African horse sickness results from infection with the African horse sickness virus (AHSV), a member of the genus Orbivirus in the family Reoviridae. There are nine serotypes of AHSV. Serotype 9 is widespread in endemic regions, while serotypes 1 to 8 are found only in limited geographic areas. African horse sickness is a serious, often fatal, arthropod-borne viral disease. The mortality rate can be as high as 95% in some forms of this disease. The African horse sickness virus can infect horses, donkeys, mules, zebras, camels and dogs. The most serious infections occur in horses and mules, which appear to be accidental hosts.