Changes

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^{7, 8, 9}. 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^{7, 8, 9}. 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, the virus travels 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.

 

Viremia in BTV-infected ruminants is highly cell associated, and viremia is prolonged but not persistent especially in cattle [4,8,31]. The virus promiscuously associates with all blood cells, thus titers of virus in each cell fraction are proportionate to the numbers of each cell type; specifically, BTV is quantitatively associated most with platelets and erythrocytes and, because of the short lifespan of platelets, virus is most associated with erythrocytes late in the course of BTV infection of ruminants. BTV infection of erythrocytes facilitates both prolonged infection of ruminants and infection of hematophagous insect vectors that feed on viremic ruminants [9,10]. Interestingly, BTV nucleic acid may be detected by polymerase chain reaction (PCR) in the blood of infected cattle and sheep for many months after it no longer can be detected by virus isolation in cell culture or inoculation of susceptible sheep. Furthermore, ruminant blood that contains BTV nucleic acid as determined by PCR assay, but not infectious BTV as determined by virus isolation, is not infectious to vector insects even by intrathoracic inoculation [8,20,34].

==Diagnosis==

==Diagnosis==