Eastern Equine Encephalitis

Learn more about how to help prevent this mosquito-borne disease.

From AQHA Corporate Partner Pfizer Animal Health

It is the best intent of every horse owner to keep their horses healthy, whether they are at home or traveling. However, when horses do get sick, they often exhibit symptoms that are similar for many different diseases, which can make diagnosis difficult. Working with a veterinarian in a preventative health care program can help protect your horses against many diseases, including Eastern equine encephalitis, a serious neurological disease that can be fatal.

What is Eastern equine encephalitis?
Eastern equine encephalitis, commonly referred to as EEE, is a viral disease that is spread from birds or small mammals to horses by infected mosquitos1. It is one of several mosquito-borne diseases that can affect the central nervous system and cause severe complications and death.

What are the symptoms of EEE?
Signs in infected horses can be varied but usually begin with fever, depression and listlessness, which then progress to more serious neurological signs such as incoordination, stumbling, circling, head pressing, coma and often death2. Once the horse becomes infected with the EEE virus and develops neurological signs, the disease is fatal in roughly 90 percent of cases2. There is no effective treatment, and seizures resulting in death usually occur within 48-72 hours of the horse’s first indication of illness3.

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How to help protect horses against EEE?
Vaccinations are the most effective way to help protect horses against EEE and other encephalitic diseases, such as Western equine encephalitis (WEE) and West Nile virus (WNV). According to the American Association of Equine Practitioner’s vaccination guidelines, Eastern equine encephalitis, Western equine encephalitis and West Nile are considered core vaccinations for all horses, along with tetanus and rabies4. For adult horses previously vaccinated against EEE, annual revaccination should be completed in the spring prior to mosquito season. In animals at high risk or with impaired immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to help induce protective immunity during periods of likely exposure. It is best to consult a veterinarian on the horse’s vaccination program, as timing and frequency may vary depending on the age, geographic location, use and whether the horse has been vaccinated before.

Pfizer Animal Health offers the trusted Fort Dodge line of vaccines, including the Mosquito Shot ™ (WEST NILE-INNOVATOR® + EWT) which helps protect against Eastern equine encephalitis (EEE) virus, Western equine encephalitis (WEE) virus and West Nile virus (WNV) in a single vaccine. The WEST NILE-INNOVATOR vaccines, along with other Pfizer Animal Health equine vaccine combinations, are included in the Equine Immunization Support Guarantee.

The Pfizer Animal Health Equine Immunization Support Guarantee program provides up to $5,000 for reasonable diagnostic and treatment costs if a horse properly vaccinated by a veterinarian contracts the corresponding equine disease. Disease protection backed by the Equine Immunization Support Guarantee includes infection from West Nile virus, equine influenza virus, tetanus, Eastern equine encephalitis virus, Western equine encephalitis virus and Venezuelan encephalitis virus. Vaccinations must be performed by a licensed veterinarian with an established client-patient relationship to be eligible.

Is EEE a threat today?
The risk of exposure and geographic distribution of EEE varies from year to year with changes in distribution of insect vectors and reservoirs important to the natural ecology of the virus4. However, unvaccinated horses may be at serious risk of contracting mosquito-borne illnesses, even in areas where the disease may not be common. In 2010, states such as Florida that monitor the development of mosquito-borne diseases through sentinel chickens, saw an upsurge in the detection of Eastern equine encephalitis virus and West Nile virus5. Though Eastern equine encephalitis typically appears in the southern and eastern parts of the country, Michigan and California reported multiple cases of the disease6. Eastern equine encephalitis is considered to be endemic in all areas of North America by the American Association of Equine Practitioners, and all horses should be vaccinated against it.

Ensuring that all horses get their spring vaccinations is the best way for horse owners to help protect their horses and do their part to keep EEE from becoming an endemic in 2011.

Whether you’re renewing your membership or joining for the first time, being an AQHA member comes with plenty of perks. You can get corporate partner discounts, receive America’s Horse magazine, show at official shows and join the Horseback Riding program!

1. Eastern Equine Encephalitis. Health Promotion and Education. 2001. Available at Accessed on March 10, 2011.

2. Putnam AH and Holt TJ. Eastern Equine Encephalitis. The Florida Department of Agriculture & Consumer Services; The Division of Animal Industry. March 11, 2011. Available at: Accessed on March 11, 2011.

3. Crans WJ. Questions Regarding Eastern Equine Encephalitis and Horse. Rutgers Center for Vector Biology. October 1 1993. Available at: Accessed on March 10, 2011.

4. Core Vaccination Guidelines. American Association of Equine Practicioners. 2008. Available at: Accessed March 1, 2011.

5. Bronson Urges Vaccinations for Horses As EEE Cases Rise. Florida Department of Agriculture and Consumer Services. June 28, 2010. Available at: Accessed 8 March 8, 2011.

6. Disease Maps 2010. U.S. Department of the Interior. U.S. Geological Survey. Available at: Updated January 11, 2011. Accessed February15, 2011.