The severity of clinical signs depends on the virus strain as well as the species and breed of animal affected. The clinical signs are described in the OIE Manual of Standards for Diagnostic Tests and Vaccines. The effects on animal welfare vary with the severity of the disease. Subclinical infections are very common in all ruminant species in infected areas. Transmission takes place via certain species of Culicoides biting midges which are biological vectors.
Transmission is only via bite.
In the Europe, Africa and the Middle East C. imicola is considered as the most important vector, in North America the major vector is C. variipennis and in Central and South America it is probably C. insignis . In Australia the major vectors are C. wadai , C. brevitarsis, C. fulvus and C. actoni . Other species of Culicoides have been shown to be less efficient vectors. The efficiency of vector species of Culicoides in transmitting BTV varies, depending on the virus strain and the local sub-population of the insect involved. Viraemia may last for up to 54 days in sheep and goats and for up to 100 days in cattle, though it is usually much shorter in all 3 species.