Clinical Evaluation & Disease

Diagnosis & Reporting

Eastern equine encephalitis (EEE) virus disease should be considered in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes, organ transplantation, or potentially blood transfusion,  especially during the summer months in areas where virus activity has been reported.

In addition to other more common causes of encephalitis and aseptic meningitis (e.g., herpes simplex virus and enteroviruses), other arboviruses (e.g., West Nile, La Crosse, St. Louis encephalitis, and Powassan viruses) should also be considered in the differential etiology of suspected EEE illness.

EEE virus disease is a nationally notifiable condition. All cases should be reported to local public health authorities in a timely manner. Reporting can assist local, state, and national authorities to recognize outbreaks and to implement control measures to reduce future infections.

Clinical Signs & Symptoms

The incubation period for EEE virus disease ranges from 4 to 10 days and can be several weeks in immunocompromised people.

Most persons infected with EEE virus have no apparent illness. Symptomatic persons typically develop a systemic febrile illness that can progress in <5% of individuals to meningitis or encephalitis. Signs and symptoms in patients with neuroinvasive disease include headache, confusion, focal neurologic deficits, meningismus, seizures, or coma. EEE neuroinvasive disease is estimated to have a 30% case fatality rate and results in neurologic sequelae (such as seizure disorders, hemiplegia, and cognitive dysfunction) in more than 50% of survivors.

Clinical Evaluation

Cerebrospinal fluid (CSF) findings include initial neutrophil-predominant then lymphocyte-predominant pleocytosis and elevated protein levels; glucose levels are normal. Neuroimaging shows brain lesions consistent with encephalitis, including neuronal destruction and vasculitis in the cortex, midbrain, and brain stem. There is minimal involvement of the spinal cord.


Approximately one third of EEE virus disease cases are fatal. Death usually occurs 2 to 10 days after onset of symptoms but can occur much later. Of those who recover, many are left with physical or mental sequelae, which can range from mild brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction. Many patients with severe sequelae require long-term care and die within a few years.