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Horse Owners Urged to Vaccinate against Eastern Equine Encephalitis

Horses at the University of Florida Horse Teaching Unit. Horse, domesticated Equines, hoofed mammals UF/IFAS Photo by Tyler Jones.

Mature horses should receive an EEE booster vaccine every spring and fall.  Photo by Tyler Jones.

Eastern Equine Encephalitis (EEE) is a disease that is transmitted by mosquitoes. EEE most often infects horses but can also infect humans.  There is no vaccine for humans and approximately 1/3 of those infected die of the disease. The majority of those who survive, suffer brain damage.  The mortality rate in horses is 70-90%, with most cases succumbing or being euthanized due to disease severity.  Fortunately for horses, there are vaccines available to help protect against infection.

Dr. Amanda House, University of Florida College of Veterinary Medicine Clinical Associate Professor, advises, “It is critical that every horse in Florida be vaccinated for EEE at least twice a year.  Horses under 4 years of age, or those new to the state should be vaccinated three times a year.  EEE is a deadly disease that vaccination can help reduce or eliminate.”

Mosquito control on the farm is also critical for decreasing the incidence of disease in both animals and humans. Dr. Carissa Wickens, University of Florida State Extension Horse Specialist, has this to say about mosquito control, “Management strategies that can help reduce exposure of horses to mosquitoes include: eliminating standing water around barns, paddocks, and pastures (e.g. cleaning water troughs regularly, emptying plastic wading pools, etc.) as removing standing water reduces mosquito breeding sites, housing horses indoors during peak mosquito activity (dusk to dawn), placing fans in the barn, and keeping barn lights turned off during the evening and overnight hours.”

Both horses and humans are “dead end” hosts, meaning the disease is not transmitted from an infected horse or human to another horse or human. Instead, birds serve as the main vector of the virus.  A mosquito feeds on an infected bird and then feeds on a human or horse to transmit the disease.  Mosquitoes also transmit other diseases such as West Nile Virus (WNV).  This virus is similar to EEE, but has a lower mortality rate (approximately 33% in horses).  A vaccine is also available to protect horses against WNV and should be administered once or twice per year in Florida.

Two additional core vaccines for horses are tetanus, which are often combined with the EEE vaccine, and rabies. Performance horses should also be vaccinated against influenza and equine herpesvirus.  A good rule of thumb is to vaccinate for EEE, tetanus, WNV, and rabies in the spring, and to re-vaccinate for EEE and WNV in the early fall, for most adult horses.  It is recommended that you consult your veterinarian to design a vaccination schedule that best suits your situation.

For more information on EEE, go to the UF/IFAS publication entitled:

Eastern Equine Encephalitis