Changes

==Pathogenesis==

==Pathogenesis==

Once PRRS virus gains entry to the host, primary replication occurs in regional or mucosal macrophages such as in the tonsil and the nasal and respiratory epithelium. After around twelve hours, viraemia occurs and the virus is disseminated systemically.  

Once PRRS virus gains entry to the host, primary replication occurs in regional or mucosal macrophages such as in the tonsil and the nasal and respiratory epithelium. After around twelve hours, viraemia occurs and the virus is disseminated systemically. The PRRS virs then seconarily replicated in the moncytes and macrophages in a wide variety of tissues. Other cells, such as fibroblasts, endothelial cell and smooth muscle may acqure PRRS infection, but only cells of the macrophage lineage have been shown to unequivocally support replication. Viraemia persists for around 4-6 weeks in most infections, but lymphoid tissues can remain infected for extended periods of time: following experimental innoculation, PRRS virus has been detected for up to 225 days. Both cell mediated and humoral immune responses are induced by PRRS virus infection, and pesistance of infection occurs despite this. The fact that virus replicates in pigs for such a long time contributes to the maintenance of infection within a herd.

Secondary replication of PRRS virus occurs in monocyte/

macrophages in many tissues. While other cell

types, including fibroblasts, endothelial cells, seminiferous

epithelium and smooth muscle, have been reported

to be PRRS virus-positive by in situ hybridization and/or

immunohistochemistry, only cells of macrophage lineage

have been unequivocally shown to support PRRS

virus replication (Sur et al., 1997).

In most swine, viremia resolves within 4-6 weeks following

infection. However, PRRS virus can persist in

lymphoid tissues, such as tonsil, for extended periods of

time and be transmitted to susceptible swine (Zimmerman

et al., 1992). PRRS virus has been detected for up

to 225 days following inoculation, though the proportion

et al., 2003). Persistence occurs in the face of neutralizing

and other antibodies, and cell mediated immune

responses, as indicated by gamma-interferon producing

cells. Studies indicate that a low T cell response

contributes to prolonged PRRS clearance (Xiao et al.,

2004). Persistence is of epidemiologic significance

by which PRRS virus is maintained within, and

transmitted between, swine herds.

==Diagnosis==

==Diagnosis==