St. Louis encephalitis is a viral disease spread by infected mosquitoes. This disease can affect the central nervous system, causing severe complications and even death. The virus was discovered in 1933. It is estimated to have been reported in 10,000 cases of illness and 1,000 deaths.

The Central Nervous System

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St. Louis encephalitis is caused by a virus with the same name. Mosquitoes are infected with this virus when they feed on birds, and infected mosquitoes can transmit the virus to humans and animals. St. Louis encephalitis is not spread from person to person.

Risk Factors

A risk factor is something that increases your chance of getting a disease or condition.

Some factors thought to increase the risk of St. Louis encephalitis include:

  • Elderly age
  • Living in or visiting the southern, central, or western United States, especially during the summer and fall


St. Louis encephalitis can result in a wide range of symptoms, or produce no symptoms at all. The disease can be mild, severe, or even fatal. The disease is fatal in 3%-30% of people who develop symptoms. Symptoms associated with St. Louis encephalitis usually appear 5-15 days after the bite of an infected mosquito and may include:

  • Headache
  • Fever
  • Neck stiffness
  • Stupor
  • Disorientation
  • Coma
  • Tremors
  • Convulsions (especially in infants)
  • Paralysis


Your doctor will ask about your symptoms and medical history, and perform a physical exam. He or she will also perform blood tests to identify the virus.


There is no specific treatment for St. Louis encephalitis. Treatment will focus on managing your symptoms and related complications.


There is no vaccine against St. Louis encephalitis. Prevention of this disease centers around controlling mosquitoes and avoiding mosquito bites. Steps you can take to avoid mosquitoes include:

  • Stay inside between dusk and dark. This is when mosquitoes are most active.
  • Wear long pants and long-sleeved shirts when outside.
  • Spray exposed skin with an insect repellent that contains up to 35% diethyltoluamide (DEET).