The appearance of BT disease in ruminants is usually the first indication that the virus is circulating in the ecosystem. Unless an active surveillance system of virus isolation or serological testing is in place, disease manifestation is the only indication that the virus is in the region. Viral identification is complicated by the need for rapid methodology, such as PCR identification. Virus isolation by chick embryo or cell culture is slow and laborious. Serological results are delayed until after an immune response has taken place; most susceptible animals will have already shown clinical signs of disease.
The means by which the virus is introduced into the ecosystem is wind, which transports BT-infected Culicoides (Sellers, 1992). This concept is thought to be the way that incursions of C. imicola carried BTVs into southern Europe from North Africa in the mid-1950s and in the 1980s. Fortunately, the vectors and the virus did not persist, as the climatic conditions did not permit their establishment in succeeding years. BTVs also may appear in new areas by transport of viremic BT-infected animals, which then serve as a source of virus-infected