Objective: To determine whether cell-mediated immunity against porcine reproductive and respiratory syndrome (PRRS) virus is correlated with protection against reproductive failure in sows during clinical outbreaks of PRRS in commercial herds.
Design: Outbreak investigation in 4 swine breeding herds.
Animals: 97 sows.
Procedures: On each farm, blood samples were collected from sows with clinical signs (abortion or increased fetal death; case sows) and from clinically normal sows (control sows). The intensity of the cell-mediated immune (CMI) response was determined by use of an interferon-gamma enzyme-linked immunospot (ELISPOT) assay. Multiple logistic regression analyses and t tests were used to compare ELISPOT assay values between case and control sows. Multiple linear regression was used to investigate associations between cell-mediated immunity and the magnitude of clinical signs.
Results: In 2 farms, case sows had lower ELISPOT assay values than control sows. A negative association between the intensity of the CMI response and the number of pigs born dead per litter was detected on 1 farm. In 1 farm, no association was detected between the intensity of the CMI response and protection against reproductive failure.
Conclusions and clinical relevance: Evidence that a strong CMI response was correlated with protection against clinical PRRS was detected in 3 of 4 farms. However, farms and sows within farms varied considerably in their immune responsiveness and in the degree to which they were protected clinically. Increasing cell-mediated immunity within infected herds has the potential to decrease clinical reproductive disease, but only if the sources of intra- and interfarm variation in the intensity of cell-mediated immunity to PRRS virus can be identified.