St. Louis encephalitis

St. Louis encephalitis (SLE), is the mosquito-transmitted virus disease historically of greatest medical importance in North America. SLE was first recognized in 1933 in St. Louis, Missouri, and epidemics have occurred sporadically and unpredictably in the subsequent decades.

In the summer, SLE virus is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and Culex mosquitoes. In Florida, the principal vector is Cx. nigripalpus, in the Midwest, Cx. pipiens pipiens and Cx. p. quinquefasciatus and in the Western United States, Cx. tarsalis and members of the Cx. pipiens complex. Humans and other mammals are not an important part of the life cycle of the virus. However, people who live in or visit an area where the virus occurs can be infected by the bite of an infected mosquito. After infection, the virus invades the central nervous system, including the spinal cord and brain.

Although periodic SLE epidemics occur only in the Midwest and Southeast, SLE virus is distributed throughout the contiguous 48 States. Since 1964, there have been 4,437 confirmed SLE infections, with an average of 193 infections per year (range 4 - 1,967). However, less than 1% of SLE viral infections are clinically apparent and the vast majority of infections remain undiagnosed. Illness ranges in severity from a simple febrile headache to meningoencephalitis, with an overall case-fatality ratio of 5-15 %. The disease is generally milder in children than in adults, but in those children who do have disease, there is a high rate of encephalitis. The elderly are at highest risk for severe disease and death.

No human vaccines are commercially available for this disease. Arboviral encephalitis can be prevented in two major ways: personal protective measures and public health measures to reduce the population of infected mosquitoes. Personal protective measures include reducing time outdoors, particularly in early evening hours; wearing long pants and long sleeved shirts, and applying mosquito repellent to exposed skin areas. Public health measures often require insecticide spraying.