Spread of pseudorabies virus among breeding swine in quarantined herds

Abstract

Summary In theory, pseudorabies virus (prv) may be eliminated from any size of breeding herd by phased test and removal if replacement gilts are not infected with prv, culling decisions are partially based on prv status, and the cull rate is higher than the incidence rate of prv. Annual cull rates are commonly at least 50%, but little information exists on the incidence of prv within enzootically infected swine herds. The purpose of this study was to develop a method by which spread of prv could be detected among breeding swine within enzootically infected herds and to determine the incidence of prv infection in these herds. Data were collected from 17 herds that were quarantined for prv and ranged in size from 120 to 1,100 sows. At each herd, within the first 5 days of introduction, a group of approximately 30 replacement gilts was identified, vaccinated with a glycoprotein X-deleted prv vaccine, and blood sample was collected. The owner of 1 herd had a nonvaccinated breeding herd and elected to leave incoming gilts nonvaccinated. After vaccination, blood samples were collected every 1 to 2 months for an average of 13.6 months. Serum samples from vaccinated gilts were tested for antiglycoprotein X antibodies by a specific differential elisa. Samples from nonvaccinated gilts were evaluated by serum neutralization test. Product-limit method was used to estimate the probability of not becoming infected with prv. Spread was detected in 7 of 8 herds that had more than 400 sows and in 2 of 9 herds that had less than 400 sows. The minimal elapsed time between introduction and detection prv infection varied from 2 to 6 months. In the 9 herds in which spread was detected, the probability that a replacement gilt did not become infected ranged from 53% at month 10 after introduction to 95% at month 15. Annual incidence ranged from 61% to 4.7%, respectively. When prv was detected, it spread slowly and sporadically. Therefore, assuming that replacement gilts remain negative and that the culling decision could be made partially on prv status, phased testing and removal would be a viable method to eliminate prv from most of these herds.