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St Louis Encephalitis Virus

Saint Louis encephalitis virus (SLEV) is an arthropod-borne virus (arbovirus) that is primarily transmitted in Indiana by mosquitoes in the genus Culex. While most people who become infected with SLEV do not develop any symptoms, some people can develop severe inflammation in the brain, spinal cord, or other parts of the nervous system.

St. Louis encephalitis virus. Centers for Disease Control and Prevention.

  • Transmission

    SLEV is transmitted in Indiana by mosquitos in the genus Culex. Mosquitoes become infected when they feed on wild birds. Infected mosquitoes can then spread SLEV to people and other mammals. Once infected, people and other mammals are “dead-end hosts,” which means that they do not pass the virus on to other biting mosquitoes.

    SLEV transmission cycle. Graphic: Centers for Disease Control and Prevention.

  • Signs and Symptoms

    Most people (about 99%) infected with SLEV do not develop any symptoms.

    Some people (about 1%) develop a fever and other symptoms. Signs and symptoms of SLEV disease usually appear within 5 – 15 days of a bite from an infected mosquito. The onset of symptoms is usually abrupt, with fever, headache, dizziness, nausea, and malaise. Signs and symptoms intensify over a period of several days to a week. Some people will spontaneously recover after this period; others develop signs of central nervous system infections. SLEV disease is generally milder in children than in older adults. Almost 90% of elderly people with SLEV disease develop encephalitis (inflammation of the brain). SLEV disease is fatal in approximately 5-15% of cases. The risk of fatal disease increases with age.

  • Diagnosis

    Diagnosis of SLEV disease is based on the patient’s signs and symptoms and appropriate laboratory testing. If you think you have SLEV disease, contact your healthcare provider.

  • Treatment

    No specific medication is available to treat SLEV disease. People with severe illness usually require hospitalization, supportive care, and/or rehabilitation.

  • Prevention

    The best way to prevent SLEV disease is to avoid mosquito bites. Please see our mosquito prevention page for more information.

    For more information about St. Louis encephalitis virus, visit the CDC SLEV webpage.

  • Maps and Statistics

    SLEV disease is rare in Indiana. No cases were reported from 2013–2017. For more information about infectious diseases in Indiana, please visit:

    For maps showing recent SLEV infections in people, horses, and mosquitoes, click here.

    National statistics for SLEV disease can be found at the CDC SLEV Statistics and Maps webpage.

  • Resources
    Information for Providers

    For SLEV disease diagnosis, treatment, and testing information, click here.