Western Equine Encephalitis - Defense Centers for Public Health

Western equine encephalitis (WEE) is a disease that is spread to humans and horses by infected mosquitoes. It is one of a group of mosquito-borne viral diseases that can affect the central nervous system and cause severe complications and even death. Other similar diseases are West Nile virus (WNV), Eastern equine (EEE), St. Louis (SLE) and La Crosse encephalitis. WEE is an infection found throughout North, Central and South America. In the United States, the disease is most commonly reported from the plains regions of the Western and Central United States. WEE is a rare disease in humans, the Centers for Disease Control and Prevention (CDC) reports a total of 639 confirmed cases in the U.S. since 1964.

Western Equine Encephalitis

Culex tarsalis. Image by Joseph Berger

How do people become infected with Western equine encephalitis?

People contract WEE primarily through the bite of infected Culex tarsalis mosquitoes. This mosquito prefers biting birds in the spring and large mammals (including humans) as the summer progresses. Culex tarsalis only bites at dawn and dusk. Mosquitoes acquire this virus by feeding on infected birds. Infected

mosquitoes then transmit the virus to more birds, as well as to humans and horses. After a bite from an infected mosquito, the virus invades the persons' central nervous system. WEE is NOT transmitted from person-to-person. For example, you cannot get the virus from touching or kissing a person who has the disease or from a health care worker who has treated someone with WEE.

Can you get Western equine encephalitis directly from birds or other animals?

There is no evidence that a person can get WEE from handling either live or dead infected animals. However, as a general precaution, always wear gloves when handling dead animals, including dead birds and use double plastic bags when disposing of them.

What are the symptoms of Western equine encephalitis infection?

WEE occurs in all age groups. Infection with the WEE virus can cause illness ranging from very mild to fatal. Symptoms usually appear 5 to 10 days after the bite of an infected mosquito. People with mild illness often experience only a headache and sometimes fever. People with the more severe form of the disease, can exhibit sudden high fever, headache, drowsiness, irritability, nausea and vomiting; followed by confusion, weakness, coma and death. Infants with WEE infection often suffer seizures. Major complications, including brain damage, are reported in about 13% of infected persons overall and in about one-third of infants. The disease is fatal in about 3% of people who develop severe symptoms.

Is there a treatment for Western equine encephalitis infection?

There is no specific medical treatment for WEE. Therapy is supportive only, directed at relieving the symptoms and preventing complications. Diagnosis is based on tests of blood or spinal fluid. There is no vaccine for humans, but a vaccine is available for horses.

What should I do if I think i have Western equine encephalitis?

If you develop symptoms such as sudden high fever, unusually severe headaches, nausea or vomiting seek medical attention immediately. Individuals with suspected WEE illness should be evaluated by a healthcare provider.

How can Western equine encephalitis be prevented?

The best way to prevent diseases spread by mosquitoes is to protect yourself from mosquito bites by using the DoD Insect Repellent System. It incorporates insect repellent on the uniform, DEET, picaridin, or IR3535 repellent on exposed skin, a properly worn uniform, and sleeping inside a insecticide-treated bed net.

Select the image for more information on the DoD Insect Repellent System.

Graphic showing the DoD Insect Repellent System