A review of evidence for immunosuppression due to porcine reproductive and respiratory syndrome virus.

Accounts of field disease and experimental studies involving porcine reproductive and respiratory syndrome (PRRS) are reviewed for evidence of immunomodulation or immunosuppression by the causative virus. The conclusion is that 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 different cells of the immune system, but its significance for the course of disease or pig health are unclear. Likewise, the current experimental evidence for any interaction of PRRSV with other pathogens is ambiguous and therefore no firm conclusions can yet be drawn. Strains of PRRSV do vary in pathogenicity, which may be related to their degree of ability to cause overt respiratory disease in the absence of other agents. Experimentally, varying degrees of interstitial pneumonia are a common histological finding. There is, as yet, no firm evidence of general immunosuppression--in fact, some contrary evidence exists in the form of observations of a transient enhancement of humoral response, possibly through polyclonal B cell activation. The basis of pathogenicity of PRRSV and of any interaction with other agents is still unknown and is likely to remain unclear. Virus interaction with the pig's immune system must be addressed before any assessment of virulence of any known or emergent strains of PRRSV can be made.