A review of evidence for immunosuppression due to porcine reproductive andrespiratory syndrome virus

Accounts of field disease and experimental studies involving porcine reprod uctive and respiratory syndrome (PRRS) are reviewed for evidence of immunom odulation or immunosuppression by the causative virus. The conclusion is th at immunomodulation through infection of alveolar macrophages is likely to occur, but that it is transient and at a local level, in the lung. There is some evidence for more subtle effects via more disseminated replication or induction of apoptosis with some isolates, but more definitive studies are needed. There is some emerging evidence of interaction between PRRSV and d ifferent cells of the immune system, but its significance for the course of disease or pig health are unclear. Likewise, the current experimental evid ence for any interaction of PRRSV with other pathogens is ambiguous and the refore no firm conclusions can yet be drawn. Strains of PRRSV do vary in pa thogenicity, which may be related to their degree of ability to cause overt respiratory disease in the absence of other agents. Experimentally, varyin g degrees of interstitial pneumonia are a common histological finding. Ther e is, as yet, no firm evidence of general immunosuppression - in fact, some contrary evidence exists in the form of observations of a transient enhanc ement of humoral response, possibly through polyclonal B cell activation. T he basis of pathogenicity of PRRSV and of any interaction with other agents is still unknown and is likely to remain unclear. Virus interaction with t he pig's immune system must be addressed before any assessment of virulence of any known or emergent strains of PRRSV can be made.