Reproductive performance of gilts following vaccination and subsequent heterologous challenge with European strains of porcine reproductive and respiratory syndrome virus

The objective of this study was to evaluate the efficacy of two commercially available modified live virus vaccines for preventing the reproductive and early postnatal consequences of infecting (challenging) pregnant gilts with virulent porcine reproductive and respiratory syndrome virus (PRRSV). For this purpose 21 crossbred gilts were allocated to one or another of four groups (Groups A–D). Group A comprised four gilts neither vaccinated nor challenged; Group B comprised five gilts that were challenged but not vaccinated; Group C comprised seven gilts that were vaccinated (AmervacPRRS®) and challenged; Group D comprised five gilts that were vaccinated (Pyrsvac-183®) and challenged. Vaccination was 24 days before conception, and challenge was at 90 days of gestation. Both vaccine viruses and the challenge virus were European strains but differed in part from one another on the basis of their genetic (nucleotide) sequence. After challenge PRRSV was isolated from five (100%), four (57%), and two (40%) of the gilts of Groups B, C and D, respectively. Although vaccination failed to prevent a detectable viremia in all of the gilts of Groups C and D after they were challenged (or congenital infection of some of their pigs), it did provide a statistically significant level of protection in regard to the incidence of congenital infection, reproductive performance, and pig health and viability. Namely, for Groups C and D the numbers of liveborn pigs/litter and healthy pigs/litter throughout the early postnatal period were similar to those of Group A (nonvaccinated and nonchallenged) and far exceeded those of Group B (nonvaccinated and challenged).