Duration of PRRS Virus Infections and Proportion of Persistently Infected Pigs
2000 University of Nebraska-Lincoln Extension Swine Report. The objective of this study was to more fully characterize persistent PRRSV infections in swine. Twentyeight 35-day-old segregated-earlyweaned pigs were inoculated intranasally with PRRSV. Serum and tonsil biopsy samples were collected on days 0, 7, 14, 28, and then about monthly thereafter until day 251 post inoculation (PI). Virus was isolated from serum and tonsil biopsy samples through days 28 and 56 PI, respectively. Viral RNA was detected in serum and tonsil biopsy samples by RT-PCR through day 251 PI, although no positive serum samples were detected on days 84-196 PI. Greater proportions of day 28 and 56 PI serum samples and tonsil biopsies were found to be PRRSV RNA positive by RT-PCR than positive by virus isolation. Although 20 of 28 tonsil biopsies collected on day 84 PI were positive by RT-PCR, only one of 28 tonsil biopsies collected one month later (day 119 PI) was positive. Three pigs returned to seronegative status on or after day 196 PI. Neither virus nor viral RNA was detected in these animals beyond day 119 PI. Conversely, five pigs that were persistently infected through day 225 or 251 PI remained seropositive throughout the study although one pig had an ELISA S/P ratio of 0.41, nearly at the cutoff point of 0.40. The results confirm RT-PCR is more sensitive than virus isolation in identifying PRRSVinfected pigs. The abrupt drop in the proportion of pigs with RT-PCR positive tonsil samples from day 84 to day 119 PI indicates most pigs clear the virus within three to four months, but some may remain persistently infected for several months.