St. Louis Encephalitis


St. Louis encephalitis is an infection from a mosquito. It can range from mild to life-threatening.

The Central Nervous System

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St. Louis encephalitis is caused by a virus. It is spread to humans from the bite of an infected mosquito. The virus is not spread from person to person.

Risk Factors

Things that may raise the risk of St. Louis encephalitis are:

  • Older age
  • Living in or visiting the southern, central, or western US, especially during summer and fall


St. Louis encephalitis can cause a wide range of symptoms. However, sometimes there are no symptoms. The disease can be mild, severe, or even fatal.

Symptoms may be:

  • Fever
  • Headache or stiff neck
  • Feeling dizzy or lightheaded
  • Nausea
  • Weakness
  • Uncontrolled body movements
  • Loss of control of muscles
  • Confusion, lack of responsiveness, or coma


The doctor will ask about your symptoms and health history. A physical exam will be done. Blood tests are usually used to diagnose the virus.


There is no specific treatment for St. Louis encephalitis. Treatment depends on how severe the disease is. The goal is to manage symptoms and problems. Depending on the symptoms, options may be:

  • Pain medicines—for headaches
  • Anti-nausea medicines
  • Fluids—by mouth or IV
  • Breathing support


To reduce the risk of St. Louis encephalitis:

  • Avoid mosquito bites by:
    • Covering up the skin
    • Using bug sprays, netting, and screens
    • Staying inside between dusk and dark


Centers for Disease Control and Prevention
National Institute of Allergy and Infectious Diseases


Alberta Ministry of Health


Curren EJ, Lindsey NP, et al. St. Louis encephalitis virus disease in the United States, 2003-2017. Am J Trop Med Hyg. 2018;99(4):1074-1079.
Measurement of St. Louis encephalitis virus antibody. EBSCO DynaMed website. Available at: Accessed April 2, 2021.
Mosquito avoidance. EBSCO DynaMed website. Available at: Accessed April 2, 2021.
Saint Louis encephalitis fact sheet. Centers for Disease Control and Prevention website. Available at:
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Accessed April 2, 2021.
Last reviewed December 2020 by EBSCO Medical Review Board David L. Horn, MD, FACP
Last Updated: 4/2/2021

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