Until the introduction of West Nile Virus into the US in 1999, St. Louis Encephalitis (SLE) was the most important epidemic mosquito-borne
viral disease in the United States and was second in the State of Colorado after
Western Equine Encephalitis (WEE). The
last major epidemic in the U.S. was in 1975 when 1,815 cases and 142 deaths were
reported. SLE continues to occur, 3,615 cases were reported nationally between
1975 and 1984. SLE is primarily found across the Midwest, The East and Gulf
Coasts, and in California. Colorado last reported 6 human cases in 1987. SLE
virus causes acute illness in man with a wide spectrum of central nervous system
manifestations ranging from fever and headache, to fatal meningoencephalitis.
SLE is not pathogenic in horses. Treatment of patients is supportive only. Case
fatality rates vary from four to twenty percent. Resultant neurologic
dysfunction has been reported in only a small percentage of cases. During
epidemics, the disease strikes urban and suburban areas, and effects primarily
older persons in lower socio-economic areas. SLE virus is also a disease of
birds and is transmitted from bird to bird by several species of mosquito.
Humans may become infected through the bites of certain mosquitoes which have
previously fed upon birds carrying the virus. The northern house mosquito, Culex
pipiens is the primary vector of SLE to man. This mosquito is a very common
species in many areas of the United States.