Changes

==Pathogenesis==

==Pathogenesis==

The pathogenesis of BTV infection is similar in sheep and cattle, and most probably, all species of ruminants [4,19,21,26]. There are marked differences in the severity of disease that occurs in different ruminant species after BTV infection, however, with cattle being especially resistant to expression of BT disease. After cutaneous instillation of virus through the bite of a BTV-infected Culicoides vector the virus travels to the regional lymph node where initial replication occurs. The virus then is disseminated to a variety of tissues throughout the body where replication occurs principally in mononuclear phagocytes 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].

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, 10}. However, the severity of disease varies greatly with species and cattle in particular express very few signs.  

 

After cutaneous instillation of virus through the bite of a BTV-infected Culicoides vector the virus travels to the regional lymph node where initial replication occurs. The virus then is disseminated to a variety of tissues throughout the body where replication occurs principally in mononuclear phagocytes 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==