Clinical Signs and Symptoms of Eastern Equine Encephalitis

Key points

  • Eastern equine encephalitis is a severe disease that occurs in <5% of people infected with the virus.
  • The incubation period ranges from 4 to 10 days but can be longer in patients who are immunocompromised.
  • About a third of patients who develop severe eastern equine encephalitis die and half of survivors have neurologic sequelae.
Doctor holding up a brain scan to a window

Clinical presentation

Eastern equine encephalitis should be considered in any person with an acute febrile or neurologic illness who has had recent exposure to mosquitoes, especially during the summer months in areas where virus activity has been reported. Other causes of encephalitis and aseptic meningitis should also be considered, as appropriate (e.g., herpes simplex viruses, enteroviruses, West Nile virus, St. Louis encephalitis virus, La Crosse virus, Powassan virus).

Signs and symptoms

Most persons infected with eastern equine encephalitis virus have no apparent illness. Symptomatic persons typically develop a systemic febrile illness that can progress in <5% of individuals to meningitis or encephalitis. The incubation period for eastern equine encephalitis ranges from 4 to 10 days but can be several weeks in patients who are immunocompromised.

Signs and symptoms in patients with neuroinvasive disease can include headache, meningismus, confusion, focal neurologic deficits, seizures, and coma.

Clinical assessment

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

Outcomes

Eastern equine encephalitis is estimated to have a 30% case fatality in people who develop severe disease. Death usually occurs 2 to 10 days after onset of symptoms but can occur much later. Of those who recover, more than half are left with physical or neurologic 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.