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Saint Louis Encephalitis

Saint Louis encephalitis virus (SLEV) is a Flavivirus that is transmitted to humans by the bite of an infected mosquito. Other viruses that belong to the genus Flavivirus include West Nile, Zika and dengue. The virus is cycled from mosquitoes, to birds, and back to mosquitoes. Once a mosquito has fed upon an infected bird, it becomes infected as well and can then spread the virus to humans. The birds that are most commonly infected with SLEV are found in urban and suburban environments including sparrows, pigeons, blue jays, and robins. The most common mosquito that vectors SLEV belongs to the species Culex. In the east United States, SLEV is vectored by Culex pipiens and Culex quinquefasciatus, Culex nigripalpus in Florida, and Culex tarsalis and Culex pipiens in the west. In the United States, the Center of Disease Control (CDC) reports that there were 14 neuroinvasive disease cases of SLEV in 2019. Three human cases of SLE have been reported in Alabama since 2008.

About Saint Louis Encephalitis Infection

  • Often when people become infected they show no symptoms.
  • The incubation period for this virus is usually 4-14 days.
  • Symptoms of onset illness include fever, headache, dizziness, nausea and malaise.
  • Most commonly, severe cases of SLEV occur in elderly people.
  • Cases of children with SLEV are generally milder than cases in elderly.
  • Symptoms intensify over a period of several days to a week, and may improve after this period.
  • In cases that do not improve after about a week, the patient may develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors and unsteadiness.
  • In severe cases coma can occur.
  • The CDC reports among patients diagnosed with SLEV, 5 to 15% die.
  • The risk of fatality from SLEV increases with age.
  • People that live in or visit areas that have the Culex mosquitos that vector SLEV are at risk of infection.


  • There is no vaccination for SLEV.
  • There are no specific antiviral medications.
  • Suspected SLEV cases should be evaluated by a healthcare provider.
  • If SLEV is suspected, diagnostic tests and appropriate serologic tests should be ordered, and proper supportive treatment administered.
  • If you or your family member develop the symptoms of SLEV contact your healthcare provider.

Detect, Test, and Report

The best preventive measure for residents living in areas infested with disease vectoring mosquito species is to eliminate the places where the mosquito lays her eggs, primarily artificial containers that hold water.

Items that collect rainwater or to store water (for example, plastic containers, 55-gallon drums, buckets, or used automobile tires) should be covered or properly discarded. Pet and animal watering containers and vases with fresh flowers should be emptied and cleaned (to remove eggs) at least once a week. This will eliminate the mosquito eggs and larvae and reduce the number of mosquitoes present in these areas. For more info on how to control mosquitos around your home Vector Control.

Using air conditioning or window and door screens reduces the risk of mosquitoes coming indoors. Proper application of mosquito repellents on exposed skin and proper clothing decreases the risk of being bitten by mosquitoes. See Avoid the Bite for more information about preventing mosquito-borne diseases. To learn more about insect repellents, visit this Environmental Protection Agency website: Insect Repellent Information from the EPA.

Related Links

For questions or concerns regarding insect-borne disease in Alabama, see the Contact Us page, or email us at

Page last updated: December 7, 2021