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.