Infection can cause a range of illnesses. For people with infection of the central nervous system, a sudden fever and severe headache can be followed quickly by seizures and coma. About half of these patients die from the disease. Of those who survive, many suffer permanent brain damage and require lifetime institutional care. Symptoms usually appear 4 to 10 days after the bite of an infected mosquito.
Diagnosis is based on tests of blood or spinal fluid.
Risk of exposure is increasing as the population expands into endemic areas. Wet summers increase the numbers of mosquitos, therefore, also increase the chance of infection. The disease is fatal to about half of those who develop severe symptoms. Of those who survive, many suffer permanent brain damage. Eastern equine encephalitis is a rare disease.