African horse sickness (AHS) is a noncontagious viral disease affecting all species of equidae. It is caused by one or more of the nine different serotypes of an orbivirus of the family Reoviridae. The disease is characterised by alterations in respiratory and circulatory functions. While the disease is usually fatal in horses, other equine species and their crossbreds may display only mild clinical signs.

Transmission : because AHS is transmitted by at least two species of Culicoides midges, its occurrence is also affected by the seasonal changes in climatic and environmental conditions of the vector's habitat.

Occurrence : all serotypes of AHS virus occur in eastern and southern Africa; this distribution reflects that of zebra, which cycle the virus asymptomatically. In West Africa only AHS serotype 9 is found. Examples of the occasional outbreaks that have occurred outside sub-Saharan Africa are the pandemic in certain countries of the Middle East up to India and Turkey (1959-1961), the outbreaks in Spain (serotype 9, 1966) and the epidemic caused by serotype 4 in Spain (1987-1990), Portugal (1989) and Morocco (1989).

Laboratory diagnosis of AHS, based on the identification of infectious virus, virus nucleic acid, viral antigens or specific antibodies, is of great importance, as the characteristic clinical signs and lesions can be confused with those of other diseases.

It is a List A disease, according to the OIE Classification of Diseases. This means it is a transmissible disease that has the potential for very serious and rapid spread, irrespective of national borders, that is of serious socio-economic or public health consequence and that is of major importance in the international trade of animals and animal products.