Introduction. Bluetongue virus (BTV) is the prototype virus of the Orbivirus genus in the Reoviridae family. Orbiviruses that infect and are transmitted by arthropod vectors (e.g. gnats, ticks, mosquitoes, etc.) include viruses that may cause disease in their vertebrate hosts with serious economic consequences in some regions of the world. The BTV group, which consists of at least 24 different serotypes (BTV-1, -2, etc.), infects field and domestic animals (e.g. sheep and cattle), occasionally with high morbidity and mortality but often with almost no apparent clinical symptoms. Other orbiviruses that may cause disease in animals include African horse sickness virus (AHSV; nine serotypes, AHSV-1, -2, etc.) and epizootic haemorrhagic disease virus (EHDV; seven serotypes, EHDV-1, -2, etc.) of deer. Although BTV and AHSV were isolated in 1900, and initial morphological and biochemical characterization were reported as early as 1969 (see review article, Gorman, 1990), much of the current knowledge on the molecular biology, genome structure and encoded products has been obtained only recently. The data are predominantly based on research on BTV.
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