African horse sickness (AHS) is a highly infectious and deadly disease caused by African horse sickness virus. It commonly affects horses, mules, and donkeys. It is caused by a virus of the genus Orbivirus belonging to the family Reoviridae. This disease can be caused by any of the nine serotypes of this virus. AHS is not directly contagious, but is known to be spread by insect vectors. Epidemiology AHS virus was first recorded south of the Sahara Desert in the mid-1600s, with the introduction of horses to southern Africa. The virus is considered endemic to the equatorial, eastern, and southern regions of Africa. Several outbreaks have occurred in the Equidae throughout Africa and elsewhere. AHS is known to be endemic in sub-Saharan Africa, and has spread to Morocco, the Middle East, India, and Pakistan. More recently, outbreaks have been reported in the Iberian Peninsula and Thailand. AHS has never been reported in the Americas, eastern Asia, or Australasia. Epidemiology is dependent on host-vector interaction, where cyclic disease outbreaks coincide with high numbers of competent vectors. The most important vector for AHS in endemic areas is the biting midge Culicoides imicola, which prefers warm, humid conditions. Larvae do not carry the virus, and long, cold winters are sufficient to break epidemics in nonendemic areas. Host The common hosts of this disease are horses, mules, donkeys, and zebras. However, elephants, camels, and dogs can be infected, as well, but often show no signs of the disease and it is very rare. Dogs usually contract the disease by eating infected horse meat, although a recent report has been made of the disease occurring in dogs with no known horse-meat ingestion. Transmission This disease is spread by insect vectors. The biological vector of the virus is the Culicoides (midges) species, but this disease can also be transmitted by species of mosquitoes including Culex, Anopheles, and Aedes including A. aegypti, and species of ticks such as Hyalomma and Rhipicephalus. Clinical signs Horses are the most susceptible host with close to 90% mortality of those affected, followed by mules (50%) and donkeys (10%). African donkeys and zebras very rarely display clinical symptoms, despite high virus titres in blood, and are thought to be the natural reservoir of the virus. AHS manifests itself in four different forms: Pulmonary form The peracute form of the disease is characterized by high fever, depression, and respiratory symptoms. The clinically affected