Equine Encephalitis

By: Tam Garland, DVM, PhD, DABVT

TVMA Member
College Station, TX

Published July 2015

Photo by Tracy Fant Colvin, DVM

What is Equine Encephalitis?

Equine encephalitis is a viral infection affecting the horse’s brain and spinal cord. There are three encephalitis viruses that occur in Texas: Eastern equine encephalitis (EEE), Western equine encephalitis (WEE) and Venezuelan equine encephalitis (VEE). Now controlled by widespread vaccination, large outbreaks of EEE and WEE rarely occur in the U.S., though isolated cases and small outbreaks have been seen, especially with EEE. While VEE has not been seen recently in the U.S., serious epidemics of the virus have occurred in South America in recent years.

How is Equine Encephalitis Transmitted?

The equine encephalitis viruses are transmitted to horses through mosquitoes. Birds are the primary reservoir hosts of the viruses, particularly EEE and WEE, while rodents are the typical hosts for VEE. Mosquitoes can transmit these viruses to humans as well.

Horses should be vaccinated against the equine encephalitis viruses annually, ideally before the spring when mosquitoes are most prevalent. Owners should discuss a vaccination schedule with their veterinarian as it is suggested that horses with increased susceptibility (e.g., limited immunity, travel stress, etc.) and those living where mosquitoes are most active year-round should be vaccinated more frequently.

What are the Signs?

The clinical signs of the equine encephalitis viruses include fever, depression, loss of appetite, reacting abnormally to stimuli, involuntary muscle movements, circling, wandering, itchiness, lying down, paddling legs, pressing its head against surfaces and diarrhea or constipation. Note that West Nile Virus has similar signs of infection and is often combined in the vaccination for the equine encephalitis viruses. Death from the viruses can often occur within days. Of the viruses, WEE has the lowest mortality rate at 50 percent, while EEE is the most deadly to horses with its 90-percent mortality rate. There is evidence suggesting that young horses are more susceptible to EEE (http://www.aaep.org/-i-165.html).

Horses sometimes recover from encephalitis but often display lasting neurological signs. Once a horse has been infected by equine encephalitis, antivirals may be prescribed, but treatment generally consists only of supportive care. Infected horses are often euthanized as they often become dangerous due to falling and stumbling.

How Can Equine Encephalitis be Prevented?

Horse owners should discuss with their veterinarian ways to prevent exposure to the virus carriers—mosquitoes. There are horse-safe or equine-specific mosquito repellant sprays owners can apply to their horse and spray in their horse’s barn. It is helpful for owners to eliminate any standing water, even puddles, as this attracts mosquitoes. Generally, the horse’s water source is deep enough and experiences enough movement to not be a mosquito attraction.

While protecting our equine friends, it is important to remember that we need to protect ourselves by wearing long-sleeve shirts and using mosquito repellents designed for human-beings.

Tam Garland, DVM, PhD, DABVT, is a graduate of Texas A&M College of Veterinary Medicine who lives in College Station, Texas. Dr. Garland practices at Texas A&M Veterinary Medical Diagnostic Laboratory.