severe viral disease of horses and mules. It sometimes affects birds, reptiles, and humans.

Of the several strains of the virus, the most prevalent are the A group, which includes the Eastern, Western, and Venezuelan strains, and the B group, which includes the Japanese and St. Louis strains. The virulent Western type has a mortality as high as 90 percent in horses and 10 percent in humans.

Immunity lasting one year is conferred by recovery from the disease or by vaccination with killed vaccine. Birds appear to harbour the disease but do not exhibit any definite symptoms. The mosquito transmits the virus from birds to horses, mules, or humans. Other animals may be infected in the same way but not show symptoms. Sheep and cats appear to be resistant.

Symptoms in horses include disturbances in equilibrium, high fever, incoordination, and paralysis; those in humans include headache, drowsiness, sweating, and mental confusion. Because of the symptoms the disease is often mistakenly called sleeping sickness. Treatment is aimed at making the victim comfortable. The disease can cause permanent brain damage. Control centres on eradication of mosquito carriers and the use of vaccines against the Eastern and Western strains.