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Saint Louis Encephalitis

Saint Louis encephalitis (SLE) is a viral disease spread to people by the bite of an infected mosquito. Most people infected with Saint Louis encephalitis virus have no apparent illness. Initial symptoms of those who become ill include fever, headache, nausea, vomiting, and tiredness. Severe neuroinvasive disease (often involving encephalitis, an inflammation of the brain) occurs more commonly in older adults. In rare cases, long-term disability or death can result. There are no vaccines to prevent nor medications to treat Saint Louis encephalitis. Care is based on symptoms. You can reduce your risk of infection with Saint Louis encephalitis virus by using insect repellent, wearing long-sleeved shirts and long pants, and taking steps to control mosquitoes indoors and outdoors.


The most effective way to prevent infection from ­­­Saint Louis Encephalitis virus is to prevent mosquito bites. Mosquitoes bite during the day and night. Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and take steps to control mosquitoes indoors and outdoors.


Less than 1% of St. Louis encephalitis virus infections are clinically apparent and the vast majority of infections remain undiagnosed. The incubation period for St. Louis encephalitis virus disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days. Onset of illness is usually abrupt, with fever, headache, dizziness, nausea, and malaise. Signs and symptoms intensify over a period of several days to a week. Some patients spontaneously recover after this period; others develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness. Coma can develop in severe cases. The disease is generally milder in children than in older adults. About 40% of children and young adults with St. Louis encephalitis virus disease develop only fever and headache or aseptic meningitis; almost 90% of elderly persons with St. Louis encephalitis virus disease develop encephalitis. The overall case-fatality ratio is 5 to 15%. The risk of fatal disease also increases with age.


No vaccine against St. Louis encephalitis virus infection or specific antiviral treatment for clinical St. Louis encephalitis virus infections is available. Patients with suspected St. Louis encephalitis should be evaluated by a healthcare provider, appropriate serologic and other diagnostic tests ordered, and supportive treatment provided.


Preliminary diagnosis is often based on the patient’s clinical features, places, and dates of travel (if patient is from a non-endemic country or area), activities, and epidemiologic history of the location where infection occurred.

Laboratory diagnosis of arboviral infections is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect virus-specific IgM and neutralizing antibodies.

In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry and virus culture of autopsy tissues can also be useful. Only a few state laboratories or other specialized laboratories, including those at CDC, are capable of doing this specialized testing.

Instructions for sending diagnostic specimens to the DVBD Arbovirus Diagnostic Laboratory can be found at the following site: Instructions for Sending Diagnostic Specimens to the DVBD Arbovirus Diagnostic Laboratory.

Test results are normally available 4 to 14 days after specimen receipt. Reporting times for test results may be longer during summer months when arbovirus activity increases. Receipt of a hard copy of the results will take at least 2 weeks after testing is completed. Initial serological testing will be performed using IgM capture ELISA, MIA (Microsphere-Based Immunoassay), and IgG ELISA. If the initial results are positive, further confirmatory testing may delay the reporting of final results. ALL RESULTS WILL BE SENT TO THE APPROPRIATE STATE HEALTH DEPARTMENT. Notify your state health department of any submissions to CDC.


St. Louis encephalitis virus is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and Culex species mosquitoes. Wild birds are the primary vertebrate hosts. Birds sustain inapparent infections but develop viremias (i.e., virus in their blood) sufficient to infect the mosquito vectors. Birds that are abundant in the urban-suburban environment, such as the house sparrow, pigeon, blue jay, and robin, are principally involved. The principal vectors are Cx pipiens and Cx quinquefasciatus in the east, Cx nigripalpus in Florida, and Cx tarsalis and members of the Cx pipiens complex in western states. Humans and domestic mammals can acquire St. Louis encephalitis virus infection, but are dead-end hosts.

Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD). Accessed July 17, 2020.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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