West Nile Encephalomyelitis in Horses

West Nile encephalomyelitis in horses is swelling of the brain and/or spinal cord caused by infection with the West Nile virus. This virus came to North America in 1999. It spreads mainly through mosquito bites.

Not all horses that get infected with West Nile virus become seriously sick. In fact, many infected horses never show signs of disease. However, horses that do develop symptoms can become very ill, and some die.

In the United States, the number of reported cases of West Nile encephalomyelitis in horses was highest in the early 2000s and later went down. This was likely because of vaccination and mosquito control.

Any horse can get infected, no matter its breed, age, or sex. Horses that are not vaccinated against the virus are at greater risk of infection. Adult horses are infected more often, and older or unvaccinated horses are more likely to become severely ill.

Signs of West Nile encephalomyelitis can vary from horse to horse and can look like symptoms of other illnesses. Early symptoms might include anxiety, a mild fever, lameness, or just seeming unwell.

As the disease affects the horse's nervous system, more serious signs can appear, including:

• muzzle twitching

• vision problems

• wandering

• pressing the head against things

• walking in circles

• trouble swallowing

• an unusual gait

• tremors

• poor coordination

• weakness

• muscle spasms

• paralysis (inability to move)

• seizures

• sometimes death

Some horses also become very sensitive to sound, light, or touch. Severe paralysis increases the risk of death.

The vet might do a blood test that checks the horse's blood for an immune system response to West Nile virus. This test, along with the horse’s symptoms, helps support the diagnosis.

There is no cure for West Nile encephalomyelitis. Treatment focuses on keeping the horse as comfortable and stable as possible while its body fights the infection. This might include giving anti-inflammatory medicines, such as nonsteroidal drugs and sometimes steroids, although results vary. Good nursing care is very important for horses with this disease.

Many horses that recover improve within 1–6 months and might return to normal. However, some horses can have long-term nerve or movement problems.

Vaccination is one of the best ways to prevent West Nile virus in horses. It greatly lowers the chance of infection, and vaccinated horses that do get sick usually have milder signs and better recovery.

Mosquito control is also important. Helpful steps include cleaning water containers weekly, removing standing water, using fans (safely) in barns, and using insect control products in the environment and on horses as directed by the product label.

Horses usually do not spread West Nile virus to people. People get infected mainly from mosquito bites, not from caring for a sick horse.