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.
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.
- Hughes HR, Velez JO, Davis EH, Laven J, Gould CV, Panella AJ, et al. Fatal human infection with evidence of intrahost variation of eastern equine encephalitis virus, Alabama, USA, 2019. Emerg Infect Dis. 2021;27(7):1886-1892. doi: 10.3201/eid2707.21035
- Brown SC, Cormier J, Tuan J, Lier AJ, McGuone D, Armstrong PM, et al. Four human cases of eastern equine encephalitis in Connecticut, USA, during a larger regional outbreak, 2019. Emerg Infect Dis. 2021;27(8):2042-2051. doi: 10.3201/eid2708.203730
- Lindsey NP, Martin SW, Staples JE, Fischer M. Notes from the Field: Multistate outbreak of eastern equine encephalitis virus – United States, 2019. MMWR Morb Mortal Wkly Rep. 2020;69:50-51. doi: 10.15585/mmwr.mm6902a4
- Lindsey NP, Staples JE, Fischer M. Eastern equine encephalitis virus in the United States, 2003–2016. Am J Trop Med Hyg. 2018;98(5):1472–1477. doi: 10.4269/ajtmh.17-0927
- Deresiewicz RL, Thaler SJ, Hsu L, Zamani AA. Clinical and neuroradiographic manifestations of eastern equine encephalitis. N Engl J Med. 1997;336(26):1867-1874. doi: 10.1056/NEJM199706263362604