St. Louis Encephalitis

Culex-Nig Mosquito

Transmission & History

St. Louis Encephalitis (SLE) virus is a flavivirus that is transmitted to humans and other vertebrates primarily by mosquitoes of the genus Culex. Infection with SLE results in inapparent infection in a variety of birds and mammals with a resultant period of viremia and lasts a matter of days. Humans represent an incidental, deadend host. The first human disease outbreak recognized to be caused by the SLE virus occurred in St. Louis, Missouri, in 1933.

Virus Cycle & Bird Population

The annual cycle of the SLE virus in Florida is still not completely understood. We know, however, that wild bird populations are central to the transmission cycle. When a bird becomes infected by the bite of a carrier mosquito, it produces more virus in its blood that can infect other susceptible mosquitoes that bite the bird. These mosquitoes can then further spread the virus in one or two days, but the virus disappears two to three days later, so a bird remains “infectious” for only a few days. Birds do not show any symptoms of disease and become immune after exposure.


For more information please visit the Florida Department of Health website and the Florida Cooperative Extension's Electronic Information website.