Bluetongue viruses (BTV) are infectious, noncontagious viruses in the family Reoviridae and genus Orbivirus that are transmitted to domestic and wild ruminants (sheep, cows, goats, deer) by biting midges (no-see-ums) in the genus Culicoides (Fig. 3) (6).   The viruses are nonenveloped and the icosohedral 85-nm-diameter capsid contains the 10 segments of the linear double-stranded RNA genome of approximately 25 kilobases (4, 5). The disease is found worldwide in sheep, goats, and cattle (ruminant species).  Due to the seasonal increase in the vector, disease is most prevalent mid summer to early fall. 

The disease is most problematic in sheep where mortality can approach 80%.  Factors such as severe weather, infection with parasites, poor diet, and stress are known to exacerbate mortality.  Mild infections often go unnoticed. Disease signs include fever (usually 104 to 106^oC), depression, and lassitude beginning 6 to 8 days postinfection.  The disease progresses to salivation, redness, and swelling of the mucous membranes of the mouth (Fig. 1), eventually leading to ulceration of the mouth and nose (Fig. 2 and 3) often giving a burnt appearance.  Vomiting may result, which leads to bacterial pneumonia, and is the most common cause of BTV mortality. Figure 4 shows a sheep that has purulent nasal discharge due to the broncho-pneumonia.  Coronitis (swelling of the tissue at the top of the hoof) is also common and may lead to lameness (Fig. 5) and/or sloughing of the hooves (Fig. 6).  The bluish appearance of the tongue from which the virus gets its name, is actually infrequently observed (Fig. 7).   If the animal survives, full recovery usually occurs within 10 to 14 days (1–3).  There are vaccines available but use is limited (2).  BTV in cattle is very similar to infections in sheep although morbidity of adult animals is much lower, usually about 5%.