COMPARISON AMONG STRAINS OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS FOR THEIR ABILITY TO CAUSE REPRODUCTIVE FAILURE

Objective-To compare the virulence of selected strains of porcine repr oductive and respiratory syndrome virus (PRRSV) relative to reproducti ve performance of pregnant gilts. Design-16 pregnant gilts (principals ) were exposed oronasally to 4 strains (vaccine strain RespPRRS, field strains VR-2385, VR-2431, and NADC-8, 4 gilts/strain) of PRRSV on or about day 90 of gestation. 4 pregnant gilts (controls) were kept under similar conditions, except for exposure to PRRSV. Samples and specime ns obtained from gilts, pigs (before ingestion of colostrum), and fetu ses were tested for PRRSV and homologous antibody. Animals-20 pregnant gilts. Procedure-The virulence of each strain of PRRSV was evaluated mainly on the clinical status of the corresponding litters at farrowin g. Results-Most gilts remained clinically normal throughout the study and farrowed normally at or near the expected farrowing time. All viru s strains crossed the placenta of principal gilts to infect fetuses in utero. The number of late-term dead fetuses (which appeared to be the best measure of relative virulence) ranged from 0 for litters of cont rol gilts and gilts exposed to strain RespPRRS, to 38 for gilts expose d to strain NADC-8. Ail principal gilts became viremic and developed a ntibody against PRRSV, Ail strains persisted in alveolar macrophages o f at least some principal gilts for at least 7 weeks after exposure; C onclusion-Strains of PRRSV vary in virulence. Clinical Relevance-The e ffects of PRRSV on reproductive performance are strain dependent and t his should be considered in making a tentative diagnosis on the basis of clinical observations.