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→Pathogenesis
When a BTV-infected midge takes a blood meal from a ruminant host, innoculated virus spreads from the skin to the regional lymph nodes. After initial replication in the lymph nodes, a cell-associated viraemia carries the virus to many tissues where further replication occurs in macrophages and endothelial cells.
When a BTV-infected midge takes a blood meal from a ruminant host, innoculated virus spreads from the skin to the regional lymph nodes. After initial replication in the lymph nodes, a cell-associated viraemia carries the virus to many tissues where further replication occurs in macrophages and endothelial cells.
Clinical signs and lesions in BTV-infected sheep likely reflect virus-mediated endothelial injury, as BTV replicates in endothelial cells causing cell injury and necrosis [21,26]. Similarly, white-tailed deer, which are highly susceptible to BT, develop consumptive coagulopathy as a consequence of BTV-induced damage to endothelial cells [18]. Consumptive coagulopathy in BTV-infected sheep and deer predisposes to the bleeding tendency that characterizes fulminant BT. Endothelial injury also is likely responsible for increased vascular permeability leading to edema in tissues such as the lung (pulmonary edema), and vascular thrombosis leads to tissue infarction.
Culicoides insects are biological vectors of BTV, thus the virus replicates within the tissues of each insect after infection from feeding on the blood of a BTV-infected ruminant [22]. Vector insects can only transmit BTV to another susceptible ruminant after an extrinsic incubation period of some 10 - 14 days, during which time the virus is disseminated from the insect's gut to its salivary glands. The external incubation period is shorter when insects are held at high ambient temperatures. Vertical transmission with transovarial transfer of BTV has not been demonstrated in Culicoides insects, however, infection of adult insects is lifelong. Furthermore, individual insects can survive for relatively long periods of time, particularly in cooler ambient temperatures [23].
==Diagnosis==
==Diagnosis==