Viral Encephalomyelitis (Encephalitis) in Horses

Viral encephalomyelitis in horses is a serious disease involving inflammation of the brain and spinal cord. It is caused by viruses that are often spread by mosquito bites. There are four main types of viral encephalomyelitis in horses:

• West Nile encephalomyelitis

• Eastern equine encephalitis (EEE)

• Western equine encephalitis (WEE)

• Venezuelan equine encephalitis (VEE)

These diseases, each of which is caused by a different virus, occur in different regions of the world: EEE occurs mainly in eastern North America and the Caribbean, WEE in western North America and parts of Central and South America, and VEE in Mexico and Central and South America. West Nile virus is present throughout North America.

The severity of illness varies by type. EEE and VEE are often severe, and horses can die quickly. West Nile disease and WEE are often less serious, and some infected horses might not have any symptoms if their immune system controls the infection.

Signs of viral encephalomyelitis can include:

• fever

• vision problems

• wandering

• head pressing

• circling

• trouble swallowing

• unsteady gait

• weakness

• paralysis (inability to move)

• seizures

• death

Horses with West Nile encephalomyelitis might also have these symptoms:

• incoordination

• muscle spasms

• sensitivity to touch

• muzzle twitching early in the disease

Veterinarians diagnose viral encephalomyelitis by considering the horse's symptoms, the geographic location and time of year (both of which influence mosquito activity), and the results of blood tests that detect antibodies against the suspected virus.

There is no specific cure, so treatment is supportive. This might include giving the horse fluids, anti-inflammatory drugs, or antiseizure medications, and providing careful nursing care.

This disease can be prevented by vaccinating horses. Vaccines are available for all four types of encephalitis. A veterinarian can recommend an appropriate vaccination schedule based on geographic location and current risk.

Mosquito control is also important for prevention and includes removing standing water, cleaning troughs and buckets weekly, and decreasing insect breeding areas.

Outcomes vary. Some horses with viral encephalomyelitis recover completely, whereas others are left with lasting neurological damage. Some horses die from the disease.

People can catch these viruses from infected mosquitoes but not directly from horses. Human illness ranges from mild flu-like symptoms to severe disease. Children, older adults, and people with immune system problems are more likely than others to become seriously ill.