St. Louis encephalitis virus (SLEV) is a type of flavivirus that is similar to West Nile virus (WNV), which are both members of the Japanese encephalitis antigenic complex. SLEV is widely distributed in the Americas, particularly South America. SLEV was first isolated in 1933 during a major epidemic in St Louis, Missouri, USA.
SLEV is transmitted to horses by infected mosquitoes, and maintained in cycles between birds and Culex mosquitoes. Horses and humans serve as the dead-end hosts of the virus.