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Horse - Eastern Equine Encephalitis FAQ’s

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Anne Lichtenwalner, DVM, PhD, University of Maine Cooperative Extension and Donald E. Hoenig, VMD, State Veterinarian, Maine Department of Agriculture, Conservation and Forestry, Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) is a preventable, but fatal, disease in horses. Unfortunately, this disease can also affect humans if they are bitten by mosquitoes that carry the virus. The mosquitoes are infected by feeding on infected birds, in which the virus replicates and which act as natural “reservoirs” for the disease.

During early August 2009, a horse in Troy, Maine died of EEE. By early September, additional horses in Unity, Stetson, Thorndike, Berwick and Gorham had died and had been confirmed positive for infection with EEE.

Previous to 2009, the last fatal equine case of EEE in Maine was during September 2008, in Lebanon. Public and animal health officials in Maine are extremely concerned with the current situation because of the large geographic jump the disease has made from far southern Maine to central Waldo County, a distance of more than 150 miles.

Frequently asked questions

  • What are the signs of EEE in horses?  Horses will show central nervous system symptoms, such as appearing to have poor balance, behaving strangely, or becoming severely lethargic. Head pressing, circling, tremors and eventual coma and seizures are also frequently seen. If you suspect EEE, contact your veterinarian, the local extension office or the state veterinarian (Dr. Don Hoenig, 207-287-3701). This is a reportable disease in Maine.
  • Is this disease preventable?  Yes, this disease is preventable by routine vaccination. Many are available, and often can be purchased at feed or pet stores for owner administration to their own horses. Often EEE vaccination can be given in combination with Tetanus, another important equine vaccination. This makes it a very available, affordable option. Generally, vaccination for EEE is carried out annually but horse owners should consult with their practicing veterinarian to decide whether a booster is needed now, due to the current increased risk.
  • How is it transmitted?  EEE is harbored in birds. Mosquitoes bite infected birds, and become carriers. The mosquitoes may then bite humans or other animals, infecting them. Horses are sensitive to the virus, but don’t “concentrate” it as do birds or mosquitoes, so are not considered a risk for transmitting infections into mosquitoes or for directly infecting humans. Thus, they are considered to be a “dead end” host for the disease.
  • Should we be concerned about EEE in humans?  This disease is most commonly reported in people in Florida, Georgia, Massachusetts, and New Jersey, but has been reported to cause human fatalities in the Northeast. Last year, one individual in Massachusetts died of EEE, which may have been contracted while vacationing in Maine. The same mosquito vector that passes the virus to horses may transmit it to humans. When human infections are seen, they generally occur approximately two weeks after an outbreak of the disease in equines. It’s critical to follow good mosquito control, including personal protection:
    • Using an effective insect repellent on skin and clothing (DEET or other EPA-registered repellent)
    • Covering up with long-sleeve shirts, pants and socks when outdoors
    • Placing mosquito netting over infant carriers when outdoors with infants
    • Being aware that mosquitoes are most active at dawn and dusk: stay in!
    • Cleaning up unnecessary standing water around the yard to reduce mosquito habitats
    • Vaccinating horses
  • Are other animals affected?  Although dogs, pigs and alpacas have been shown to become antibody-positive to EEE (seroconversion), they are not in general considered to be susceptible to the virus. With the possible exception of pet birds, companion animals are not expected to get this disease. Though chickens and quail can be infected under experimental conditions, they are not expected to become infected under field conditions. In contrast, pheasants, pigeons, chukar partridges, turkeys and ducks have been reported to contract EEE and to exhibit paralysis, depression, reduction in egg laying and mortality in young birds. You should consult with your veterinarian about using any vaccine in a species for which it is not labelled.
  • Will this simply go away?  Usually encephalitis viruses such as EEE are less of a problem once the first frost has occurred. You may wish to consider getting advice from a mosquito control company (a list is available through your local Extension office). You may wish to plan ahead for next year by discussing the use of larvicides (which are used in spring) or adulticides (which are used in summer and fall) with the department of Environmental Protection and the Maine Department of Agriculture.

Other information sites

Maine Center for Disease Control and Prevention EEE Website

US Centers for Disease Control and Prevention EEE Website
This site discusses symptoms and onset of the disease in humans.

Maine Department of Agriculture Animal Health Website

For more information on educational programs in your area, contact your county Extension office.


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