Encephalitis in Horses

Kaela Schraer, DVM
By Kaela Schraer, DVM on Oct. 11, 2022
Horse in stables

In This Article


What is Encephalitis in Horses?

Encephalitis is inflammation of the brain. In horses, there are four main viruses that cause encephalitis:

  • Eastern Equine Encephalitis Virus (EEE)

  • Western Equine Encephalitis Virus (WEE)

  • Venezuelan Equine Encephalitis Virus (VEE)

  • West Nile Virus (WNV)

All four of these diseases are transmitted by mosquitoes–meaning any horse can be infected but unvaccinated horses are most likely to become clinically ill. That’s why it’s important to maintain a regular vaccination schedule for your horse every year.

EEE and WNV are also transmissible to humans through mosquito bites. EEE, WEE, and WNV are all found in the United States and may be reportable diseases depending on the state you live in. VEE is considered a Foreign Animal Disease because there are no natural reservoirs of it in the United States, and therefore it is reportable everywhere.

Symptoms of Encephalitis in Horses

Early symptoms, as listed below, may resemble many other infectious diseases:

  • Fever

  • Lethargy or depression

  • Lack of appetite

Neurologic signs are the hallmark of encephalitis and can include:

  • Changes in vision

  • Circling

  • Disorientation

  • Changes in behavior

  • Muscle twitching

  • Head pressing

  • Dysphagia (inability to swallow)

  • Paralysis

  • Seizures

Ultimately, if symptoms become severe enough, these diseases can lead to death.

Causes of Encephalitis in Horses

Encephalitis is primarily caused by four viruses: EEE, WEE, VEE and WNV. Wild bird and mammal populations act as reservoirs for the viruses. Mosquitos pick up the virus by taking a blood meal from an infected animal and then transmit the virus to a horse through a bite.

How Veterinarians Diagnose Encephalitis in Horses

An ELISA or PCR test can be used on samples of blood or spinal fluid to isolate the virus particles to diagnose symptomatic cases.

Treatment of Encephalitis in Horses

There is no cure for encephalitis in horses, so treatment primarily focuses on supportive care:

  • Non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone and banamine

  • Steroids

  • IV fluids

  • Nutritional support

  • Sedation for circling erratic patients

  • Sling support for unstable or paralyzed patients

Recovery and Management of Encephalitis in Horses

As frightening as these diseases may sound, most horses who have been vaccinated for these diseases recover without significant issues. If symptoms were mild, no long-term changes may be necessary for your horse’s lifestyle. If your horse experiences any of the more severe neurologic signs, these can linger, and your veterinarian may advise you to stop riding and to use caution when handling your horse.

Prevention of Encephalitis in Horses

Vaccinations are the first line of defense against all these viruses. Horses who are vaccinated are much more likely to survive an infection and typically have much milder symptoms.

Mosquito control is also very important to help prevent encephalitis in horses. This includes emptying and cleaning water buckets regularly and eliminating any sources of unused, standing water.

Encephalitis in Horses FAQs

Can horses recover from encephalitis?

Yes, most horses recover from encephalitis. The likelihood of a full recovery with no long-term complications is higher in vaccinated horses.

How long does encephalitis last in horses?

Encephalitis can last days to weeks in horses and in severe cases, the side effects may last years.

Is encephalitis contagious?

The viruses that cause encephalitis are transmitted by mosquitoes. They cannot be transmitted through direct contact between horses or between horses and humans.


Eastern & Western Equine Encephalomyelitis. AAEP.

Equine Encephalitis (EEE/Wee/Vee). USDA APHIS | Equine Encephalitis (EEE/WEE/VEE).

Equine Encephalitis. The Center for Food Security and Public Health. 2009.

Young, Amy. West Nile Virus. UC Davis Veterinary Medicine Center of Equine Health. 2021.

Featured Image: iStock.com/VladGans


Kaela Schraer, DVM


Kaela Schraer, DVM


Dr. Kaela Schraer graduated from Tufts Cummings School of Veterinary Medicine in 2017 with her doctorate in veterinary medicine. After...

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