Infection with EEE is asymptomatic in about 95% of cases, but severe cases are seen with a mortality rate of 33%. A high fever, headache, and vomiting are the beginnings of inflammation of the brain. It can progress to disorientation, seizures, and/or coma. Significant brain damage can be seen in survivors. Immunity is lifelong.
In the United States, annual cases average eight, but in 2012 there were 15 cases. Larger outbreaks were also seen that year in West Nile virus and other encephalitis viruses. Most illness with EEE is seen from June to October. The five people who died had a median age of 76 years.
There is no treatment beyond supportive measures and there is no vaccine is available.