Effect of porcine reproductive and respiratory syndrome virus infection on the clearance of Haemophilus parasuis by porcine alveolar macrophages.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection in young piglets is frequently associated with secondary infection due to various pathogens, especially those of the respiratory tract. One of the most important mechanisms in respiratory diseases is related to the alteration of function of porcine alveolar macrophages (PAMs). The objective of this study was to determine how PRRS virus infection affects the capabilities of PAMs in the phagocytosis and destruction of Haemophilus parasuis. Phagocytosis percentages were determined in vitro and ex vivo, after collected PAMs were directly exposed to the virus of if PAMs were collected from piglets previously infected with PRRSV. In vitro experiments demonstrated that H. parasuis uptake by PAMs is only increased in the early stages of PRRSV infection (2 h post-infection). In contrast, in the ex vivo experiments it was shown that PAMs from PRRSV-infected piglets do not seem to change in their phagocytic rate until the later stages of infection. Together with a decrease in the phagocytic rate, a marked decrease in the functional ability of PAMs to kill bacteria was observed 7 d post-infection. It is hypothesized that when animals are exposed to PRRSV, there is a marked decrease in the functional ability of PAMs to kill bacteria through the release of superoxide anion, indicating a possible negative effect of the virus, at least at the macrophage level.