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Porcine Reproductive and Respiratory Syndrome (view source)
Revision as of 18:34, 27 August 2010
, 18:34, 27 August 2010→Pathogenesis
==Pathogenesis==
==Pathogenesis==
Transmission of PRRS virus to swine occurs most efficiently
through direct contact. Blood, saliva, urine, milk
and semen from infected animals have been shown to
transmit PRRS virus to susceptible swine (Wills et al.,
1997). Aerosol transmission may occur but has proven
difficult to demonstrate even over relatively short distances
(meters). Transmission by semen, either
through natural service or artificial insemination is well
established (Yaeger et al., 1993).
Following exposure to PRRS virus, primary replication
occurs in regional or mucosal macrophages (Rossow,
1998; Rossow et al., 1996a). Typically macrophages of
the tonsil, nasal and respiratory epithelium are sites of
primary replication after oronasal exposure. Viremia
can occur within 12 hours following exposure resulting
in systemic dissemination of the virus. Lymphatic transport
of PRRS virus and virus-infected macrophages to
regional lymph nodes is also thought to occur.
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
of positive animals appears to drop off between 80 and
100 days (Albina et al., 1994; Allende et al., 2000; Wills
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
since persistently infected animals are capable of transmitting
PRRS virus to susceptible pigs via direct contact
(Albina et al., 1994). This is likely an important mechanism
by which PRRS virus is maintained within, and
transmitted between, swine herds.
==Diagnosis==
==Diagnosis==