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).
The first historical reference to horse sickness was in Yemen in 1327. It was mistaken for anthrax (1881 and 1895), then for a malaria strain (1888, 1895, 1900, 1911), then heartwater (1900, 1904), because of the high degree of mortality. M'Fadyean (1900), Theiler (1901) and Nocard (1901) isolated the causal agent as belonging to the group of ultravisible viruses. First attempts to immunize horses: 1900, 1918, 1925.
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.
High fatality rate: 50-95%. In enzootic areas disruption to agriculture and transport has been met by the replacement of horse-drawn traffic by mechanization or other means, but when the disease appears for the first time, agriculture is disrupted with resulting widespread hardship (Iran, India, Middle East, Pakistan). Development of irrigation systems and conservation of surface water, creating favourable conditions for the perpetuation of insect populations, provides a serious threat to the continued use of horses and mules in farming systems.