Project Title: Use of tonsilar crypt exudates to evaluate PRRSV shedding and transmission following a MLV PRRS vaccine or a live PRRS virus inoculation

Institution: University of Missouri-Columbia and Albany Veterinary Services

Outcome/Results: Tonsilar crypt exudate can be used effectively to test for PRRSv harbored on the tonsils of infected pigs.

Summary: Persistently infected pigs are silently infected with PRRSv and do not show the clinical signs associated with the acute phase of PRRSv infection. Rather, these pigs have fully recovered from the acute phases of PRRSv infection, yet they will continue to carry a low-level viral infection for an extended period of time. These persistently infected pigs may shed the PRRSv either continuously or intermittently and naïve pigs may become infected from either direct or indirect contact. In this experiment, we exposed pigs to a field isolate or a vaccine isolate. Following exposure, we used a technique scraping the tonsils to collect tonsilar crypt exudate. We monitored shedding of PRRSv over a 5-month period in order to determine the duration of time that PRRSv can persist in tonsilar crypt exudates and to determine if a 90% tonsilar clearance rate is adequate to prevent transmission of PRRSv to naïve animals. We discovered that PRRSv may persist in the tonsilar exudate for 160 days post exposure. PRRSv was not transmitted to naïve pigs after 130 days post exposure even though 4 animals (out of 80) still possessed RT-PCR positive tonsilar crypt exudate at this time.

Bioassay results (infecting pigs with PRRSv positive serum) suggest that tonsilar crypt exudate materials collected from pigs 130 days post exposure and injected into 21 day old pigs would elicit an antibody response 30 days after inoculation. There was no difference in circulating antibodies of PRRSv or in the rate of PRRSv elimination from the tonsil between the LVI and MLV vaccinated groups in this study.