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Western Equine Encephalitis

Where Is Western Equine Encephalitis Found?

This disease is found in North, Central, and South America, but most cases have been reported from the plains regions of the western and central United States.

How Is It Transmitted?

The virus that causes western equine encephalitis has a complex life cycle involving birds and a 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 western equine encephalitis virus. In rare cases, however, people who live in or visit an area where the virus lives can be infected by the bite of an infected mosquito. Horses are common in these regions and can also be infected.
After infection, the western equine encephalitis virus invades the central nervous system, including the spinal cord and brain.

What Are the Symptoms of Western Equine Encephalitis?

Infection can cause a range of illnesses, from no symptoms to fatal disease. People with mild western equine encephalitis often have only a headache or sometimes, a fever. People with more severe western equine encephalitis can have sudden high fever, headache, drowsiness, irritability, nausea, and vomiting, followed by confusion, weakness, and coma. Young infants often have seizures.
Symptoms usually appear in 5 to 10 days after the bite of an infected mosquito.

Making a Diagnosis

Diagnosis of western equine encephalitis is based on tests of blood or spinal fluid.

Who's at Increased Risk for Western Equine Encephalitis?

While anyone can get western equine encephalitis, some people are at increased risk, including those who:
  • Live in or visit areas where the disease is common
  • Work outside in areas where the disease is common
  • Participate in outdoor recreational activities in areas where the disease is common.
Western equine encephalitis occurs in all age groups.
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