Bacterial infections are potentiated by porcine reproductive and respiratory syndrome virus infection: Fact or fiction?

Since 1987, clinicians and scientists have consistently observed an increase in bacterial diseases in conjunction with porcine reproductive and respiratory syndrome virus (PRRSV) outbreaks. Most early studies attempting to demonstrate the potentiation of secondary disease by PRRSV infection used a common experimental design: PRRSV inoculation of a weaned pig, followed by challenge 5 to 10 days later with any of a number of bacterial pathogens. Most of these experiments were unsuccessful. On the other hand, switching the order of inoculation, i.e., pathogenic bacteria followed by PRRSV, reproduced the additive disease effects observed in the field. Furthermore, alternate models of clinical disease, such as in utero infection, have shown that PRRSV can cause lesions in lymphoid tissues of piglets and produce alterations in peripheral blood leukocyte populations and leukocyte function suggestive of an immunosuppressive effect. These observed changes may explain why bacterial infections cause much more severe disease in PRRSV-infected pigs. Concurrent infection with other potentially immunosuppressive agents, such as porcine circovirus type 2, may aid, or be necessary for, the expected increase in susceptibility to disease following PRRSV infection under field conditions.