Objective: To compare the virulence of selected strains of porcine reproductive and respiratory syndrome virus (PRRSV) relative to reproductive 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 specimens obtained from gilts, pigs (before ingestion of colostrum), and fetuses 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 farrowing.
Results: Most gilts remained clinically normal throughout the study and farrowed normally at or near the expected farrowing time. All virus 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 control gilts and gilts exposed to strain RespPRRS, to 38 for gilts exposed to strain NADC-8. All principal gilts became viremic and developed antibody against PRRSV. All strains persisted in alveolar macrophages of at least some principal gilts for at least 7 weeks after exposure.
Conclusion: Strains of PRRSV vary in virulence.
Clinical relevance: The effects of PRRSV on reproductive performance are strain dependent and this should be considered in making a tentative diagnosis on the basis of clinical observations.