St. Louis encephalitis virus (SLE), a member of the flavivirus family, was the most important mosquito-borne arbovirus in North America up until 1999, when West Nile virus (WNV) was introduced into the United States. Wild birds are the maintenance and amplifying hosts of SLE, which is transmitted among birds and to humans by primarily Culex mosquitoes. Human infection with SLE can result in mild to severe illness, with case-fatality rates ranging from 3%-30%.
SLE was first recognized in 1933 in St. Louis, Missouri, where it caused over 1000 human cases and 200 fatalities. The virus is found throughout the U.S., Canada, and nothern Mexico, and causes periodic epidemics of encephalitis, with outbreaks generally occuring between August and October. An average of 128 cases are reported annually in the U.S. The most recent outbreak occured in 1999, in New Orleans, Louisiana, with 20 reported cases.
SLE in California
Since 1945, 597 human cases of SLE have been reported in California. The most recent outbreaks occurred in 1984 and 1989 in the Los Angeles Basin (26 cases) and the southern San Joaquin Valley (29 cases), respectively. The last human case reported was in 1997, from Los Angeles County. SLE activity has not been detected in mosquito pools or sentinel chickens since 2003.