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→Pathogenesis
The pathogenesis of BTV infection has been shown to be similar in sheep and cattle, and is assumed to be similar in other species of ruminants<sup>7, 8, 9</sup>. However, the severity of disease varies greatly with species and cattle in particular express very few signs.
The pathogenesis of BTV infection has been shown to be similar in sheep and cattle, and is assumed to be similar in other species of ruminants<sup>7, 8, 9</sup>. However, the severity of disease varies greatly with species and cattle in particular express very few signs.
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. In the process of reproducing, bluetongue virus causes endothelial cell injury and necrosis<sup>9</sup>. In turn, an increase in vascular permeability causes oedema, and thrombosis can arise which may lead to tissue infarction. A consumptive coagulopathy can occure sheep and deer<sup>2</sup>.
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. In the process of reproducing, bluetongue virus causes endothelial cell injury and necrosis<sup>9</sup> which can increase vascular permeability to cause oedema. Endothelial damage can also give thrombosis, leading to tissue infarction, and (in sheep and deer) a consumptive coagulopathy<sup>2</sup>.
As ''Culicoides'' midges are biological vectors of BTV, the virus replicates in insect tissue after feeding on an infected host. 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].
As ''Culicoides'' midges are biological vectors of BTV, the virus replicates in insect tissue after feeding on an infected host. 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].