disease of horses that is caused by a non-oncogenic (non-cancer-causing) retrovirus. Bloodsucking insects, especially horseflies, transmit the disease. Signs, which appear about two weeks after exposure, include fever, progressive weakness, weight loss, edema, and anemia. An attack lasts three to five days. In the chronic form the fever recurs at intervals that vary from days to months. The affected animal is apt to have a shaggy coat, to be thin, weak, and sluggish, and to have swollen legs. An asymptomatic animal may carry the virus. No specific treatment or vaccine is available, but a highly accurate serologic test, the Coggins test, enables identification of infected horses. Human infection, although rare, has been reported.