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. Fewer than 5 cases are reported in most years. Although small outbreaks of human disease have occurred in the United States, epidemics in horses can be common during the summer and fall.