Immunologic responses and reproductive outcomes following exposure to wild-type or attenuated porcine reproductive and respiratory syndrome virus in swine under field conditions

Objective - To compare immunologic responses and reproductive outcomes in sows housed under field conditions following controlled exposure to a wild-type strain of porcine reproductive and respiratory syndrome virus (PRRSV strain WTV) or vaccination with a modified-live virus (MLV) vaccine. Design -Randomized controlled trial. Animals - 30 PRRSV-naïve 10-week-old female pigs. Procedure - Humoral and cell-mediated immune responses were monitored while pigs were held in isolation for 84 days after inoculation with the WTV strain (n = 10), inoculation with the WTV strain and 42 days later vaccination with a killed-virus vaccine (10), or vaccination with an MLV vaccine (10). Reproductive outcomes were measured after pigs were released into the farm herd. Results - Inoculation with the WTV strain, regardless of whether a killed-virus vaccine was subsequently administered, elicited faster and more substantial production of strain-specific neutralizing antibodies, as well as a more rapid generation of interferon-γ secreting cells, than did vaccination with the MLV vaccine. Despite the enhanced immune responses in pigs inoculated with the WTV strain, animals vaccinated with the MLV vaccine produced a mean of 2.45 more pigs than did sows exposed to the WTV strain, mainly because of a lower rate for failure to conceive. Conclusions and Clinical Relevance -Results suggest that current assays of immunity to PRRSV correlate only imperfectly with degree of clinical protection and that the practice of controlled exposure of sows to a circulating PRRSV strain should be reconsidered in light of negative clinical outcomes.