Western Equine Encephalitis

Western equine encephalitis is a disease that is spread to horses and humans by infected mosquitos. It is one of a group of mosquito-borne virus diseases that can affect the central nervous system and cause severe complications and even death. Other similar diseases are eastern equine encephalitis, St Louis encephalitis, and La Crosse encephalitis.

Infectious Agent

Western equine encephalitis is caused by the western equine encephalitis virus, an arbovirus. arbovirus is short for arthropod-borne virus. Arboviruses are a large group of viruses that are spread by certain invertebrate animals, mainly blood-sucking insects. In the United States, arboviruses are usually spread by infected mosquitos. Birds are often the source of infection for mosquitoes, which can sometimes spread the infection to horses, other animals, and in rare cases, people.

Where is western equine encephalitis found?

Western equine encephalitis is found in North, Central and South America, but most causes have been reported from the plains regions of the western and central United States.    

How do people get western equine encephalitis?

The virus that causes western equine encephalitis has a complex life cycle involving birds and specific type of mosquito. Culex tarsalis, that is common in farming areas and around irrigated fields. Humans, horses, and other mammals are not an important part of the life cycle of the virus. In rare cases, however, people who live in or visit an area where the virus lives can be infected. After inflection, the virus invades the central nervous system, including the spinal cord and brain.

What are the signs and symptoms of western equine encephalitis?

People with mild illness will often have  a headache and sometimes fever. People with more severe disease can have sudden high fever, headache, drowsiness, irritability, nausea, vomiting, followed by confusion, weakness and coma.

Who is at risk for western equine encephalitis?

Anyone can get western equine encephalitis, but some people are at increased risk:

  • People living in or visiting areas where the disease is common.
  • People who work outside or participate in outdoor recreational activities in areas where the disease is common.  ​​​

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