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  • Author or Editor: David G. Thawley x
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Summary

We compared 3 modified-live pseudorabies virus (PRV) vaccine strains, administered by the intranasal (IN) or IM routes to 4- to 6-week-old pigs, to determine the effect of high- and low-challenge doses in these vaccinated pigs. At the time of vaccination, all pigs had passively acquired antibodies to PRV. Four experiments were conducted. Four weeks after vaccination, pigs were challenge-exposed IN with virulent virus strain Iowa S62. In experiments 1 and 2, a high challenge exposure dose (105.3 TCID50) was used, whereas in experiments 3 and 4, a lower challenge exposure dose (102.8 TCID50) was used. This low dose was believed to better simulate field conditions. After challenge exposure, pigs were evaluated for clinical signs of disease, weight gain, serologic response, and viral shedding.

When vaccinated pigs were challenge-exposed with a high dose of PRV, the duration of viral shedding was significantly (P < 0.05) lower, and body weight gain was greater in vaccinated pigs, compared with nonvaccinated challenge-exposed pigs. Pigs vaccinated IN shed PRV for fewer days than pigs vaccinated IM, but this difference was not significant. When vaccinated pigs were challenge-exposed with a low dose, significantly (P <0.05) fewer pigs vaccinated IN (51%) shed PRV, compared with pigs vaccinated IM (77%), or nonvaccinated pigs (94%). Additionally, the duration of viral shedding was significantly (P < 0.05) shorter in pigs vaccinated IN, compared with pigs vaccinated IM or nonvaccinated pigs. The high challenge exposure dose of PRV may have overwhelmed the local immune response and diminished the advantages of the IN route of vaccination. Repeatable difference in the effect of vaccine strain was not detected in pigs challenge-exposed with low- or high-PRV doses. These findings suggested that the dose of challenge virus is critical for determining the effectiveness of vaccination in reducing the prevalence and duration of viral shedding after PRV infection, and that the route of vaccination plays a more important role than vaccine strain.

Free access
in Journal of the American Veterinary Medical Association

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.

Free access
in Journal of the American Veterinary Medical Association

Summary

Knowledge of the factors that place susceptible gilts at highest risk of pseudorabies virus (prv) infection in a quarantined herd is crucial to reduce spread of prv within the herd. Cohorts of prv seronegative gilts were monitored in 17 herds that were endemically infected with prv to determine the location of breeding females at the time of infection with prv and identify herd characteristics and management and housing factors that may influence spread of prv in the breeding section of swine herds endemically infected with prv. Blood samples were collected every 1 to 2 months for an average of 13.6 months. In addition, blood was collected from a representative sample of finishing pigs (≥ 20 weeks old) 3 times per year to determine their serologic prv status. Incidence rates and relative risks of prv infection were estimated for 4 areas of the breeding section: gestation barn, gilt pool, farrowing room, and breeding area. Overall, 28, 11, 8, and 2 females became infected with prv in each of these areas, respectively. The greater number of females infected in the gestation barns, compared with the number of females infected in other locations, is probably a consequence of being at risk for a longer period rather than of a higher incidence rate. Herd size, common housing for gilts in the gilt pool and sows, and serologic pattern of prv infection in finishing pigs were associated with the detection of spread of prv in the breeding section of the 17 herds. An implication of these findings is that attempts to control the spread of prv among females in the breeding herd must be made in all phases of production.

Free access
in Journal of the American Veterinary Medical Association

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.

Free access
in Journal of the American Veterinary Medical Association

Summary

Data were collected from 104 Minnesota swine farms quarantined for pseudorabies virus (prv) infection. Each herd was serologically evaluated for the presence of antibodies to prv in finishing pigs. Herd management practices, swine housing design, and disease profiles were described for each farm. Multiple logistic regression analysis was used to determine which factors were associated with circulation of prv in the finishing pigs of farrow-to-finish farms. Sixty-seven (64%) of the herds had no serologic evidence of prv circulation in the finishing section, whereas 37 herds (36%) contained at least one prv seropositive finishing pig. The odds of a given finishing herd being seropositive for prv were 2.85 times higher if the finishing pigs were housed in confinement (P = 0.01), 2 times higher if Actinobacillus (Haemophilus) pleuropneumoniae was a clinical problem in the herd (P = 0.03), 1.36 times less for each year that passed since the herd quarantine was issued (P = 0.01), 1.74 times higher if clinical signs of prv were reported (P = 0.04), and 1.52 times higher if animal protein was included in at least one of the rations (P = 0.08).

Free access
in Journal of the American Veterinary Medical Association