Pathogenesis of plaque variants of porcine reproductive and respiratory syndrome virus in pregnant sows

Objective - To determine whether porcine reproductive and respiratory syndrome virus plaque variants vary in their pathogenicity in causing late-term reproductive failure. Design - Four groups of 2 sows each at 86 days of gestation were inoculated intranasally with PRRSV small (MN-Hs) and large (MN-HI) plaque variants, field isolate, and cell culture medium, respectively. In addition, 2 sows each at 86 days of gestation were inoculated intranasally or IM with MN-Hs virus. Animals - 14 pregnant sows. Procedure - Inoculated sows were allowed to deliver at term, and each litter was examined for clinical abnormality and presence of virus. Results - Two sows infected with MN-Hs virus delivered 14 live and 5 dead pigs, whereas 2 sows infected with MN-HI virus delivered 0 live and 25 dead pigs. Two sows inoculated with a field isolate (MN-W) delivered 10 live and 20 dead pigs. Two control sows had 26 normal fetuses at slaughter at 107 days of gestation. Virus was isolated from 16 (66.7%) of 24 liveborn pigs, 9 (64.3%) of 14 stillborn pigs, and 3 (12.0%) of 25 mummified fetuses of the 6 infected sows. Subsequently, 4 MN-Hs-infected sows delivered 40 live and 11 dead pigs. Conclusions - Marked difference in the pathogenicity in pregnant sows between porcine reproductive and respiratory syndrome virus strains was documented The MN-Hs virus is considered to be of low pathogenicity, but the other viruses are highly pathogenic for late-term pregnant sows.