Tonsilar crypt exudate to evaluate shedding and transmission of porcine reproductive and respiratory syndrome virus after inoculation with live field virus or vaccination with modified live virus vaccine

Two isolated groups of 44 seven-week-old principal pigs were confirmed negative for porcine reproductive and respiratory syndrome virus (PRRSV) by testing blood samples by ELISA and reverse transcription-polymerase chain reaction (RT-PCR). Pigs were then inoculated (Day 0) with serum containing a field strain of PRRSV (LVI) or were vaccinated with a commercial modified live virus PRRS vaccine (MLV). Blood samples and tonsilar scrapings were collected on Day 14 and tonsilar scrapings on Days 28, 42, 56, 70, 100, 130, and 160 for testing by RT-PCR soon after collection. When 90% of pigs were PCR-negative (Day 130), three PRRSVnaive sentinel pigs were introduced per pen of LVI and MLV pigs. No sentinels had seroconverted when tested by ELISA 30 days later. Tonsilar crypt exudate and RT-PCR-positive serum samples from the principal pigs were pooled to create homogenates for a swine bioassay. Threeweek-old PRRSV-naive pigs were inoculated intramuscularly with PRRSV-positive tonsilar crypt exudate pools (15 pigs), PRRSV-positive serum pools (five pigs), or PRRSV-negative tonsilar crypt exudate pools (five pigs). When tested by ELISA 30 days later, all 20 pigs inoculated with PRRSV-positive pools were seropositive and the five pigs inoculated with PRRSVnegative pools were seronegative.