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

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

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three horses in pasture Eastern Equine Encephalitis (EEE) is a preventable, but fatal, disease. Horses (and other equine species) are the most sensitive to the disease, but other domestic animals, including llamas and alpacas and some bird species, can be affected by EEE. 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 2009 in Maine, over a dozen unvaccinated horses died due to EEE. During 2013, we have had one equine death due to EEE in Maine. These equine losses could have been prevented by proper vaccination. Both EEE and tetanus are easily preventable by proper vaccination, and both should be included in your horses’ yearly boosters. Any vaccine needs an initial “booster” (second shot given at a specific time interval after the first one, usually a month), and for many vaccines (like EEE) at least an additional yearly booster (same shot, but given 6 months to a year following the initial “booster”). During mosquito season, it’s recommended that horses be revaccinated for EEE if more than 6 months have elapsed since the last vaccination. Check with your veterinarian for specific recommendations for equine vaccines in your area.

Frequently asked questions

What are the signs of EEE in horses?

Horses with EEE 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 see any of these symptoms, veterinary help is needed. Other diseases, such as rabies, can cause similar signs. If you suspect EEE, contact your vet, the state veterinarian, and the local UMaine Extension County Offices. EEE is one of the reportable diseases 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 at 1 month or so 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 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, 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 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, 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.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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