size and the number of malformed pigs observed. Using this grouped data, the average herd size was estimated for herds where zero, one and two or more defective pigs were observed. The estimated number of pigs farrowed was 116.1, 124.3 and 254.7, respectively. The rate of births of malformed pigs per 1000 total births was also calculated for each category of herd size. This rate did not vary significantly as the herd size increased. Next, the reports received from the producers were grouped according to season. The defects reported from October through
March were labeled fall-winter and those reported from April through September were labeled spring-summer. There were no major differences between the seasons for the total number of pigs born. The number of congenital malformations reported by body system and season for Missouri swine for the three year period, October 1967 through September 1970, is presented in Table III. The rate of the various types of malformations was greater in the fall-winter season than in the spring-summer season. These differences might, in part, have been due to a seasonal effect or to closer observation by producers in the fall-winter. For all seasons the most common defect observed was ruptures (hernias), with a rate of 2.77 per 1000 total births. The next most common types of defects reported were rectal defects and leg defects, with a rate of 0.76 and 0.67 respectively. Two hundred seventy-eight direct inter-
g Mofformation Rate Significantly greater (p 0.001) than statewide overage
( 27counties)
0 Maiformation Rote
Significantly less (p 0.001) than stotewide overage ( 2 2 counties)
MISSOURI COUNTIES Malformation Rote oal def ects a 6.7 per J1OO births Fig. 1. Counties with a swine malformation rate significantly higher or lower (at the p statewide mean rate of 6.7 per 1000 total births.
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=
0.001 level) than the
139
views were used to validate the results of the mailed questionnaire. In these herds 9,065 female pigs farrowed 89,200 pigs. The estimated stillbirth and malformation rate per 1000 total births was 77.0 and 24.9 respectively. The breed distribution of the malformed offspring and their dam and sire was compared. Twenty-seven percent of the offspring, 38% of their dams and 96% of their sires were purebred. The most common breeds encountered were Hampshire, Yorkshire and Duroc, in descending order. There was no significant difference in the incidence of stillbirth or congenital defects in offspring of purebred or crossbred sires or dams. The number and percentage of sows versus gilts was then compared. The percentage of defects was similar for pigs from both sows and gilts except that more rectal defects were observed in the offspring of gilts and more ruptures (hernias) were observed in the offspring of sows. A comparison of the defects by type reported on the mailed questionnaire with defects described by direct interview are presented in Table IV. Approximately 86% of the defects were verbally confirmed. The discrepancy between the number of defects reported on the mailed questionnaire and by direct interview was due to the erroneous classification of mummified pigs as congenitally malformed pigs.
DISCUSSION Observations of "natural experiments" usually are not under the epidemiologist's control and are frequently only descriptive studies without extensive statistical analysis. The view that these limitations preclude the experiments' usefulness is not entirely true. For example, in the initial phase of the swine study our methods included mailed questionnaires, i.e. observational studies, but later, the field investigations progressed, and we used more sophisticated study methods (21, 22). Because congenital malformations are "rare events" it is doubtful that a prospective studv would have been practical, especially in a study area the size of Missouri. The ultimate aim of most epidemiological studies is to define the etiology of the dis140
TABLE IV. Comparison of Reported versus Confirmed Defects by Type for the 278 Direct Interview Farms
Type Defect Head Spine Eyes Ears
Mouth Rectum ... Belly Hem ia Sex organs. Legs Tail Hair Skin Others Total
%
Reported Confirmed Confirmed 142 241 58.9 88 73 82.9 82 103 79.6 73 92.4 79 92 103 112.0 375 111.7 419 39 35 89.7 409 439 107.3 188 67.0 126 406 71.2 570 250 254 101.6 34 52 65.4 69 37 53.6 69 31 44.9 85.9 2,624 2,254
ease but this objective does not have to be included in the first step of the study design. It has been suggested that an animal population study be conducted in three steps (18). First, identify the problems, second, quantify the disease problem and third, evaluate and determine possible causal associations between etiological factors and the disease. In such a stepwise study design one may progress from development of a simple surveillance system, e.g. a short mailed questionnaire (12, 13) or preliminary direct interview of animal producers (21) to the more detailed collection of specimens for further laboratory studies (19, 21). Thus, in the statewide study of defects in swine, etiological studies were not initially the primary focal point. A more important problem was to define the population-at-risk and its population dynamics, e.g. density in a study area the size of Missouri, as well as to validate the data collected (21). Before the statewide study the lack of names and locations for swine producers precluded the use of sampling techniques. Also, the incidence and distribution of malformations by type were unknown. Swine malformations had been reported so infrequently that it was impossible to evaluate their impact on swine populations (8). Therefore, we decided to use the mailed questionnaire as a basis for the statewide study. Comparison of the total pigs born as reported on the mailed questionnaire with estimates obtained by another Missouri investigator (5) showed that our study population represented approximately 48% of
Can. J. comp. Med.
Missouri's swine population. Further, comparative evaluations of the total pigs born by year for each of the 114 counties were calculated by Kendall's tau (23). This latter evaluation, in essence a method for validating the mailed questionnaire data, indicated that the annual swine populations in our study were representative of the statewide swine populations. In subsequent field investigations we were able to demonstrate that the defects reported by the mailed questionnaire gave us a fairly accurate picture of the numbers, general types and locations of defective pigs. Further data obtained during these studies suggested a refusal rate of only 5% (20, 21). The mailed questionnaire data on swine malformations are being used to pinpoint areas for more detailed studies such as specific comparative studies with select environmental factors. To find those counties with malformation rates significantly different (at the p = 0.001 level) from the state mean rate individual county rates were compared against the state mean rate. Forty-nine county rates were significantly different (at the p = 0.001 level). By chance alone we would have expected less than one county rate to be significant. The data obtained in the three year statewide study of swine congenital malformations employing techniques that have been used in epidemiological studies of human populations are being used as baseline material for further studies. These include comparative studies with human malformations and selected environmental factors.
ACKNOWLEDGMENTS Appreciation is expressed to the Area Extension Directors and their staffs, especially the Extension Specialist (agents) responsible for the swine birth defect study in each county, for their interest, cooperation and efforts related to the study.
REFERENCES
1. ACHESON, E. D. Medical Record Linkage. Oxford University Press. 1967.
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2. BACKSTROM, C. H. and G. D. HURSH. Survey Research. Noirthwester-n Univelrsity Press. 1963. 3. CANNELL, C. F. The reliability of survey data. In Genetics and the Epidemiology of Chronic Diseases. J. V. Neel, M. W. Show and W. J. Shell, Eds. Public Health Service Pub. So. 1163. U.S. Government Printing Office, Washington. 1963. 4. GORDON, J. E. Field epidemiology. Am. J. med. Sci. 246: 354-376. 1963. 5. GRIMES, G. A. Hog inventories 1968-1971 data for Missouri counties. Mlissouri Extension Division. University of Missou±i. Columbia. 6. HUGH-JONES, M. E. The uses and limitations of animal disease surveillance. Vet. Rec. 92: 11-15. 1973. 7. KISH, L. Survey Sampling. New York: John Wiley and Son. 1965. 8. KITCHELL, R. L., C. R. STEVENS and C. C. TURVES. Cardiac and aortic arch anomalies, hydrocephalus and other abnor malities in newborn pigs. J. Am. vet. med. Ass. 130: 453-457. 1957. 9. LILIENFIELD, A. M. Epidemiology of infections and non-infectious disease: some considerations. Am. J. Epidem. 97: 135-147. 1973. 10. MacMAHON, B. and T. F. PUGH. Epidemiology Principles and Methods. Boston: Little, Brown and Company. 1973. 11. MANTEL, N. and W. HAENSZEL. Statistical aspects of the analysis of data from retrospective study of disease. J. natn. Cancer Inst. 22: 719-748. 1959. 12. MARIENFELD, C. J. A field study relating geochemical environment to health and disease. Ann. N.Y. Acad. Sci. 199: 335-347. 1972. 13. MARIENFELD, C. J.. S. L. SILBERG, R. W. MENGES, W. T. CRAWFORD and H. T. WRIGHT. Multi-species study of congenital malformations in Missouri. Missouri Med. 64: 230-233. 1967. 14. NATIONAL RESEARCH COUNCIL. Committee on Animal Health. A historical survey of animal-disease morbidity and mortality reporting. Pub. 1346. U.S. National Academy of Science. 1966. 15. NIE, N., D. H. BENT and C. H. HULL. SPSSStatistical Package for the Social Sciences. New York: McGraw-Hill Book Co. 1970. 16. PARRISH, H. M., D. C. BLENDEN and T. P. WEIL. Some missing measurements in comparative medicine. Can. J. comp. Med. 32: 483-485. 1968. 17. PRIESTER, W. A., A. G. GLASS and N. S. WAGGONER. Congenital defects in domesticated animals: general considerations. Am. J. vet. Res. 31: 18711879. 1970. 18. RUPPANNER, R. Measurement of disease in animal populations based on interviews. J. Am. med. Ass. 161: 1033-1038. 1972. 19. SELBY, L. A., H. C. HOPPS and L. D. EDMONDS. Comparative aspects of congenital malformations in man and swine. J. Am. vet med. Ass. 159: 14851490. 1971. 20. SELBY, L. A., C. J. MARIENFELD, W. T. HE1DLAGE, H. T. WRIGHT and V. E. YOUNG. Evaluation of a method to estimate the prevalence of congenital malformations in swine using a mailed questionnaire. Cornell Vet. 61: 203-213. 1971. 21. SELBY, L. A., L. D. EDMONDS, D. W. PARKE, R. W. STEWART, C. J. MARIENFELD and W. F. HEIDLAGE. Use of mailed questionnaire data in a study of swine congenital malformations. Can. J. comp. Med. 37: 413-417. 1973. 22. SELBY, L. A., L. D. EDMONDS, R. W. STEWART, W. R. LOWER and D. W. PARKE. Effects of swine husbandry on the incidence of congenital malformations: a matched-paired study. Envir. Res. 6: 77-83. 1973. 23. SERVICE, J. A User's Guide to the Statistical Analysis System. Raleigh, North Carolina: North Carolina State University. 1972. 24. SIEGEL, S. Nonparametric Statistics of the Behavioral Sciences. New York: McGraw-Hill. 1956. 25. SILBERG, S. L., C. J. MARIENFELD, H. T. WRIGHT aad R. C. ARNOLID. Surveillance ')f congenital malformations in Missouri. 1953-1964: A preliminary report. Archs envir. Hlth 13: 641644. 1968. 26. SNEDECOR, G. W. Statistical Methods. Ames: Iowa State University Press. 1956. 27. WRIGHT, H. T., C. J. MARIENFELD and S. L. SILBERG. The definition of "place" in environmental epidemiology: A rectangular -coordinate method. Pub]. Hlth Rep., Wash. 83: 427-433. 1968.
141
Numerous survey designs have been developed for epidemiological field studies of human populations, most of which are also applicable to field studies of animal poulations. Each design has its own advantages and disadvantages. The final design selected for a particular study depends upon such factors as the overall purpose of the study, the geographic dimensions of the study area, the disease incidence or prevalence and species to be studied as well as the planned use for the data. Population dynamics including the distribution and density of the species to be studied are factors that should also be considered in the initial design of a study. A surveillance system, using mailed questionnaire data and a subsequent survey using direct interviews to validate the data in a statewide study of swine birth defects are used to illustrate some of the techniques that can be applied to domestic animal populations in a fairly large geographic area. The type of data collected, its use and its limitations are also considered.
maines. La plupart de ces plans peuvent aussi servir a des etudes similaires chez des populations animales. Chacun d'entre eux comporte cependant ses propres avantages et inconvenients. Le choix final d'un plan en vue d'une etude particuliere depend de facteurs tels que l'objectif global de l'etude, 1'etendue geographique impliquee, l'incidence ou la predominance des maladies, 1'espece qu'on desire etudier et l'usage auquel on destine les renseignements recueillis. La dynamique d'une population, y compris la distribution et la densit- de l'espce sur laquelle portera l'etude, constituent des facteurs dont il faut tenir compte lors de l'elaboration d'un plan d'6tude. Pour illustrer certaines des techniques qu'on peut appliquer 'a des populations d'animaux domestiques dans une region geographique relativement etendue, les auteurs utilisent l'exemple d'un programme de surveillance utilisant les donnees obtenues 'a l'aide d'un questionnaire postal et d'une enquete ulterieure, basee sur des entrevues destinees a verifier l'exactitude de ces donnes, lors d'une etude portant sur les anomalies congenitales observees chez les porcelets naissants sur tout le territoire de l'letat du Missouri. Ils commentent aussi le genre de donn4es recueillies, ainsi que leur utilisation et leurs limitations.
RASUME Des chercheurs ont deja developpe plusieurs plans d'6tude applicables 'a des enquetes epidemiologiques portant sur des populations hu-
*Department of Veterinary Microbiology, College of Veterinary Medicine, Department of Community Health and Medical Practice, School of Medicine (Selby), 31nvironmental Health Surveillance Center (Edmonds) and Environmental Trace Substances Center (Hyde), University of Missouri, Columbia, Missouri 65201. Dr. L. A. Selby is a former E.I.S. officer and L D. Edmonds is presently an E.I.S. officer with the Center for Disease Control, Atlanta, Georgia 30333. L D. Hyde's present address is American Industrial Hygiene Association, Akron, Ohio 44313. Submitted September 10, 1975.
Volume 40
-
April, 1976
INTRODUCTION
An epidemiological study by definition is a population study. Because it is a population study rather than a study of an individual, methods have been developed that apply to population(s) dispersed over geographic areas of different sizes and dimensions. Each survey design has certain advantages and disadvantages. The final selection of the methods used depends upon such factors as the purpose of the study, 135
the geographic dimensions of the study area, the disease incidence or prevalence and the species to be studied. Of major importance is the consideration of how the data collected will be used. This report demonstrates how some techniques developed to study human diseases have been applied to the field study of animal diseases. A surveillance system developed to study swine congenital malformations illustrates the type of data one can collect in a field study of animal populations. The uses and limitations of these data are described. LITERATURE REVIEW
Most epidemiological studies are designed to evaluate or establish possible causal associations between a condition (i.e. disease) and various characteristics (e.g. etiological agent) (10). In addition to descriptive characteristics, the distribution of diseased individuals and the population-at-risk is of concern as well as variations in the distribution pattern. The spectrum of epidemiological study designs varies from the observational descriptive approach of "natural experiments" to the more complex experimental-controlled study (9-11). Each study design, (9, 10) or sampling method (7) has its own biases. The conceptual framework and genera] r:iethods for epidemiological field investigations of human populations have been published (4). A general discussion of the reliability of survey data and their limitaticns has been p)resented by a number of individuals (3, 6, 7, 16). Existing reporting systems on animal populations' morbidity and mortality (14) may be of limited value in epidemiological field studies because of variations in the quality of data, definition of terms and categorization of survey observations. Some investigators have been hindered by the lack of an accepted system of classifying diseases (16). Yet, a classification system devised for human populations (25) might be applicable to animal populations (19). Another problem in field studies has been the lack of methods to define animal populations at various times (16) or at variouLs localities. These problems have been minimized due to the rapid advances
136
in computer science. Such techniques as record linkage (1) and the ability to locate animal (s) geographically at different times (27) are techniques that can be applied to field studies of animal populations. Also, if there are problems in determining the population-at-risk, the relative risk technique developed by Mantel and Haenzel (11) is one possible solution that has been used (17). Designing a field survey of an animal population requires as much careful, concise planning, execution and evaluation of data as is required in experimental or laboratory studies. Existing data may be considered as a basis for designing a field study but before using them one must evaluate their completeness, accuracy and meaningfulness. Depending on the data to be collected appropriate survey instruments need to be designed and methods of analysis selected (7). In designing survey questionnaires one must write instructions for each question, pretest the qtuestionnaire and revise it before its extensive field use (2). As with laboratory studies, appropriate methods, equipment and trained personnel should be used in conducting the field survey. Data collected in a statewide study of congenital malformations in domestic swine will be presented to show how an epidemiological field study was designed to collect data from a fairly large study area where the population-at-risk and the prevalence of the disease of interest were unknown variables.
MATERIALS AND METHODS In the spring of 1966 a statewide multispecies study of birth defects was begun to evaluate the occurrence of defects in human beings and domestic swine. Two methods were established to determine the prevalence, distribution and type of malformations that occurred in domestic swine: a questionnaire survey and on site investigations. The questionnaire was mailed to each swine producer semiannually to collect data on the overall prevalence, distribution and general types of malformations observed. An evaluation of a pilot study that used this system (20) and an-
Can. J. comp. Med.
other study designed to evaluate the representativeness and accuracy of data collected have been reported (21). A computerized register of all known swine producers in Missouri was established and updated semiannually. University of Missouri Extension Specialists (county agents) with offices in each of Missouri's 114 counties served as the primary focus point for the mailed questionnaire study. Every six months (April and September) a questionnaire (12) and a cover letter written by the local Extension Agent were mailed to each producer. They were requested to report the number of pigs born and the number of birth defects by type for the previous six-month period. Questionnaires were returned to the Environmental Health Surveillance Center in Columbia via the local Extension office. At the Center each questionnaire was reviewed for completeness and questions in the comments section of the questionnaire were answered by telephone or letter. Jn addition, a 5%c' sample of producers reporting congenital defects in two or more pigs were interviewed at the farm by trained interviewers. Thus, the authenticity of the information on the original questionnaire was verified verbally. Because of limited time-cost factors it was decided to exclude herds where fewer than 50 pigs had been farrowed during the reporting period. Producers were asked to describe the defects observed, as well as to verify the total females bred and the total pigs born, stillborn and defective as reported on their mailed questionnaire. In addition, the breed of the dam, sire and malformed offspring was obtained. Statistical Programs for the Social Sciences (SPSS) (15) and Statistical Analysis System (SAS) (23), both com-
puterized systems of data analysis, were used in the data evaluation. In addition, the rates of congenitally malformed pigs were evaluated by the binomial test or analysis of variance (ANOV) (26). The Z value for each county rate was calculated.
Those counties with a rate that differed = 0.001 level) from the statewide mean rate were graphed. In instances where a nonparametric statistical test was considered more appropriate, e g. -2 test, it was applied to the data (24).
significantly (at the p
RESULTS Returns from the statewide mailed questionnaires survey in Missouri ranged from 34% to 40%7c for the six semiannual periods (October 1967-September 1970). Table I presents the total pigs born and the average litter size for the 13,036 producers reporting malformations and the 36,736 producers reporting no malformations. The number and rate per 1000 total births of stillborn and defective pigs is also presented. Using the two sample binomial test, the rate of stillbirths did not vary significantly (at the p = 0.05 level) between farms reporting malformations and farms reporting no malformations. The stillborn and malformed pig rates per 1000 pigs born were 59.2 and 6.7, respectively. Approximately one out of four farms reported that they had observed one or more defective pigs in their swine herds. Of the farms reporting one or more defective pigs 38.5% observed only one defective pig. The rate of births of congenitally malformed pigs per 1000 total
TABLE I. Total Pigs Born, Average Litter Size, Stillbirth Ratetand Malformation Rate for Missouri (Oct. 1967 - Sept. 1970)
Producers who Reported No Malformations Malformations Total
Farms ................................... 13,036 36,736 49,772 Total Pigs Bom . ... 2,843,030 3,794,346 . 6,637,374 Pigs Born Alive ........................... 2,664,659 3,579,532 6,244,191 Pigs Stillborn. 178,371 214,812 393,183 Stillbirth Rate Per 1000 Total Birthsa .' 62.7 56.6 59.2 Pigs with Malformations ................... 44,445 44,445 Malformation Rate Per 1000 Total Births .... 6.7 Average Litter Size ........................ 9.5 9.2 9.3 sNo significant difference between the rates for herds with and without malformations at the p = 0.05 level ...........................
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TABLE II. Number of Farms Reporting Birth of Malformed Pigs by Size of Herd Number of Malformed Pigs Observed
Size of Herd 1-99
100-199 200-499
500-998 456 130 133 96 70 89 67 53 47 23 54 118 23 9 3 4
999+ 48 15 17 12 8 13 9 7 11 5 11 46 16 3 0 4 225
Total Number of Farms
0.................... 23,921 36,736 8,240 4,071 1.................... 5,017 1,033 2,361 1,478 888 748 953 2.................... 2,739 431 525 559 3.................... 1,623 194 311 400 983 4.................... 123 188 278 691 5.................... 77 123 215 491 6.................... 33 54 97 244 7.................... 40 66 109 273 8.................... 25 29 49 131 9.................... 10 .................... 29 43 112 249 11- 20 .................... 35 186 71 456 21- 30 .................... 3 11 37 90 2 31- 40 .................... 2 11 27 41- 50 .................... 0 2 3 8 4 1 14 51-100 ...........1 Totals .................... 7,912 28,228 1,375 12,032 49,772 Rate of Defective 6.4 6.1 5.8 Pigs per 1000 Births' .. 7.3 8.9 6.3 'Calculated from grouped data for defective pigs presented in this table. This is the reason the overall rate of 6.3 is less than the rate of 6.7 in Table I
TABLE III. Comparison of the Number and Rate of Swine Congenital Malformations per 1000 Total Births by Body System and Type of Defect for Fall-Wintera Season versus Spring-Summerb Season Farrowings for Missouri (Oct. 1967 - Sept. 1970)
Body System
Fall-Winter Number Rate
Spring-Summer Number Rate
All Four Seasons Number Rate
Central Nervous 0.57 0.36 1,176 1,882 3,058 System 0.31 841 Head 0.26 1,019 863 0.26 335 0.10 Spine Special Sense 0.44 0.32 1,044 1,451 2,495 Organs 550 0.17 465 0.14 Eye 901 0.27 579 0.18 Ears Alimentary & 1.12 0.85 3,722 2,821 6,543 Respiratory... 685 0.21 513 0.16 Mouth 0.87 0.64 Rectum 2,903 2,124 134 0.04 184 0.05 Open Belly... 3.05 2.49 Rupture (hernia) 10,174 8,246 18,240 Genito-urinary 0.44 0.39 1,285 1,484 2,769 (sex organs).... 1.24 1.07 Bones and Joints 3,546 4,128 7,674 0.70 0.64 2,343 2,114 Legs 0.54 0.43 Tail 1,785 1,432 0.72 0.63 2,075 Other. 2,409 4,484 ... 0.36 985 0.30 Hair 1,201 722 0.22 498 0.15 Skin 468 0.14 592 0.18 Monster, etc. Total Malformations Re6.11 7.58 45,443 20,193 25,250 ported...... 3,306,428 Total Pigs Born. 6,637,374 3,330,946 October 1 through March 31 *Fall-Winter includes bSpring-Summer includes April 1 through September 30 Using total numbers by body system; x2 = 76.77 with 6 d.f. p < 0.001
0.46
........
1,860 1,198 0.38
........
........
........
........
........
138
0.28 0.18
1,015 1,480
1,198 5,027
0.16 0.22
0.99
318
4,457 3,217 2,186
2.77 0.42 1.16
0.68
1.220 1,078
0.18 0.76 0.05
0.67 0.49 0.33 0.18 0.16
6.85
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births on a county basis was then compared. The county rates were then subdivided into extremes. Those counties with rates that differed significantly (at the p = 0.001 level) above or below the average statewide rate of 6.7 total defects were shaded and are presented in Fig. 1. Table II presents the number of farms reporting the birth of malformed pigs by herd size and the number of malformed pigs observed. Using this grouped data, the average herd size was estimated for herds where zero, one and two or more defective pigs were observed. The estimated number of pigs farrowed was 116.1, 124.3 and 254.7, respectively. The rate of births of malformed pigs per 1000 total births was also calculated for each category of herd size. This rate did not vary significantly as the herd size increased. Next, the reports received from the producers were grouped according to season. The defects reported from October through
March were labeled fall-winter and those reported from April through September were labeled spring-summer. There were no major differences between the seasons for the total number of pigs born. The number of congenital malformations reported by body system and season for Missouri swine for the three year period, October 1967 through September 1970, is presented in Table III. The rate of the various types of malformations was greater in the fall-winter season than in the spring-summer season. These differences might, in part, have been due to a seasonal effect or to closer observation by producers in the fall-winter. For all seasons the most common defect observed was ruptures (hernias), with a rate of 2.77 per 1000 total births. The next most common types of defects reported were rectal defects and leg defects, with a rate of 0.76 and 0.67 respectively. Two hundred seventy-eight direct inter-
g Mofformation Rate Significantly greater (p 0.001) than statewide overage
( 27counties)
0 Maiformation Rote
Significantly less (p 0.001) than stotewide overage ( 2 2 counties)
MISSOURI COUNTIES Malformation Rote oal def ects a 6.7 per J1OO births Fig. 1. Counties with a swine malformation rate significantly higher or lower (at the p statewide mean rate of 6.7 per 1000 total births.
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=
0.001 level) than the
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views were used to validate the results of the mailed questionnaire. In these herds 9,065 female pigs farrowed 89,200 pigs. The estimated stillbirth and malformation rate per 1000 total births was 77.0 and 24.9 respectively. The breed distribution of the malformed offspring and their dam and sire was compared. Twenty-seven percent of the offspring, 38% of their dams and 96% of their sires were purebred. The most common breeds encountered were Hampshire, Yorkshire and Duroc, in descending order. There was no significant difference in the incidence of stillbirth or congenital defects in offspring of purebred or crossbred sires or dams. The number and percentage of sows versus gilts was then compared. The percentage of defects was similar for pigs from both sows and gilts except that more rectal defects were observed in the offspring of gilts and more ruptures (hernias) were observed in the offspring of sows. A comparison of the defects by type reported on the mailed questionnaire with defects described by direct interview are presented in Table IV. Approximately 86% of the defects were verbally confirmed. The discrepancy between the number of defects reported on the mailed questionnaire and by direct interview was due to the erroneous classification of mummified pigs as congenitally malformed pigs.
DISCUSSION Observations of "natural experiments" usually are not under the epidemiologist's control and are frequently only descriptive studies without extensive statistical analysis. The view that these limitations preclude the experiments' usefulness is not entirely true. For example, in the initial phase of the swine study our methods included mailed questionnaires, i.e. observational studies, but later, the field investigations progressed, and we used more sophisticated study methods (21, 22). Because congenital malformations are "rare events" it is doubtful that a prospective studv would have been practical, especially in a study area the size of Missouri. The ultimate aim of most epidemiological studies is to define the etiology of the dis140
TABLE IV. Comparison of Reported versus Confirmed Defects by Type for the 278 Direct Interview Farms
Type Defect Head Spine Eyes Ears
Mouth Rectum ... Belly Hem ia Sex organs. Legs Tail Hair Skin Others Total
%
Reported Confirmed Confirmed 142 241 58.9 88 73 82.9 82 103 79.6 73 92.4 79 92 103 112.0 375 111.7 419 39 35 89.7 409 439 107.3 188 67.0 126 406 71.2 570 250 254 101.6 34 52 65.4 69 37 53.6 69 31 44.9 85.9 2,624 2,254
ease but this objective does not have to be included in the first step of the study design. It has been suggested that an animal population study be conducted in three steps (18). First, identify the problems, second, quantify the disease problem and third, evaluate and determine possible causal associations between etiological factors and the disease. In such a stepwise study design one may progress from development of a simple surveillance system, e.g. a short mailed questionnaire (12, 13) or preliminary direct interview of animal producers (21) to the more detailed collection of specimens for further laboratory studies (19, 21). Thus, in the statewide study of defects in swine, etiological studies were not initially the primary focal point. A more important problem was to define the population-at-risk and its population dynamics, e.g. density in a study area the size of Missouri, as well as to validate the data collected (21). Before the statewide study the lack of names and locations for swine producers precluded the use of sampling techniques. Also, the incidence and distribution of malformations by type were unknown. Swine malformations had been reported so infrequently that it was impossible to evaluate their impact on swine populations (8). Therefore, we decided to use the mailed questionnaire as a basis for the statewide study. Comparison of the total pigs born as reported on the mailed questionnaire with estimates obtained by another Missouri investigator (5) showed that our study population represented approximately 48% of
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Missouri's swine population. Further, comparative evaluations of the total pigs born by year for each of the 114 counties were calculated by Kendall's tau (23). This latter evaluation, in essence a method for validating the mailed questionnaire data, indicated that the annual swine populations in our study were representative of the statewide swine populations. In subsequent field investigations we were able to demonstrate that the defects reported by the mailed questionnaire gave us a fairly accurate picture of the numbers, general types and locations of defective pigs. Further data obtained during these studies suggested a refusal rate of only 5% (20, 21). The mailed questionnaire data on swine malformations are being used to pinpoint areas for more detailed studies such as specific comparative studies with select environmental factors. To find those counties with malformation rates significantly different (at the p = 0.001 level) from the state mean rate individual county rates were compared against the state mean rate. Forty-nine county rates were significantly different (at the p = 0.001 level). By chance alone we would have expected less than one county rate to be significant. The data obtained in the three year statewide study of swine congenital malformations employing techniques that have been used in epidemiological studies of human populations are being used as baseline material for further studies. These include comparative studies with human malformations and selected environmental factors.
ACKNOWLEDGMENTS Appreciation is expressed to the Area Extension Directors and their staffs, especially the Extension Specialist (agents) responsible for the swine birth defect study in each county, for their interest, cooperation and efforts related to the study.
REFERENCES
1. ACHESON, E. D. Medical Record Linkage. Oxford University Press. 1967.
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2. BACKSTROM, C. H. and G. D. HURSH. Survey Research. Noirthwester-n Univelrsity Press. 1963. 3. CANNELL, C. F. The reliability of survey data. In Genetics and the Epidemiology of Chronic Diseases. J. V. Neel, M. W. Show and W. J. Shell, Eds. Public Health Service Pub. So. 1163. U.S. Government Printing Office, Washington. 1963. 4. GORDON, J. E. Field epidemiology. Am. J. med. Sci. 246: 354-376. 1963. 5. GRIMES, G. A. Hog inventories 1968-1971 data for Missouri counties. Mlissouri Extension Division. University of Missou±i. Columbia. 6. HUGH-JONES, M. E. The uses and limitations of animal disease surveillance. Vet. Rec. 92: 11-15. 1973. 7. KISH, L. Survey Sampling. New York: John Wiley and Son. 1965. 8. KITCHELL, R. L., C. R. STEVENS and C. C. TURVES. Cardiac and aortic arch anomalies, hydrocephalus and other abnor malities in newborn pigs. J. Am. vet. med. Ass. 130: 453-457. 1957. 9. LILIENFIELD, A. M. Epidemiology of infections and non-infectious disease: some considerations. Am. J. Epidem. 97: 135-147. 1973. 10. MacMAHON, B. and T. F. PUGH. Epidemiology Principles and Methods. Boston: Little, Brown and Company. 1973. 11. MANTEL, N. and W. HAENSZEL. Statistical aspects of the analysis of data from retrospective study of disease. J. natn. Cancer Inst. 22: 719-748. 1959. 12. MARIENFELD, C. J. A field study relating geochemical environment to health and disease. Ann. N.Y. Acad. Sci. 199: 335-347. 1972. 13. MARIENFELD, C. J.. S. L. SILBERG, R. W. MENGES, W. T. CRAWFORD and H. T. WRIGHT. Multi-species study of congenital malformations in Missouri. Missouri Med. 64: 230-233. 1967. 14. NATIONAL RESEARCH COUNCIL. Committee on Animal Health. A historical survey of animal-disease morbidity and mortality reporting. Pub. 1346. U.S. National Academy of Science. 1966. 15. NIE, N., D. H. BENT and C. H. HULL. SPSSStatistical Package for the Social Sciences. New York: McGraw-Hill Book Co. 1970. 16. PARRISH, H. M., D. C. BLENDEN and T. P. WEIL. Some missing measurements in comparative medicine. Can. J. comp. Med. 32: 483-485. 1968. 17. PRIESTER, W. A., A. G. GLASS and N. S. WAGGONER. Congenital defects in domesticated animals: general considerations. Am. J. vet. Res. 31: 18711879. 1970. 18. RUPPANNER, R. Measurement of disease in animal populations based on interviews. J. Am. med. Ass. 161: 1033-1038. 1972. 19. SELBY, L. A., H. C. HOPPS and L. D. EDMONDS. Comparative aspects of congenital malformations in man and swine. J. Am. vet med. Ass. 159: 14851490. 1971. 20. SELBY, L. A., C. J. MARIENFELD, W. T. HE1DLAGE, H. T. WRIGHT and V. E. YOUNG. Evaluation of a method to estimate the prevalence of congenital malformations in swine using a mailed questionnaire. Cornell Vet. 61: 203-213. 1971. 21. SELBY, L. A., L. D. EDMONDS, D. W. PARKE, R. W. STEWART, C. J. MARIENFELD and W. F. HEIDLAGE. Use of mailed questionnaire data in a study of swine congenital malformations. Can. J. comp. Med. 37: 413-417. 1973. 22. SELBY, L. A., L. D. EDMONDS, R. W. STEWART, W. R. LOWER and D. W. PARKE. Effects of swine husbandry on the incidence of congenital malformations: a matched-paired study. Envir. Res. 6: 77-83. 1973. 23. SERVICE, J. A User's Guide to the Statistical Analysis System. Raleigh, North Carolina: North Carolina State University. 1972. 24. SIEGEL, S. Nonparametric Statistics of the Behavioral Sciences. New York: McGraw-Hill. 1956. 25. SILBERG, S. L., C. J. MARIENFELD, H. T. WRIGHT aad R. C. ARNOLID. Surveillance ')f congenital malformations in Missouri. 1953-1964: A preliminary report. Archs envir. Hlth 13: 641644. 1968. 26. SNEDECOR, G. W. Statistical Methods. Ames: Iowa State University Press. 1956. 27. WRIGHT, H. T., C. J. MARIENFELD and S. L. SILBERG. The definition of "place" in environmental epidemiology: A rectangular -coordinate method. Pub]. Hlth Rep., Wash. 83: 427-433. 1968.
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