Bluetongue: the disease in cattle.

Most researchers in South Africa found that although BT virus could be isolated from apparently healthy cattle and from inoculated cattle the virus did not produce overt clinical disease in cattle. However, when epizootics were reported outside Africa, clinical signs were observed in cattle in Israel, Palestine, Syria, Portugal, and Spain. Most natural BT infections in cattle in the United States do not result in overt clinical signs. However, in certain infected herds, approximately 5% of the cattle show from mild to severe disease. Except for severe cases, spontaneous recovery is usual. The clinical diagnosis of BT in cattle is difficult and requires laboratory assistance. Culicoides variipennis can serve as a vector of BT virus from cattle to cattle, cattle to sheep, sheep to cattle, and sheep to sheep. In utero transmission occurs in cattle and can result in abortion, hydraencephaly, congenital deformity, and birth of viraemic calves which may or may not develop BT antibody. Calves inoculated in utero or those born to infected dams may have a persistent viraemia with or without BT antibody. tone such animal has been held in insect-secure quarters and has continued to harbour virus for 3 years. Bluetongue virus was isolated from the semen of experimentally infected bulls. Calves inoculated with BT virus and also given an immuno-suppressant developed marked clinical disease in 8 to 12 days. Bluetongue virus is very closely associated with the erythrocytes of infected cattle, sheep, and goats. Cattle are considered important and relatively long-term virus reservoirs. In attempts to determine the maximum period of viraemia in cattle it is necessary to inoculate washed erythrocytes, rather than whole blood, and to use susceptible sheep as the assay system rather than embryonated chicken eggs.