African horse sickness is a highly infectious and devastating disease that causes great suffering and many fatalities in equids. It commonly affects horses, mules, donkeys and dogs. The disease is caused by nine different serotypes of the African horse sickness virus, ASHV, genus Orbivirus (Reoviridae), and it is spread mainly by hematophagous arthropods of the Culicoides. Some crossreactions are observed between 1 and 2,3 and 7,5 and 8, and 6 and 9 serotypes. Clinical forms of AHS include peracute pulmonary, subacute cardiac, and mixed as well as subclinical (horse sickness fever). The most severe, with mortality rates exceeding 95%, is the pulmonary form, accompanied by fever, mild depression, sweating, spasmodic coughing, anorexia and respiratory distress. The subacute cardiac form with a mortality of about 50%, is characterized by fever, swelling of the head, neck and supraorbital fossae and sometimes, petechial hemorrhages in the eyes. The mildest form of the disease is generally not fatal and is accompanied by a low grade fever, anorexia, depression and congestion of the mucous membranes. The most common cases with a 70% mortality rate, are mixture of the pulmonary and cardiac forms. Differential diagnosis include equine encephalosis virus (also Orbivirus), equine infectious anemia, equine viral arteritis, anaplasmosis, babesiosis, or theileriosis. Clinical AHS cases have also been described in dogs, with acute respiratory distress syndrome or sudden death. Diagnosis of the disease is based on typical clinical signs and lesions, a history consistent with vector transmission and confirmation by laboratory detection of virus and/or anti-virus antibodies. Currently, prevention and control of African horse sickness is based on administration of live attenuated vaccines and control of the arthropod vectors.