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.