bluetongue

n.
A viral disease of sheep and cattle that is transmitted by biting insects and is characterized by fever, the formation of oral lesions, and swelling and cyanosis of the lips and tongue.

An arthropod-borne disease of ruminant species. Its geographic distribution is dependent upon a susceptible ruminant population and climatic conditions that favor breeding of the primary vector, a mosquito (Culicoides species).

Bluetongue virus is the prototype of the genus Orbivirus (family Reoviridae). The viral genome exists as 10 segments of the double-stranded ribonucleic acid (RNA) that encode for seven structural and three nonstructural proteins. The viral particle has a double capsid, with the outer coat (morphologically poorly defined) being composed of two proteins. Twenty-four serotypes of bluetongue virus have been defined, and their distribution throughout the world is varied. See also Animal virus.

While multiple ruminant species can become infected, only sheep and deer typically display clinical bluetongue disease. Severity of the disease is dependent upon multiple factors, including virus strain, animal breed, and environmental conditions. Upon infection by a gnat bite, the virus apparently replicates in the local lymphatic system prior to the viral particles moving into the blood (viremia). Viral replication occurs in the endothelial cells of small vessels, resulting in narrowing of the vessel, release of proteinaceous material into the surrounding tissues, and possibly hemorrhage, with the respiratory tract, mucous membranes, cardiac and skeletal musculature, and skin being most affected. Animals experiencing acute clinical symptoms typically die from pneumonia or pulmonary failure; hemorrhage at the base of the pulmonary artery indicates the presence of a vascular lesion.

Control of bluetongue disease requires the application of vaccines and modulation of the farm environment. While blue-tongue virus vaccines are available, efficacy is often incomplete and variable, in part because of the multiplicity of serotypes active throughout the world and limited cross-serotype protection. Furthermore, use of polyvalent (multiple-serotype) vaccines in the United States has been discouraged because of potential genetic reassortment between vaccine viruses and wild-type viruses, a process that could possibly lead to pathogenic variants. Relative to environment, elimination of vector breeding sites can also facilitate control of virus transmission. With the multiplicity of serotypes typically active in an endemic area, and the minimal cross-serotype protection observed, administration of vaccine in the face of an outbreak may be of limited value. See also Vaccination.

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An infectious, non-contagious disease of sheep and occasionally cattle, transmitted by Culicoides spp. Caused by an Orbivirus with at least 24 serotypes worldwide. Cattle are the reservoir and amplification hosts. Severe disease is restricted to fine wool and mutton breeds of sheep. Infection, but not disease, is endemic in tropical and subtropical regions. Disease occurs in epidemic and incursive areas when climatic conditions allow the expansion of vector occurrence. Currently, this is occurring in southern Europe associated with global warming. Manifest with fever, catarrhal stomatitis, rhinitis, enteritis and lameness due to a coronitis and myositis. High case fatality can occur in sheep. Congenital infections with wild or vaccine virus may result in defects in the nervous system, the type and severity depending on the stage of gestation.

• b. virus (BTV) — a member of the family Reoviridae, genus Orbivirus. Causes bluetongue in sheep and other ruminants. Within the BTV serogroup there at least 24 serotypes of BTV and considerable genetic variability