Western Equine Encephalitis Fact Sheet
Revised July 2013
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What is Western equine encephalitis?
Western equine encephalitis (WEE) is an illness transmitted to people and horses through the bite of a mosquito infected with WEE virus.
WEE is normally maintained between Culex tarsalis mosquitoes (1 of 50 species in Minnesota) and birds. People and horses are bitten by Culex tarsalis during the late summer months (mid-July through early September).
How serious is Western equine encephalitis?
Most people infected with WEE virus will have no symptoms or a very mild illness. A small percentage of people, especially infants and possibly elderly people may develop encephalitis (inflammation of the brain). Approximately 5-15% of these encephalitis cases are fatal, and about 50% of surviving infants will have permanent brain damage.
Most of the severe human cases begin with a sudden onset of fever, headache, stiff neck, vomiting, and lethargy. Within two to four days, the illness may progress to disorientation, irritability, seizures and coma. There is no treatment for WEE other than supportive care until the acute phase of the illness is over.
Horses and humans are often referred to as "dead-end" hosts for WEE, because the virus does not build to high enough levels in our blood to infect other mosquitoes that bite us.
What is the risk of a Western equine encephalitis outbreak in Minnesota?
During 1941 there was a large regional outbreak (including several states and Canada) of WEE. There may have been as many as 791 cases in Minnesota that year with 90 deaths. In recent years, Minnesota has had infrequent and smaller outbreaks of WEE (15 human cases in 1975, single cases in 1983 and 1999).
WEE is most commonly reported from states and Canadian provinces west of the Mississippi River. During past Minnesota outbreaks, the virus has been found over much of western and southern Minnesota. Culex tarsalis mosquitoes are often abundant in this area because they are able to use semi-permanent grassy wetlands in agricultural parts of the state as breeding sites.
How can people prevent Western equine encephalitis?
If people engage in outdoor activities at dusk and dawn they can wear long sleeved shirts and long pants. They can also use mosquito repellents containing DEET (up to 30% DEET) according to label instructions.
People can reduce their risk of WEE significantly by avoiding outdoor activities at dusk and dawn (the primary feeding period of Culex tarsalis mosquitoes).A WEE vaccine is available for horses. Please contact your veterinarian for vaccine recommendations. However, there is no Western equine encephalitis vaccine for humans.