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St. Louis Encephalitis

  • Cynthia M. Johnson, MA
Publication Type:


St. Louis Encephalitis


St. Louis encephalitis is an infection spread by a mosquito bite. The disease can be mild, severe, or even deadly.

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

St. Louis encephalitis is more common in older adults. It is also more common in people living in or visiting the southern, central, or western United States, especially during the summer and fall.


Some people may not have symptoms of St. Louis encephalitis. Others may have a wide range of problems, such as:

  • Fever
  • Headache
  • Stiff neck
  • Lightheadedness
  • Nausea
  • Weakness
  • Uncontrolled body movements
  • Confusion
  • A loss in alertness
  • coma


The doctor will ask about symptoms and past health. A physical exam will be done.

Blood tests will be done to look for signs of infection.


There is no specific treatment for St. Louis encephalitis. Treatment depends on how severe the disease is. The goal is to manage symptoms. Options are:

  • Pain relievers
  • Anti-nausea medicine
  • Fluids—by mouth or IV
  • Breathing support


The risk of St. Louis encephalitis can be lowered by taking steps to avoid mosquito bites, such as:

  • Using insect repellent
  • Wearing long-sleeved shirts and pants that have been treated with insect repellent
  • Taking steps to control mosquitos indoors and outdoors, such as with screens and netting and by emptying any containers that may contain water where mosquitos could breed




  • 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: https://www.dynamed.com/lab-monograph/measurement-of-st-louis-encephalitis-virus-antibody.
  • Mosquito avoidance. EBSCO DynaMed website. Available at: https://www.dynamed.com/prevention/mosquito-avoidance.
  • St. Louis encephalitis virus. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/sle.


  • David L. Horn, MD, FACP
Last Updated:

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.