Eastern Equine Encephalitis (EEE)


Eastern Equine Encephalitis, also known as “sleeping sickness,” is caused by a virus, which results in inflammation of the tissues around the brain (encephalitis). The virus is spread from wild reservoirs to horses and humans, most often by mosquitoes, but it does not pass from horses to humans, or vice versa. In addition, it does not spread between horses, though outbreaks may occur in
mosquito-infested areas.

Clinical Signs

EEE is typically seen in unvaccinated horses where the disease is most prevalent (Eastern, South Eastern, Southern US). EEE has also been diagnosed in the Midwest and as far north as Canada. EEE is relatively common in Florida, where the mosquito population is active virtually year- round.

The first clinical sign is generally an intermittent fever, which is then followed by neurologic signs. A horse with EEE will most frequently appear lethargic and drowsy. Some horses with EEE may wander aimlessly or may even become aggressive and excitable. As the disease progresses, a horse may head-press, become blind, circle and have muscle fasciculation. Paralysis of the pharynx, larynx and tongue results in the inability to eat or drink. During the terminal stages of the disease the horse may become recumbent (unable to rise) and comatose. Horses that develop neurologic disease have a mortality rate of over 75% and those that survive often have some residual neurologic deficits.


The treatment of EEE is supportive, as there is no specific drug(s) that will inactivate the EEE virus. The primary goal of therapy is to decrease inflammation in and around the brain. Intravenous anti- inflammatory agents such as Banamine®, phenylbutazone and DMSO are the primary medications used to decrease inflammation. Other treatments are directed towards supporting the horse and its metabolic demands. Intravenous nutrition, the administration of vitamins and laxatives and eye medications are examples of supportive care a horse with EEE may require. Recumbent horses need extra nursing care in order to avoid complications such as muscle and nerve damage, sores and self-trauma.


Generally blood from a suspect case of EEE is collected on the first day of the illness and again 7-10 days later. A horse with EEE will generally demonstrate at least a 4-fold increase in the antibody formed against EEE virus. A single, elevated antibody concentration against EEE in an unvaccinated animal is usually sufficient to confirm a diagnosis of EEE. Additionally, a sample of spinal fluid (CSF) may be obtained and tested for the presence of antibody to EEE. While this diagnostic method is more conclusive, it is certainly a more
invasive test.


Vaccination against EEE is extremely safe and effective, and should be considered a core vaccine for any horse owner. In an unvaccinated adult horse, 2 doses of vaccine should be administered 3-6 weeks apart. In Florida, where disease is very prevalent, horses are generally vaccinated twice annually for EEE. Pregnant mares should be boostered 4-8 weeks prior to foaling so that their foals will ingest colostrum with a high concentration of antibody directed against EEE. Foals should be vaccinated against EEE at 4-5 months of age and again 8-16 weeks later for maximal protection against this disease. Another tactic to help prevent the disease is providing mosquito control on your farm. This will also help prevent other mosquito-born diseases such as West Nile Virus Encephalitis and Western Equine Encephalitis. The use of insect repellants, elimination of standing water and turning off barn lights at night will help reduce the mosquito population on your farm.

Lauren Danskin, DVM

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F: (352) 873-7700


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Equine Medical Center of Ocala
7107 West Hwy 326
Ocala, Florida 34482


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