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Bluetongue disease is a non-contagious disease affecting ruminants, particularly sheep. It is caused by the Bluetongue virus (BTV) and transmitted by biting midges of the genus Culicoides. Bluetongue disease can devastate livestock populations, but there have been no reported cases of human infection.
Bluetongue disease has been documented in Africa, Asia, Australia, Europe, the Middle East, and the United States. It has been spreading northward since October 1998. Bluetongue disease is seasonal in Mediterranean climates, where it subsides during the winter, as midges cannot survive in the cold. The survival of the disease past the winter season is due either to midges that survive the winter in a dormant state or to transmission of the disease to the offspring of infected ruminants. If the latter is the case, the affected offspring would be asymptomatic carriers, and they would spread the disease to others through midges in the summer.
Bluetongue disease is so named because infected animals sometimes develop cyanosis, or blue coloration, of the tongue. Other common symptoms include high fever, swelling of the face, and excessive salivation. Some animals experience other symptoms, such as nasal discharge or difficulty breathing. In advanced cases, an animal with Bluetongue disease may have torsion of the head and/or lesions on the feet so severe they inhibit walking.
Some infected animals have no symptoms, but for those who do, the disease progresses quickly. After an incubation period of five to twenty days, all symptoms usually present within a month. In some breeds of sheep, the mortality rate is as high as 90%, and the sickest animals can die within a week of showing symptoms. Recovery is often a months-long process.
There is no treatment for Bluetongue disease, but it can be controlled through quarantine, vaccination, and control of the midge vector. Vaccinations are only available for some strains of BTV. Midges can be controlled by preventing midge breeding sites, often dung heaps and moist soil, from proliferating, and by keeping animals sheltered during dusk through dawn, when midges are most active.