Bluetongue virus is the prototype virus of the genus Orbivirus , family Reoviridae. There are 24 serotypes worldwide, and 5 have been reported in the USA. The four serotypes currently found in the USA are 10, 11, 13, and 17. Serotype 2 was briefly reported in Florida and Alabama in the early 1980's. The distribution of the disease is limited by the culicoid insect vector that serves as the principal means of transmission between ruminant species. The viruses are biologically transmitted between ruminants by the biting midge Culicoides variipennis sonorensis , in the USA. This limits the regional distribution of the viruses to southern and western USA.

The usual incubation period in sheep is 5-10 days. In chronological sequence of appearance, clinical signs include dyspnea with panting; hyperemia of the lips, muzzle, and ears; pyrexia (reaching 107.5°F [42°C]); depression; and inflammation, erosions, and ulceration of the oral mucous membranes, particularly the dental pad. Other signs include swollen cyanotic tongue, lameness due to coronitis and widespread muscle necrosis, torticollis, vomiting, pneumonia, conjunctivitis, and alopecia. In sheep in the USA, mortality may range up to 30%.

In California, PHL produces vaccines to serotypes 10, 11, and 17 for use in sheep. Past surveys indicate that the predominant serotypes causing infection are 11 and 17. The modified live virus vaccines should not be used during the vector seasons because the culicoid vectors may pick up the vaccine virus and transmit it to other animals. Pregnant ewes should not be vaccinated during the first 100 days of gestation; otherwise, fetal malformations may occur. Passive immunity in lambs may last 4-6 mo. In an outbreak, the decision to vaccinate depends on the viruses that are causing infection at the time.