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Eastern equine encephalitis

Eastern equine encephalitis virus is a rare cause of brain infections (encephalitis). Only a few cases are reported in the United States each year. Most occur in eastern or Gulf Coast states. Approximately 30% of people with Eastern equine encephalitis die and many survivors have ongoing neurologic problems.


Eastern equine encephalitis virus is a member of the genus Alphavirus, family Togaviridae. Other medically important alphaviruses found in the Americas include, Venezuelan equine encephalitis virus, chikungunya virus, Mayaro virus, and Madariaga. Eastern equine encephalitis virus has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 60-65 nm. Of the four lineages of Eastern equine encephalitis antigenic complex, Group I consists of only Eastern equine encephalitis virus and is endemic in North America and the Caribbean and causes most human disease cases. The other three groups (IIA, IIB, and III) are caused by related Madariaga (formerly known as South American Eastern equine encephalitis virus) and cause primarily equine illness in Central and South America.


Eastern equine encephalitis virus is maintained in a cycle between Culiseta melanura mosquitoes and avian hosts in freshwater hardwood swamps. Cs. melanura is not considered to be an important vector of Eastern equine encephalitis virus to humans because it feeds almost exclusively on birds. Transmission to humans requires mosquito species capable of creating a “bridge” between infected birds and uninfected mammals such as some Aedes, Coquillettidia, and Culex species.

Horses are susceptible to Eastern equine encephalitis virus infection and many cases are fatal. Eastern equine encephalitis virus infections in horses, however, are not a significant risk factor for human infection because horses (like humans) are considered to be “dead-end” hosts for the virus (i.e., the concentration of virus in their bloodstreams is usually insufficient to infect mosquitoes). There is a vaccine to prevent Eastern equine encephalitis virus infection in horses.

Symptoms and Treatment

Symptoms. The incubation period for Eastern equine encephalitis virus disease (the time from infected mosquito bite to onset of illness) ranges from 4 to 10 days. Eastern equine encephalitis virus infection can result in a systemic febrile illness or neurologic disease, including meningitis (infection of the membranes that surround the brain and spinal cord) or encephalitis (infection of the brain). The type of illness will depend on the age of the person and other host factors. Some people who become infected with Eastern equine encephalitis virus may be asymptomatic (will not develop any symptoms).

Systemic infection is characterized by fever, chills, malaise, arthralgia, and myalgia. The illness lasts 1 to 2 weeks, and most people recover completely when there is no central nervous system involvement. Signs and symptoms of neurologic disease include fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness, and coma. In infants, neurologic disease often occurs soon after onset; in older children and adults, encephalitis may occur after several days of systemic illness.

Approximately a third of all people with encephalitis due to Eastern equine encephalitis virus infection die. 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.

Treatment. There is no approved human vaccine or specific antiviral treatment for Eastern equine encephalitis virus infections. Patients with suspected Eastern equine encephalitis virus disease should be evaluated by a healthcare provider, appropriate serologic and other diagnostic tests ordered, and supportive treatment provided.

Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD). Accessed July 17, 2020.

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