Bluetongue is a viral disease of ruminants transmitted by midges in the genus Culicoides. Bluetongue virus is very diverse: there are more than two dozen serotypes, and viruses can reassort to form new variants. This virus is endemic in a broad, worldwide band of tropical and subtropical regions from approximately 35°S to 40°N; however, outbreaks also occur outside this area, and the virus may persist long-term if the climate and vectors are suitable. While overwintering in regions with cold winters is unusual, bluetongue virus recently demonstrated the ability to survive from year to year in central and northern Europe.

Bluetongue virus can replicate in many species of ruminants, often asymptomatically. Clinical cases tend to occur mainly in sheep, but cattle, goats, South American camelids, wild or zoo ruminants, farmed cervids and some carnivores are occasionally affected. Cases range in severity from mild to rapidly fatal, and animals that survive may be debilitated. Additional economic costs result from reproductive losses, damaged wool and decreased milk production. Control of this vector-borne disease is difficult, except by vaccination. The existence of multiple serotypes complicates control, as immunity to one serotype may not be crossprotective against others.