African horse sickness is an acute or subacute febrile disease of equines found in humid or subhumid zones of tropical Africa caused by a virus with a number of antigenially different strains. It is transmitted by night-flying Culicoides spp and probably has a reservoir host. It results in the impaired function of the circulatory and respiratory systems, and gives rise to serious effusions accompanied by haemorrhages in various organs and tissues of the body. Recovered animals develop immunity to the particular strain of virus. Because of the association with the insect vector, the disease prevails in certain areas (warm, moist, low-lying or coastal regions, valleys, swamps or river basins) and during certain seasons (rainy season - intermittent rain followed by summer temperatures).
Horse sickness originated in the African continent and is widely distributed over the central and southern parts. In the Near East a major epidemic occurred in 1959 and spread to Pakistan and India; mortality was high.