Equine Encephalitis Viruses (EEE, VEE, and WEE) in Horses

Published on
Last updated on
5 min read

Key takeaways

Equine encephalitis viruses are a group of severe, life-threatening infections affecting the brain.

  • The three major viruses responsible for equine encephalitis are Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE)
  • Common symptoms of viral encephalitides include fever, blindness, head pressing, uncoordinated movement, and paralysis
  • A horse presenting symptoms of viral encephalitides requires blood and cerebrospinal fluid tests
  • There is no cure for equine encephalitis viruses
  • Treatment is of a supportive nature, and includes IV fluid and pain and inflammation management medications
  • The prognosis for viral encephalitides is extremely poor
  • If horses recover, they are likely to suffer from life-long neurological deficits
Are you concerned?

Connect with a vet to get more information about your pet’s health.

Book an online vet

A closer look: Equine Encephalitis Viruses (EEE, VEE, and WEE) in Horses

There are three equine encephalitis viruses, which have similar symptoms and presentations.

  • EEE is the most common form of equine encephalitis. Symptoms of EEE usually progress quickly, and are very severe with most horses becoming recumbent. Most horses die within 2-3 days of showing symptoms.
  • WEE is considered less severe than EEE. Horses develop fever initially, and many recover before progression to neurologic symptoms. Most horses with neurologic symptoms can recover with supportive care, as long as they do not become recumbent.
  • VEE is among the most severe kinds of equine encephalitis virus. The progression of symptoms is similar to EEE, however it occurs more rapidly, often within 1-2 days after infection.

The prognosis depends on the virus causing the encephalitis. VEE and EEE are generally more severe and have an extremely poor prognosis. Regardless of the kind of virus, emergency medical attention is warranted as infected horses often require hospitalization in order to manage pain and inflammation.

Risk factors

Viral encephalitides are a group of life-threatening viral conditions that rarely affect horses. To be infected, horses have to live in or travel to endemic areas. EEE and WEE can be found in eastern Canada and US states west of the Mississippi. VEE is not endemic in the United States, and primarily occurs in South America. The viruses are spread by mosquitoes, so cases are more common during seasons where mosquitoes are prevalent.

Possible causes

Viral encephalitides are caused by alphaviruses. These kinds of viruses are generally transmitted to mosquitoes by birds and rodents. Birds and rodents do not develop clinical signs of the disease. Once infected, the mosquitoes can pass on the virus to humans and to horses.

Both humans and horses are referred to as “dead-end” hosts, as they cannot transmit the virus to a feeding mosquito. The exception is epizootic VEE, which can spread from a horse to a mosquito and may result in human infection with the virus.

Main symptoms

Horses usually develop clinical signs of viral encephalitides 3 days to 2 weeks after the bite of the infected mosquito.

Testing and diagnosis

A horse presenting with symptoms of equine encephalitis viruses requires the following diagnostics:

  • Physical examination
  • Neurologic examination
  • Blood tests
  • Cerebrospinal fluid testing

Steps to Recovery

There is no specific cure for equine encephalitis viruses. Treatment is of a supportive care nature and involves:

  • IV fluids
  • Anti-inflammatory drugs
  • Anticonvulsant drugs

The prognosis for equine encephalitis viruses is extremely poor, with most deaths occurring within 2-3 days after the onset of symptoms.

Mortality rates of the three main kinds of equine encephalitis viruses are as follows:

  • EEE is between 85-100%
  • VEE is between 40-80%
  • WEE is between 20-40%

The majority of horses that recover from equine encephalitis viruses suffer from long-term neurological deficits and have a low probability of returning to athletic careers.


Annual vaccination against alphaviruses is a core preventative measure. Equine encephalitis virus vaccinations are extremely effective in preventing the condition. Vaccination for equine encephalitis viruses has to be repeated annually. Vaccination for VEE is not recommended for horses living in non-endemic areas.

In addition to vaccination, environmental controls to limit exposure to mosquitoes help reduce the likelihood of transmission. Strategies include:

  • Eliminate still-water sources
  • Limit outdoor activities at sunrise and sunset when mosquito activity is most frequent
  • Apply veterinarian-approved insect repellent

Is Equine Encephalitis Viruses (EEE, VEE, and WEE) in Horses common?

Equine encephalitis is a rare condition in horses. In 2021 a total of 111 EEE cases were reported in the United States.

Typical Treatment

There is no specific cure for equine encephalitis. Supportive care includes:

  • IV fluids
  • Anti-inflammatory drugs
  • Anticonvulsant drugs


Robert J. MacKay , BVSc, PhD, DACVIM, / Demia de Tonnerre , BVSc, DACVIM-LAIM - Writing for Merck Veterinary Manual
Maureen T. Long , DVM, PhD, DACVIM - Writing for Merck Veterinary Manual
KAREN BRIGGS - Writing for The American Quarter Horse Association
- Writing for Animal and Plant Health Inspection Service
Les Sellnow - Writing for The Horse

Our editorial committee

Our medical review team is responsible for validating and maintaining the quality of our medical information.