Factors associated with the seroprevalence of pseudorabies virus in breeding swine from quarantined herds

Robert B. Morrison From the Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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 DVM, PhD
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William E. Marsh From the Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Paul L. Anderson From the Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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David G. Thawley From the Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108.

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Summary

Strategies for the elimination of pseudorabies virus (prv) from swine herds include test and removal, offspring segregation, and depopulation/repopulation. The prevalence of prv in a herd is a major factor in selection of the most appropriate strategy. The purpose of the study reported here was to describe the prevalence of prv in adult swine in prv quarantined herds in Minnesota, and to determine herd factors associated with the seroprevalence. Questionnaires describing the health history of the herd, management practices, and design of the swine facilities were obtained from the owners of 142 quarantined herds. Blood was collected from 29 finishing pigs over the age of 4 months, up to 29 adult females, and all herd boars. Factors considered to be significant in a bivariate analysis were combined in a stepwise multiple logistic regression analysis. The prevalence of prv-seropositive adults in each herd was bimodally distributed among the 142 herds. In 42 (30%) of the herds, none of the females tested was seropositive, which represented the lower mode. At least 90% of the adults tested were seropositive in 30 (21%) of the herds and represented the higher mode. The odds of the breeding swine of a given herd having a prv seroprevalence of ≥ 20% as compared with having a seroprevalence of < 20% was 1.654 times higher per 50 adults in the herd, 13.550 times higher if the finishing pigs were seropositive, 2.378 times higher if sows were housed inside during gestation, and 1.481 times lower per number of years since the imposition of quarantine. These findings indicate that a large proportion of quarantined herds may have a low seroprevalence of prv, making them prime candidates for test and removal. Pseudorabies virus might also be eliminated from these low-prevalence herds by a method referred to as management/vaccination, which is described. These methods are inexpensive, compared with offspring segregation or depopulation/repopulation, and represent a substantial cost savings for the swine industry.

Summary

Strategies for the elimination of pseudorabies virus (prv) from swine herds include test and removal, offspring segregation, and depopulation/repopulation. The prevalence of prv in a herd is a major factor in selection of the most appropriate strategy. The purpose of the study reported here was to describe the prevalence of prv in adult swine in prv quarantined herds in Minnesota, and to determine herd factors associated with the seroprevalence. Questionnaires describing the health history of the herd, management practices, and design of the swine facilities were obtained from the owners of 142 quarantined herds. Blood was collected from 29 finishing pigs over the age of 4 months, up to 29 adult females, and all herd boars. Factors considered to be significant in a bivariate analysis were combined in a stepwise multiple logistic regression analysis. The prevalence of prv-seropositive adults in each herd was bimodally distributed among the 142 herds. In 42 (30%) of the herds, none of the females tested was seropositive, which represented the lower mode. At least 90% of the adults tested were seropositive in 30 (21%) of the herds and represented the higher mode. The odds of the breeding swine of a given herd having a prv seroprevalence of ≥ 20% as compared with having a seroprevalence of < 20% was 1.654 times higher per 50 adults in the herd, 13.550 times higher if the finishing pigs were seropositive, 2.378 times higher if sows were housed inside during gestation, and 1.481 times lower per number of years since the imposition of quarantine. These findings indicate that a large proportion of quarantined herds may have a low seroprevalence of prv, making them prime candidates for test and removal. Pseudorabies virus might also be eliminated from these low-prevalence herds by a method referred to as management/vaccination, which is described. These methods are inexpensive, compared with offspring segregation or depopulation/repopulation, and represent a substantial cost savings for the swine industry.

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