Eastern Equine Encephalitis Virus in Maine 2010: Frequently Asked Questions

Anne Lichtenwalner DVM PhD: University of Maine Extension and Donald E. Hoenig, VMD, State Veterinarian, Maine Department of Agriculture

Maine Horse Owners

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Eastern Equine Encephalitis (EEE) is a preventable, but fatal, disease in some livestock species, and in humans. The most sensitive species are horses, but other domestic animals, including llamas and alpacas and selected bird species, can be affected by the disease. 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 summer 2009, 15 horses and pheasants in three flocks in Maine died of EEE. By the end of the mosquito season in late October, equine cases in Troy, Unity, Stetson, Thorndike, Berwick, Winslow, Jackson, Newport, Acton, and Gorham had been confirmed positive for infection with EEE. In addition, there was one confirmed case in a llama in the town of York.

Previous to 2009, the last fatal equine case of EEE in Maine was during September 2008, in Lebanon. The large geographic shift is seen between the mosquito season of 2008 to that of 2009, from far southern Maine to central Waldo County, a distance of more than 150 miles, raised real concerns for public health officials. Although increased awareness of vaccinations for horses, and of general mosquito control, may curb this disease during the 2010 mosquito season, the following FAQ’s are offered to help animal owners.

Frequently Asked Questions:

  • Question: What are the signs of EEE in horses?

Answer: 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.

  • Question: Is this disease preventable?

Answer: 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.

  • Question: How is it transmitted?

Answer: 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.

  • Question: Should we be concerned about EEE in humans?

Answer: 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. 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 yards to reduce mosquito habitats


  • Question: Are other animals affected?

Answer: Although some domestic animals have been shown to become antibody-positive to EEE (seroconversion), they are not considered to be at high risk of getting ill due to EEE. With the possible exception of pet birds, companion animals other than horses 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 labeled.

  • Question: Will this simply go away?

Answer: 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 from the Board of Pesticides Control in the Maine Department of Agriculture ). 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.

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