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

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Abstract
Objective: To determine clinical consequences of exposing pregnant gilts to strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) isolated from field cases of “atypical” or “acute” PRRS in vaccinated herds. Animals: 20 pregnant gilts and their pigs and fetuses. Procedure: 8 pregnant gilts (principals: 4 groups [2 gilts/group]) were exposed 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 gilts) were kept under otherwise similar conditions. The experiment was repeated, except that principals were exposed at or approximately 90 days of gestation and allowed to farrow. Clinical observations were made at least twice daily, and samples and specimens from gilts and their fetuses and pigs were tested for PRRSV and homologous antibody. Results: Exposure of pregnant gilts to PRRSV at or approximately 45 days' gestation resulted in low prevalence of transplacental infection and fetal death. Exposure of pregnant gilts to PRRSV at or approximately 90 days' gestation resulted in higher prevalence of transplacental infection and fetal death. 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 that fit the strictest definition of atypical PRRS. Controls remained clinically 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. Clinical PRRS in vaccinated herds suggests need for a new generation of vaccines. (Am J Vet Res 1998;59:1540-1544)

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