Two Commercial Type 1 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)-Modified Live Vaccines Reduce Seminal Shedding of Type 1 PRRSV but not Type 2 PRRSV in Infected Boars

The objective of this study was to compare the effects of two commercial type 1 porcine reproductive and respiratory syndrome virus (PRRSV)-modified live vaccines on type 1 and type 2 PRRSV shedding in the semen of experimentally infected boars. Upon challenge with PRRSV, unvaccinated boars exhibited an increase in daily rectal temperature (39.4-39.7°C). Vaccination of boars with type 1 PRRSV significantly reduced the amount of type 1 PRRSV load in blood and semen after challenge with type 1 PRRSV, but barely reduced the amount of type 2 PRRSV load in blood and semen after the type 2 PRRSV challenge. There were no significant differences in the reduction of viremia and seminal shedding of type 1 and type 2 PRRSV between the two commercial vaccines. The seminal shedding of PRRSV is independent of viremia. The reduction of type 1 PRRSV seminal shedding coincided with the appearance of type 1 PRRSV-specific interferon-γ secreting cells (IFN-γ-SC) in vaccinated type 1 PRRSV-challenged boars. The frequencies of type 1 PRRSV-specific IFN-γ-SC induced by type 1 PRRSV vaccine are relatively high compared to type 2 PRRSV-specific IFN-γ-SC induced by the same vaccine which may explain why type 1 PRRSV vaccine is more effective in reducing seminal shedding of type 1 PRRSV when compared to type 2 PRRSV in vaccinated challenged boars. These results provide clinical information on how to reduce seminal shedding of type 1 PRRSV in boars using type 1 PRRSV-modified live vaccine.