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.

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.

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.

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

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

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

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

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

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

• IV fluids
• Anti-inflammatory drugs
• Anticonvulsant drugs