Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Eastern, Western & Venezuelan Equine Encephalomyelitis

Synonyms: Sleeping Sickness EEE, WEE, VEE


Encephalitis is inflammation of the brain. Encephalomyelitis is inflammation of the brain and spinal cord. A number of different types of viral encephalitis/encephalomyelitis can occur in horses, including among others: Eastern, Western and Venezuelan Equine Encephalomyelitis (EEE, WEE, and VEE, respectively).

These infections are transmitted to horses by infected mosquitos. The reservoir of the disease in the wild is birds. A mosquito bites an infected bird, takes in the virus, and then bites a horse and transmits the virus. Certain snakes have been shown as carriers of EEE (cottonmouth vipers, copperheads)

It is difficult to differentiate between the encephalitis types and West Nile Encephalomyelitis (without an antibody titer) because they all cause similar signs of disease. Symptoms include fever, depression and signs of neurologic disease (wobbliness, weakness), among many others.

EEE, WEE and VEE are reportable diseases in the USA, meaning that if a horse has or is suspected of having either of these disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

Viral encephalitis are all zoonotic. Infected horses cannot directly pass these diseases to other horses, animals or humans. EEE and WEE CAN be transmitted from infected birds to humans via mosquitos. VEE is the only one of these that can be transmitted from infected horses to humans via mosquitos. Because of this, if you have been exposed to a horse with (or suspected of having) EEE, WEE or VEE or West Nile Virus, you should always contact your doctor. Infection by these viruses can cause life-threatening disease in humans.

Diagnosis is through blood antibody testing.

Treatment is primarily anti-inflammatories and supportive care.

my vet's role


The prognosis is poor for horses infected with EEE, as it has a very high fatality rate. The prognosis is better but still guarded for horses infected with WEE.

Horses that recover from these diseases tend to have a high incidence of residual neurologic deficits, including general dullness.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
more observations

Questions To Ask Your Vet:
  • Why do you think my horse has viral encephalitis?
  • What type of encephalitis do you think my horse has?
  • What is its severity and what are the treatment options?
  • What vaccination do you recommend in our area?

Reduction of horses' exposure to mosquitos is important. This includes good facilities management to decrease the mosquito population. This includes the use of fans and screens to reduce mosquito exposure, and mosquito/insect repellents applied to susceptible horses.

Citronella products are helpful but should be used in combination with other strategies. Keep horses in protected stalls during times of high mosquito activity.

Reduce mosquito breeding areas. Eliminate all pools of low lying stagnant water around equine facilities. Replace water for animals every three days (at minimum). Stock ornamental ponds with mosquito eating fish.

Vaccination is the key to prevention of these diseases. Vaccinate horses for EEE and WEE at least annually and in accordance with your vet's recommendation. These vaccines are recommended as "core" vaccines by the American Association of Equine Practitioners (AAEP). Talk to your vet about vaccinating your horse for VEE if you live in States bordering Mexico.

Vaccination has prevented severe epidemics of EEE and WEE in the United States, but small outbreaks still occur. VEE has not been identified in the U.S. for several decades as of the time of this writing, but the risk of contracting VEE is higher in States that border Mexico, where outbreaks have historically occurred.

further reading & resources

Related References:

Higgins AJ, Snyder JR eds. The Equine Manual. 2nd Ed. Edinburgh: Elsevier Saunders 2006.

Author: Doug Thal DVM Dipl. ABVP