Eastern Equine Encephalitis Fact SheetRevised July, 2013
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What is Eastern equine encephalitis?
- Eastern equine encephalitis (EEE) is an illness people and horses can develop after being bitten by a mosquito infected with EEE virus.
- EEE is maintained in a transmission cycle involving Culiseta melanura mosquitoes (1 of 50 mosquito species in Minnesota) and birds. People and horses are infected by other mosquito species that feed on the infected birds and subsequently feed on mammals. People cannot contract EEE directly from their horses.
How serious is Eastern equine encephalitis?
- Fortunately EEE is a rare illness in humans, and only a few cases are reported in the United States each year. Many people are bitten by infected mosquitoes but never develop any symptoms of the illness. Cases of EEE are more common in children.
- Severe cases of EEE begin with the sudden onset of headache, high fever, chills, and vomiting. The illness may then progress into disorientation, seizures, and coma. The disease is the most severe mosquito-transmitted disease in the United States with between 50-75% mortality in cases with symptoms, and significant brain damage in about 80% of survivors.
- In horses, EEE is also quite severe with mortality rates ranging from 70-90 percent. EEE can also cause high mortality in non-native bird species (e.g., emus, and ring-necked pheasants).
What is the risk of an Eastern equine encephalitis outbreak in Minnesota?
- EEE is typically found in the eastern United States (as far west as Wisconsin), and in the south along the Gulf Coast.
- The risk of an EEE outbreak in humans is low in Minnesota. No human cases have been reported here, but three horse cases were reported in 2001 (first evidence of this virus in Minnesota in recent memory). These horse cases were part of a regional outbreak which included approximately 70 horses in Wisconsin, and a single horse in Iowa.
- Most of the horse cases occurred near tamarack bogs or hardwood swamps. These are the types of wetlands that produce Cs. melanura mosquitoes.
How can people prevent Eastern equine encephalitis?
- People can reduce their risk of EEE significantly by avoiding outdoor activities at dusk and dawn (the primary feeding time for several mammal-feeding mosquitoes).
- 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%) is sufficient, according to label directions).
- An EEE vaccine is available for horses. Please contact your veterinarian for vaccine recommendations. There is not a EEE vaccine for humans.