Bulletin #1003, Preventing Eastern Equine Encephalitis (EEE) Virus in Maine

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By Anne Lichtenwalner, DVM Ph.D., University of Maine Extension and
Michele Walsh, DVM, Maine Department of Agriculture, Conservation and Forestry

Revised by Dr. Dana Hill, DVM, MS, PhD, DACVP, University of Maine Cooperative Extension

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three horses in pasture

Eastern Equine Encephalitis (EEE) is a preventable, but fatal, viral disease transmitted by mosquitoes. Mosquitoes become infected by feeding on infected birds, which act as natural reservoirs for the disease. Horses (and other equine species) are the most sensitive to the disease, but other domestic animals, including llamas, alpacas, and some bird species can be affected by EEE. In particular, ratites (emus and ostriches) and non-native game birds (pheasants and quail) are susceptible to developing disease from EEE. Unfortunately, this disease can also affect humans but only if they are bitten by mosquitoes that carry the virus.

Throughout the last decade, Maine has seen increased numbers of EEE cases in domestic animals:

  • 2009: Over a dozen unvaccinated horses died due to EEE in Maine
  • 2013: One horse died due to EEE in Maine
  • 2023: Four horses and 12 emus succumb to EEE

Both equine and ratite losses may be prevented through proper vaccination. Both EEE and tetanus are easily preventable by proper vaccination, and both should be included in your horses’ yearly boosters. The majority of vaccinations require a “booster” shot (second shot given approximately 4-6 weeks after the initial vaccine), followed by a yearly booster (same shot, but given 6 months to a year following the first “booster”). During mosquito season, it’s recommended that horses be revaccinated for EEE if more than 6 months have elapsed since the last vaccination. The vaccination used for horses is commonly used off-label by veterinarians in other susceptible species, including emus and ostriches. Check with your veterinarian for specific vaccine recommendations. 

Both EEE and tetanus are easily preventable by proper vaccination, and both should be included in your horses’ yearly boosters.

Frequently asked questions

What are the clinical signs of EEE in horses?

Horses with EEE most commonly show neurological symptoms, such as appearing to have poor balance, behaving strangely, or becoming severely lethargic. Head pressing, circling, tremors, eventual coma, and seizures are also frequently seen. Most bird species remain asymptomatic but might develop mild neurological signs. In emus, EEE may present differently by causing gastrointestinal signs (vomiting and profuse bloody diarrhea).

If any of these symptoms are observed, call your veterinarian right away. Other diseases, such as rabies and West Nile virus, can cause similar signs and are all zoonotic (transmissible to humans). If you suspect EEE, contact your vet and the state veterinarianEEE is a reportable disease in Maine.

Is EEE preventable?

Yes, EEE is preventable by routine vaccination and good mosquito control. Many equine vaccination combinations are available, and can be given by your vet, or can be purchased at feed or pet stores for owner administration to their own horses. Often EEE vaccination is available in combination with tetanus, another important equine vaccination. This makes it a very available, affordable option. It’s vitally important to give a “booster” shot 4-6 weeks following the first vaccination, then booster again every 6 to 12 months. Generally, vaccination for EEE is done annually, but horse owners should consult with their practicing veterinarian to decide whether spring boosters, to protect during the high-risk summer season, are needed.

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 may develop disease but don’t “amplify” the virus as do birds or mosquitoes, so are not considered a risk for transmitting infections into mosquitoes or for directly infecting humans. Thus, although very sensitive to EEE, horses 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. 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
  • Use mosquito control for your horse, as well.  Face nets help, along with barn mosquito control, using midday turn-out times, and flysheets or sprays.

For more information, see UMaine Extension’s fact sheets on Insect Repellents (Pest Management Fact Sheet #5108), Mosquito Biology (Pest Management Fact Sheet #5109), and Mosquito Management (Pest Management Fact Sheet #5110).

Are other animals affected?

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 and ratites 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 natural 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, and especially in a species for which it is not labeled.

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 from the Board of Pesticides Control in the Maine Department of Agriculture, Conservation and Forestry). If mosquitoes are a big problem on your farm, 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 your local UMaine Extension County Offices, the Maine Department of Environmental Protection or the Maine Department of Agriculture, Conservation and Forestry.

Additional Resources

The Maine Department of Agriculture, Conservation and Forestry

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