St. Louis Encephalitis

A close-up, anterior view of a female Culex tarsalis mosquito, as it was about to begin feeding, after having landed on the skin of what would become its human host

St. Louis encephalitis (SLE) virus is spread to people by the bite of an infected mosquito. Most people infected with SLE virus do not have symptoms. Those people who do become ill may experience fever, headache, nausea, vomiting, and tiredness. Some people may develop neuroinvasive disease, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord). In rare cases, long-term disability or death can occur. There are no vaccines to prevent or medicines to treat SLE. You can reduce your risk of infection with SLE virus by using insect repellent, wearing long-sleeved shirts and long pants, and taking steps to control mosquitoes indoors and outdoors.