African horse sickness (AHS) is a serious arthropod-borne viral disease of equids, with a mortality rate that can reach 95% in some species, such as horses. At present, AHS virus (AHSV) is only endemic in Africa; however, suitable vectors exist outside this area, and infected animals or vectors may carry the virus into AHS-free regions. The potential for dissemination is especially high in animals that tend to develop mild or subclinical infections, such as zebras (Equus burchelli) and donkeys, or horses with partial immunity. One extensive epidemic in 1959–1961 affected the Middle East and parts of Asia, as well as Africa, and is thought to have been responsible for the deaths of 300,000 equids. An outbreak in Spain lasted from 1987, when the virus was introduced in imported zebras, to 1990, and spread to Portugal and Morocco. Within Africa, additional AHSV serotypes have recently spread to some areas where only one serotype was previously found. Although vaccines are available, cross-protection between serotypes is limited, and the introduction of a new serotype into an area may result in outbreaks.