Eastern Equine Encephalitis
(Sleeping Sickness)

Genevieve Fontaine, DVM, MS, DACVIM, Equine Extension Veterinarian, UF College of Veterinary Medicine

Equine Eastern Encephalitis (EEE) is a viral disease that is often fatal in horses. This disease occurs in areas and at times where the mosquito population is abundant. Even though outbreaks are generally limited to a certain geographic perimeter due to the pattern of distribution of the vector, horses located in other areas in Florida may be at risk of contracting the disease, due to the current climatic conditions.

Transmission and Cycle

Transmission of the virus to horses occurs through the bite of an infected mosquito. The "normal" cycle of the virus involves mosquitoes and swamp birds, while horses and people are "accidental" hosts. When bitten by an infected mosquito, people and horses are in fact "dead-end" hosts that cause the cycle to stop because the concentration of viral particles in their blood is insufficient to infect another mosquito. Infected horses therefore do not represent a source of virus for people or other horses.

Birds and wild animals constitute the reservoir species for the virus: the virus multiplies in their body but they do not get sick. However, their blood contains enough viral particles for mosquitoes to become infected during subsequent blood meals. Other domestic species such as burros, mules, pigs and calves may be affected like horses, but the severity of the disease is lesser than in horses.

Clinical Signs

The incubation period for EEE is approximately one week. Early signs are generally subtle and are often undetected; they involve fever (mild or severe), depression, lack of appetite and stiffness. These signs may last up to 5 days during which viremia (presence of the virus in the blood) occurs.

Signs of neurological disease usually follow: horses may become profoundly depressed (therefore the name "Sleeping Sickness") or exhibit abnormal behavior such as propulsive walking, head-pressing, aggressiveness, circling, or hyperexcitability. Some horses show signs of facial nerve dysfunction such as blindness, head tilt, and paralysis of the muscles of the face, mouth and throat. Some horses become comatose, seizure, or die suddenly.

The disease is associated with a high mortality (95% in non-vaccinated horses). The few horses that survive show gradual improvement over weeks to months, but residual neurological deficits such as ataxia, depression and abnormal behavior are not uncommon.


A presumptive diagnosis is made based on clinical findings and circumstances (location, time of the year, presence of similar cases in the area). The diagnosis may be confirmed by analyzing blood samples to determine the concentration of antibodies against EEE virus, but the peak in antibody concentration usually occurs before horses show signs of neurological disease, which is the time where the blood samples are generally collected. Therefore blood samples are not always a reliable way to diagnose EEE. A second sample collected during convalescence may reveal increased titers, and a four-fold increase is considered diagnostic.

On the other hand, a second sample may reveal decreased titers if taken when the antibody concentration is in the decreasing phase. Vaccinated horses may have high titers due to recent vaccination. Various tests such as viral isolation from cerebro-spinal fluid (CSF) and brain tissue, and PCR (a test that detects viral DNA) are used to confirm the diagnosis at post mortem examination. Examination of tissue sections collected during necropsy usually shows signs of inflammation of the central nervous system.


There is no known treatment for this disease. The only treatments used in horses with neurological signs associated with viral encephalitis are primarily supportive and include antiinflammatories, anti convulsants, and fluid therapy. Affected horses are placed in a padded stall, or on heavy bedding to prevent self-inflicted trauma. Leg wraps and padded helmet may be used to the same effect. Recumbent animals may temporarily be supported in a sling.


An equine vaccine is available. It is a killed vaccine that is safe to use in all horses, including pregnant mares and foals. It is strongly recommended to vaccinate all horses at least twice a year, and up to 4 times in Florida where the vector season is prolonged. Foals from unvaccinated mares may be vaccinated at any age, but they should be revaccinated at 6 months and one year of age to ensure adequate protection. Even though vaccinated horses are less likely to get EEE, protection provided by the vaccine is not absolute.

Avoiding exposure to the vector is another preventive measure that should be implemented. Horses should be kept away from mosquito breeding sites such as marshes, ponds, and wetlands. Stagnant water commonly found in birdbaths, dog bowls, old tires, unused swimming pools, plastic tarp or the likes should be eliminated from the horses’ environment. Insect repellents should be used as needed.

If practical, horses should be kept in stalls with screens and fans at dusk and dawn, the times where mosquitoes are the most active. People are also encouraged to limit exposure to mosquitoes by avoiding outdoor activities at dusk and dawn, and by wearing long sleeved shirts, pants, socks and shoes if outdoors at times of vector activity.

Due to the recent medical alert issued following the outbreak of equine cases in the Panhandle, suspect cases of encephalitis should be reported to the State Department of Agriculture and Consumer Services.


Brown and white horse



For additional information about eastern encephalitis or West Nile Disease, contact the Extension Office at 482-9620 or e-mail Doug Mayo.