Eastern Equine Encephalitis
Information for Health Professionals

All EEE and West Nile virus disease cases should be reported to the Vermont Department of Health.

Cause of EEE

EEE is caused by the most neuropathogenic arbovirus present in the United States. For patients that develop encephalitis, the mortality rate is high and survivors often have severe neurological damages.


EEE is relatively rare with only 5 to 10 cases reported nationally each year.

Two human cases of EEE were detected in Vermont in 2012. These were the first ever reported cases in the state. Both people were from the same area in southern Addison and northern Rutland Counties where EEE virus has subsequently been detected in mosquitoes.

No human cases were detected in 2013, but two horses in Franklin County died from this disease. In addition, EEE virus was detected in a mosquito pool collected in northern Chittenden County. EEE virus is likely circulating in many parts of the state, and it is possible that the virus will be found in people or animals in additional areas.

In 2010, deer and moose blood collected during hunting season tested positive for antibodies against EEE virus. Out of almost 500 samples tested, about 11% were positive. Positive samples came from deer and moose from all parts of the state. This was the first evidence that EEE virus is present in Vermont. The deer serosurvey was repeated from 2011 to 2013. The deer appear to have a stable seroprevalence at about 10%. One more year of data will be collected.

In the northeast, Massachusetts has had the most human cases, and outbreaks have historically occurred in the southeastern part of the state every 10 to 20 years. New Hampshire has had a recent increase in reported cases. Since 2005, New Hampshire has recorded 9 human cases.

For more information about EEE in Vermont, go to Surveillance and Data.


The virus is maintained in a bird-mosquito cycle that occasionally spills over to cause illness in mammals, including humans. Mammals are dead-end hosts and do not contribute to the transmission cycle of the virus. The mosquito that is the enzootic vector feeds mostly on birds and lives in hardwood acidic swamps. Transmission to mammals is thought to occur when either the enzootic vector takes a rare mammalian meal or when a “bridge species” of mosquito (one that routinely feeds on both birds and mammals) feeds on an infected bird and then feeds on a susceptible mammal.


 EEE virus infection in people can be asymptomatic, or it can result in either a systemic or encephalitic infection. Systemic infection has an abrupt onset and is characterized by chills, fever, malaise, arthralgia, and myalgia. The illness lasts 1 to 2 weeks. Recovery is complete when there is no central nervous system involvement.

Most of the reported cases are caused by an encephalitic infection. In infants, the encephalitic form is characterized by abrupt onset; in older children and adults, encephalitis occurs after a few days of systemic illness. Signs and symptoms in encephalitic patients include fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, cyanosis, convulsions, and coma. Approximately 1/3 of people that develop encephalitis die, and 50% of those who recover have significant neurologic sequelae.

Testing and Diagnosis

Arboviral Diseases

All EEE and West Nile virus disease cases should be reported to the Vermont Department of Health.

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More Information

Contact Us

Department of Health
Tel: 800-640-4374 (Vermont only) or 802-863-7240
Email the Department of Health (general mailbox)

Public Health Laboratory
Tel: 800-660-9997, extension 7560

Epidemiology Field Unit
800-640-4374 (in VT only), or 802-863-7240.

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