Frequently Asked Questions


What is St. Louis encephalitis?

St. Louis encephalitis (SLE) is a rare disease that is caused by a virus spread by infected mosquitoes. SLE virus is one of a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis).

How do people get infected with SLE virus?

SLE virus is transmitted by the bite of an infected mosquito. SLE virus is not transmitted directly from person to person.

Where and when have most cases of SLE occurred?

Cases have been reported throughout the country, but periodic outbreaks and epidemics have primarily occurred in the Mississippi Valley and along the Gulf Coast and more recently in the Southwest. In temperate areas of the United States, SLE cases occur primarily in the late summer or early fall. In southern states, cases can occur year-round.

Who is at risk for infection with SLE virus?

Anyone bitten by a mosquito in an area where the virus is circulating can get infected with SLE virus. The risk is highest for persons who engage in outdoor work and recreational activities and those living in low-income areas. Elderly persons are at increased risk of severe disease if they are infected.

How soon do people get sick after getting bitten by an infected mosquito?

It takes 4 to 14 days after the bite of an infected mosquito to develop symptoms of SLE.

What are the symptoms of SLE?

Most people who are infected with SLE virus have no symptoms or only mild non-specific flu-like illness. However, in some individuals, especially the elderly, SLE virus can cause serious illness that affects the central nervous system. Symptoms often include fever, headache, stiff neck, disorientation, and altered level of consciousness. Coma, convulsions, and paralysis may also occur.

How is SLE diagnosed?

Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.

What is the treatment for SLE?

There is no specific treatment for SLE. Antibiotics are not effective against viruses. Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections.

How can people reduce the chance of getting infected with SLE virus?

Prevent mosquito bites. There is no vaccine or preventive drug.

  • Use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing. The repellent/insecticide permethrin can be used on clothing to protect through several washes. Always follow the directions on the package.
  • Wear long sleeves and pants when weather permits.
  • Have secure, intact screens on windows and doors to keep mosquitoes out.
  • Eliminate mosquito breeding sites by emptying standing water from flowerpots, buckets, barrels, and other containers. Drill holes in tire swings so water drains out. Empty children’s wading pools and store on their side after use.

What should I do if I think a family member might have SLE?

If you or anyone in your household has symptoms that are causing you concern, consult a healthcare provider for proper diagnosis.

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