Western equine encephalitis (WEE) is a viral infection spread by infected mosquitoes. This disease can affect the central nervous system, causing severe complications and even death.

The Central Nervous System

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WEE is caused by the western equine encephalitis virus, which is one of a group of viruses called arboviruses. In the United States, these viruses are usually spread by infected mosquitoes.

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 WEE include:

  • Living in or visiting the plains regions of the western and central United States
  • Working outdoors
  • Participating in outdoor activities


WEE results in a wide range of symptoms and may produce no symptoms at all. The disease can be mild, severe, or even fatal. The disease is fatal in about 3% of people who develop symptoms. Symptoms associated with WEE usually appear 5-10 days after the bite of an infected mosquito and may include:

  • Headache
  • Fever
  • Drowsiness
  • Irritability
  • Nausea
  • Vomiting
  • Confusion
  • Weakness
  • Coma
  • Seizures (in young infants)


Your doctor will ask about your symptoms and medical history, and perform a physical exam. Other tests may include:


Because the infection is viral, there is no specific treatment for WEE. Treatment will focus on managing your symptoms and related complications.


There is no vaccine against WEE for humans, although there is a vaccine for horses. Prevention of WEE focuses on controlling mosquitoes and avoiding mosquito bites. Steps you can take to avoid mosquito bites include:

  • Stay inside between dusk and dark, 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).