Bluetongue (BT) disease is an insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle. BT is caused by a reovirus, which is transmitted by a specific species of gnat. Cattle are the main carriers of the bluetongue virus, however infected cattle generally don't show any clinical signs of disease. Sheep are more severely and most frequently affected by BT.
Bluetongue virus is transmitted through biting insects and can be transferred between different animal species. A cow can pass on the infection to the calf in utero. The fetus is most susceptible at 60 to 140 days of pregnancy. Infected semen can infect a cow resulting in the disease being transferred to the cow and then to the calf.