Eastern Equine Encephalitis Basics
What is Eastern equine encephalitis?
Eastern equine encephalitis (EEE) is a viral illness transmitted to people and horses through the bite of an infected mosquito.
EEE is maintained in a transmission cycle involving Culiseta melanura mosquitoes 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. This 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).
EEE is typically found in the eastern United States (as far west as Michigan), and in the south along the Gulf Coast.
Most of the 2001 horse cases occurred near tamarack bogs or hardwood swamps. These are the types of wetlands that produce Cs. melanura mosquitoes.
Signs and symptoms
Eastern equine encephalitis 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 eastern equine encephalitis are more common in children.
Severe cases of eastern equine encephalitis 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.
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 (less than 30% DEET is sufficient for adults, and no more than 10% for children) according to label instructions.
An EEE vaccine is available for horses. Please contact your veterinarian for vaccine recommendations.
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