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St. Louis encephalitis (SLE) virus normally infects birds and is transmitted between birds by mosquito vectors. Thus it is an arthropod-borne virus (arbovirus). SLE virus is found throughout the Americas, and human disease has been reported from the nor- thern part of the hemisphere. Human epidemics have been reported only from northern Mexico (Gonzalez Cortes, et al. 1975), the United States, and southern Canada (CDC 1977). Other viruses closely related to SLE (dengue, yellow fever, and Rocio viruses) cause epidemics in Central and South America. In the United States, SLE is the most important mosquito-borne disease of humans in terms of the number of cases and fatalities, and birds are the usual source of SLE virus for mosquitoes that infect humans. Human cases have been reported from all regions of the U.S. and from most of the states, but epidemics occur predominantly in the Mississippi and Ohio River watersheds, Texas, and Florida. The intensity of human cases oscillates dramatically over a 6-10 year period, with epidemic levels of disease spanning several years. The last peak of human SLE occurred in 1975; and only a few sporadic cases have been reported in the past three years (Vector-Borne Diseases Division 1977a, 1978a, 1979a). The infection in humans has a spectrum of responses: no apparent illness; mild illness with slight fever; febrile headache, often with nausea and vomiting; aseptic meningitis with sudden onset of fever and neck stiffness; and encephalitis with fever and symptoms of disorientation, confusion, stupor or coma, tremors of the face or hands, neck rigidity with or without paralysis, and occasionally death, particularly in individuals over 55 years of age (Brinker and Monath 1980). Human infection and disease are a result of an extension of SLE virus transmission out of the natural bird-mosquito cycle. Humans are dead- end hosts for the virus and therefore do not contribute to additional transmission or maintenance.