Clinical consequences of exposing pregnant gilts to strains of porcine reproductive and respiratory syndrome (PRRS) virus isolated from field cases of "atypical" PRRS

Objective-To determine clinical consequences of exposing pregnant gilts to strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRS V) isolated from field cases of "atypical" or "acute" PRRS in vaccinated he rds. Animals-20 pregnant gilts and their pigs and fetuses. Procedure-8 pregnant gifts (principals: 4 groups [2 gilts/group]) were expo sed oronasally at or about 45 days of gestation to 1 of 4 strains of PRRSV and necropsied 6 weeks later. Nonexposed controls (2 additional pregnant gi lts) were kept under otherwise similar conditions. The experiment was repea ted, except that principals were exposed at or approximately 90 days of ges tation and allowed to farrow. Clinical observations were made at least twic e daily, and samples and specimens from gifts and their fetuses and pigs we re tested for PRRSV and homologous antibody. Results-Exposure of pregnant gilts to PRRSV at or approximately 45 days' ge station resulted in low prevalence of transplacental infection and fetal de ath. Exposure of pregnant gilts to PRRSV at or approximately 90 days' gesta tion resulted in higher prevalence of transplacental infection and fetal de ath. Moreover, 1 gilt aborted and many liveborn pigs of other litters were weak and unthrifty. Clinical signs of disease and reproductive failure were especially severe for a field strain of PRRSV isolated from an epizootic t hat fit the strictest definition of atypical PRRS. Controls remained clinic ally normal and free of PRRSV. Conclusion and Clinical Relevance-Some strains of PRRSV now circulating in US swine herds are more virulent than those encountered in the past. Clinic al PRRS in vaccinated herds suggests need for a new generation of vaccines.