Eastern Equine Encephalitis
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Eastern Equine Encephalitis


En Español (Spanish Version)


Eastern equine encephalitis (EEE) is a virus affecting some wild birds. It is carried by certain mosquitoes. It is occasionally transmitted to horses, and very rarely, to people. EEE affects the brain and central nervous system.

Although it is more dangerous to horses than to humans (many people infected with the EEE virus do not have any apparent health problems), in some cases, people infected with EEE virus can become suddenly and seriously ill and may experience severe damage to the nervous system, sometimes resulting in death.

If you are in an area where EEE is known to be present, take extra precaution to avoid areas of mosquito activity and protect yourself from being bitten by mosquitoes.

Effect Encephalitis on Brain

Swollen brain

© 2008 Nucleus Medical Art, Inc.


EEE, an arbovirus, is spread by infected invertebrate animals, mostly blood-sucking insects. Arboviruses are usually spread by infected mosquitoes. Mosquitoes most often get the EEE virus by biting infected birds, and then spreading the virus to horses and other mammals, including humans.

Risk Factors

Because the only known way for humans to contract EEE is by being bitten by an infected mosquito, the risk factor most commonly associated with EEE is exposure to mosquito bites, or living near or visiting a wetland area or an area known to have incidents of EEE. Age is also a risk factor. People over age 50 or under 15 seem to be more susceptible to the infection.


If you experience any of these symptoms do not assume it is due to EEE. These symptoms may be caused by other, less serious health conditions. If you experience any one of them, see your physician.

  • Fatigue
  • Fever
  • Headache
  • Nausea
  • Restlessness or irritability
  • Difficulty walking or unstableness
  • Confusion, impaired judgment, or an altered mental state
  • Seizures


Your doctor will ask about your symptoms and medical history, and perform a physical exam. Your doctor may order the following tests:

  • Blood tests to find out if the virus is present
  • A spinal tap to remove a small amount of spinal fluid to check for signs of infection
  • An electroencephalogram (EEG) to measure the brain’s activity
  • A neurological exam to access reflexes, memory, and other brain function
  • CT scan—a type of x-ray that uses computers to make pictures of structures inside the head
  • MRI scan—a test that uses magnetic waves to make pictures of structures inside the head



© 2008 Nucleus Medical Art, Inc.


Talk with your doctor about the best treatment plan for you. There are no drug options to treat the EEE virus in humans, so medical treatment focuses on treating the symptoms of the infection. Such treatments may include:

  • Antibiotics to treat secondary infections
  • Anticonvulsants to treat seizures
  • A respirator to help with breathing
  • Pain relievers to treat headache, fever, and body aches
  • Corticosteroids to reduce swelling in the brain
  • Sedatives for restlessness or irritability


To help reduce your chances of getting EEE, take the following steps:

  • Avoid areas of mosquito activity, if possible.
  • Stay inside when mosquitoes are most active (at dawn and at dusk).
  • When outside, wear insect repellent, long pants and long-sleeved shirts to limit exposure to bites.

To help limit mosquito populations in and around your home, eliminate the insects’ breeding areas. Those may include standing water such as pet water bowls, rain barrels, and other containers.


Centers for Disease Control and Prevention

Mayo Clinic

National Institute of Neurological Disorders and Stroke


Canadian Cooperative Wildlife Centre

Public Health, Grey Bruce Health Unit


Arboviruses–eastern equine encephalitis. North Carolina Public Health website.
Available at http://www.epi.state.nc.us/epi/arbovirus/eee.html . Accessed September 19, 2006.

Crans W. Questions regarding eastern equine encephalitis and horses. Rutgers Cooperative Extension Fact Sheet # FS737. Available at http://www.rci.rutgers.edu/~insects/heee.htm . Accessed September 19, 2006.

Eastern equine encephalitis fact sheet. Centers for Disease Control website. Available at http://www.cdc.gov/ncidod/dvbid/arbor/eeefact.htm . Accessed September 21, 2006.

Eastern equine encephalomyelitis. Veterinary Services, US Department of Agriculture Animal and Plant Health Inspection Service (APHIS) website. Available at: http://www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/fs_aheasterne.html. Accessed September 21, 2006.

NINDS meningitis and encephalitis information page. National Institute of Neurological Disorders and Stroke website. Available at http://www.ninds.nih.gov/disorders/encephalitis_meningitis/encephalitis_meningitis.htm. Accessed September 22, 2006.

Stull JW, Talbot EA, MacRae S, et al. Eastern equine encephalitis—New Hampshire and Massachusetts, August-September 2005. JAMA . 2006;296:645-646. Available at:
http://jama.ama-assn.org/cgi/content/full/296/6/645 . Accessed September 21, 2006. (Also published in Morbidity and Mortality Weekly Report, available at Center for Disease Control website.)

What you should know about…eastern equine encephalitis. New Jersey Department of Health and Senior Services website. Available at http://www.state.nj.us/health/cd/f_eee.htm . Accessed September 22, 2006.

Last reviewed February 2008 by David Horn, MD

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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