PERSISTENCE OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS-INFECTION IN A SWINE OPERATION

A herd of Quebec seedstock pigs experienced in early 1992 a typical ou tbreak of porcine reproductive and respiratory syndrome (PRRS) associa ted with lesions of interstitial, proliferative and necrotizing pneumo nia in weaned piglets. The nature of the infection was confirmed by se rology using indirect immunofluorescence (IIF) and virus isolation in primary cultures of porcine alveolar macrophages (PAM). Farm productio n recovered after eight weeks of losses. In order to evaluate the pers istence of infection in the herd, five SPF-piglets were introduced in two different sections of the PRRS-affected barn four months after the disappearance of clinical symptoms, and two others were placed in a n eighboring building with apparently healthy farrow-to-finish pigs. Cli nical signs, body temperature, humoral immune response, virological an d histopathological findings were recorded over a 42-day period. Clini cal signs were evident in all of the sentinels and prolonged fever (gr eater than or equal to 140 degrees C) was recorded one day postexposur e (PE). Antibody titers to PRRS virus could be detected by IIF on PAM seven days PE, and reached 1:1024 by day 21 PE. Three of the sentinels developed significant virus neutralizing antibody titers (>1:8 to les s than or equal to 1:128) by day 35 PE. In all cases, the virus could be isolated from the serum between day 7 and 42 PE. Thus, the virus an d specific antibodies coexisted for several weeks. Lesions of intersti tial pneumonia was demonstrated in few animals. In experimental inocul ation studies, the viral strain isolated from the sentinel pigs produc ed severe reproductive disorders in two sows inoculated at 95 days of gestation. This study confirms that subclinical PRRS virus infection p ersists for a significant period in different age groups and locations and may, therefore, be transmitted by contact to replacement animals or via seedstock sold to healthy farms.