Eastern equine encephalitis (EEE), otherwise known as Triple E or sleeping sickness, is a rare and lethal virus in horses that causes inflammation of the brain.

EEE is transmitted through mosquito bites and attacks the central nervous system (CNS). It is most prevalent in the eastern regions of North America. [1]

The prognosis for this encephalitis is very poor, especially once neurological symptoms have developed. While there is no cure for EEE, supportive treatment may be administered to make the horse comfortable and help them fight infection. [2]

If you think your horse may have contracted the EEE virus, contact your veterinarian immediately for examination and diagnosis. Prompt intervention is critical for survival and for protecting other horses in the area.

Eastern Equine Encephalitis (EEE)

The terms encephalitis and encephalomyelitis are often used interchangeably when referring to the Eastern Equine Encephalitis virus.

Encephalitis refers to inflammation of the brain, whereas encephalomyelitis refers to inflammation of the brain and spinal cord. [3]

Horses are affected by several encephalitis viruses transmitted by insects that can lead to nervous system dysfunction. These viruses include West Nile, Western Equine Encephalitis (WEE) and Venezuelan Equine Encephalitis (VEE).

The clinical signs of EEE and VEE are typically very severe compared to the other viral encephalitides. [4]

Clinical Signs

During a viral epidemic of EEE, horses tend to be the first animals to show signs of infection.

After a horse becomes infected with EEE, there is a brief incubation period lasting two to three weeks before symptoms begin to appear. These symptoms are usually severe and often result in death within a few days. [4][5][6][7]

Clinical signs of EEE include:

  • Neurological signs (impaired vision, wandering, head pressing)
  • Ataxia (poor coordination and irregular gait)
  • Mild fever
  • Dullness and depression
  • Tremors
  • Inability to urinate or defecate
  • Seizures
  • Paralysis
  • Death

In the event of an encephalitis outbreak among horses or livestock, a viral infection should be investigated as the primary cause. The presence of mosquitoes in the area impacts both the investigation of the outbreak and the diagnosis of the infection. [3]

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Where is the EEE Virus Found?

The EEE virus is predominantly found in the Great Lakes regions and the Atlantic coast states of North America. There have been documented cases in the Caribbean islands as well. Transmission of the virus primarily takes place in regions with freshwater swamps. [6][8]

Cases of of EEE are commonly reported in regions of the United States, including Florida, Georgia, New Jersey, New Hampshire, Maine, and Massachusetts.

In Canada, equine fatalities caused by the EEE virus have been reported in Alberta; however, they are more prevalent in Ontario and the eastern provinces.

Viral Transmission

The EEE virus circulates between avian species and mosquitoes, whereby the latter become infected when they feed on birds carrying EEE. Horses, classified as ‘dead-end hosts’, do not harbor high levels of the virus in their blood and are unable to transmit it to other animals or insects. [9]

The mosquito species typically responsible for outbreaks in North America is Culiseta melanura, although there are more than 20 mosquito species that may transmit the virus during the spring and summer seasons. [6]

During winter, the mosquito’s inactive season, birds, reptiles, and amphibians serve as reservoir hosts. However, most of these animals are not highly susceptible to the EEE virus, and they may not develop an infection. [8]

The C. Melanura mosquito is believed to sustain the circulation of the EEE virus among wild birds until it lands and feeds on a susceptible dead-end host, such as a horse.

EEE Virus in Humans

While rare, humans can contract the EEE virus from mosquitoes. Both humans and horses are dead-end hosts, so they cannot transmit the virus to other mosquitoes or animals. In humans, the infection carries a significant risk of mortality, and there is no licensed vaccine available. [6][8]

Symptoms of EEE infection in humans resemble those of the flu, including fever, chills, muscle aches, and vomiting. In severe cases, the virus can lead to death. EEE stands out as the most lethal mosquito-borne disease in North America for both humans and horses.

It’s important to note that humans cannot catch the EEE virus from horses. However, precautionary measures should be taken when dealing with symptomatic horses. EEE can cause abnormal and unpredictable behaviors in horses, altering their demeanor. [10]

EEE Virus in Other Animals

EEE infection has been documented in various mammals, such as pigs, cows, cats, and dogs. Additionally, the virus can affect a range of other animals, including birds, reptiles, and amphibians. [5]


Vaccines are available to safeguard horses against the EEE virus. These immunizations are classified as core prophylaxis, indicating that they are recommended or mandatory for all horses, depending on the region. [11]

Core vaccines demonstrate a high level of effectiveness against severe or life-threatening diseases and pose a low risk of adverse effects.

In the United States and Canada, there are four core vaccines recommended for horses residing or traveling in these regions. These vaccines include:

  • Eastern & Western Equine Encephalomyelitis (EEE/WEE)
  • Tetanus
  • West Nile Virus
  • Rabies

EEE Vaccine

The horse can begin to develop protective antibodies against the EEE virus within as little as 3 days following vaccination.

Regular immunization has effectively reduced the occurrence of the EEE virus in horses across North America. Unfortunately, epidemics still arise, most likely due to the relatively brief duration of immunity (4 – 6 months) provided by the vaccine. [12]

Horses that have successfully recovered from Eastern equine encephalitis should receive an annual booster, as the duration of immunity after this illness remains uncertain. [5]

During particularly wet or mild years, the mosquito season may last longer than the usual 4 – 6 months. In these situations, some horses may require multiple booster shots, at the discretion of your veterinarian.


Foals should receive three doses of the EEE vaccine, with the initial dose administered between 4 – 6 months of age. The second dose should follow 4 – 6 weeks after the first, and the third dose is recommended between 10 – 12 months of age. [11]

Pregnant Mares

Inactivated vaccines are safe for pregnant mares. Unvaccinated pregnant mares or those with unknown vaccine history should be given two doses of an inactivated vaccine 3 – 6 weeks apart.

It is recommended to administer a booster shot to the mare 4 – 6 weeks before the birth or before the start of the mosquito season. This will help to transfer immunity to the developing foal via colostrum intake after birth. [5][11]

Adult Horses

Adult horses should be vaccinated yearly when mosquito season starts. Horses residing in high-risk, swampy areas or those with compromised immune systems may need more frequent vaccinations. [11]

Unvaccinated horses should receive two doses of the vaccine, spaced 3 – 6 weeks apart as directed by the manufacturer’s instructions. If the vaccination history of a horse is unknown, it is best to consider them as unvaccinated.

Biosecurity Measures

Good biosecurity policies should be adopted to prevent infection with mosquito-borne disease in horses. Fly sheets, masks and sprays offer protection by deterring mosquitoes from landing on your horse. [13]

Mosquitoes lay their eggs in still water, including puddles, stock tanks and standing water in creeks. Sources of drinking water should be dumped and refilled daily to prevent larvae from forming and reduce mosquito breeding grounds.

Most mosquito species are crepuscular, meaning they are most active at dusk and dawn. Mosquitoes avoid sunlight because exposure to too much sun can dehydrate and kill them. Turn out during daylight can help to reduce the risk of mosquito bites.

Mosquitoes also avoid windy conditions, because wind makes flying and landing more difficult. Installing fans in the barn to generate a gentle breeze and allowing horses to be turned out on windier days can be effective in preventing mosquito bites. [13]


Diagnosing EEE virus usually involves a combination of physical and neurological examinations, along with laboratory testing. Since several conditions that cause brain swelling present similar clinical signs, the diagnosis of the virus is often determined based on the geographic region.

Given the overlap of symptoms with other conditions, laboratory testing is crucial for confirming EEE in horses. Blood work and cerebrospinal fluid (CSF) analysis can be conducted to detect specific antibodies (IgM) that indicate EEE virus infection. [4]

Differential Diagnosis

A differential diagnosis refers to the process of considering and evaluating multiple potential conditions or diseases that could explain the observed clinical signs, in order to determine the most likely cause.

Several other conditions in horses affect the central nervous system, leading to neurological symptoms that are similar to EEE virus. Some of these diseases and infections include, but are not limited to: [9]

Your veterinarian will consider these other causes of neurological symptoms to determine whether your horse is affected by Eastern equine encephalomyelitis or another condition.

Neurological Examination

Your veterinarian will conduct a thorough assessment of your horse’s nervous system to evaluate any potential abnormalities or diseases affecting the brain, spinal cord, or peripheral nerves. Neurological exams are helpful, but difficult to standardize, which can lead to diagnostic inconsistencies. [14]

The exam usually involves walking the horse in a straight line, circling, and backing. Your veterinarian will observe your horse’s posture, movement, and reflexes, as well as assess their behavior and responses to stimuli to determine disease progression and prognosis.

Before conducting a hands-on examination, it is beneficial to observe horses in their stall or pasture to view their behavior in the absence of humans.

Encephalopathic horses often display abnormal behaviors such as compulsive circling, head-pressing against walls or corners, and noticeable changes in mental well-being. Depressive behavior and decreased responsiveness to stimuli are commonly observed in horses with neurological infections.

Horses with encephalitis typically present clear signs of neurological dysfunction. The challenging aspect lies in determining the specific virus causing the brain swelling. Once behavioral abnormalities are identified by a veterinarian, differential diagnosis is used to determine the cause. [14]

Horses experiencing ataxia, characterized by severe lack of coordination, should never be examined or observed under saddle due to safety concerns for both the horse and rider. Movement abnormalities can be assessed in-hand to minimize the risk of injury.


There is no cure for horses affected by EEE. Treatment focuses on providing supportive care, to control pain and reduce inflammation of the central nervous system.

Horses with neurological symptoms should be kept in a dark stall with as little light and noise as possible. Provide deep and comfortable bedding to give the affected horse a safe and restful environment. [10]

Supportive Care

If the affected horse experiences difficulty drinking due to neurological symptoms and becomes dehydrated, administration of intravenous (IV) fluids may be required. In some cases, a feeding tube may be necessary to ensure sufficient food intake. [4][5]

Precautions should be taken to avoid prolonged periods of recumbency (lying down) and self-inflicted injuries resulting from abnormal behaviors. In some cases, a sling may be used to support the horse during the recovery process. [5]


Medications may be administered to minimize pain, and reduce inflammation associated with EEE virus infection. It’s important to note that these treatments address the symptoms and provide comfort, but are not a cure for EEE.

Infected horses may be treated with anti-inflammatories, such as dexamethasone, to reduce inflammation in the brain. Antipyretics can be used to prevent fever during recovery.

If a horse develops seizures, they may be controlled with barbiturates, diazepam or other anti-seizure medications. [5]

In addition, diuretics such as furosemide may be used to reduce fluid retention and swelling in the body. Further research is needed on the efficacy of diuretics for treating inflammation in the brain.


The prognosis for horses diagnosed with EEE is very poor, especially for those with severe neurological symptoms that cannot stand up. [2]

The EEE virus has a high mortality rate, with approximately 90% of infected horses succumbing to the infection within a short timeframe of around 2 – 3 days. [2][4]

Many horses that recover from EEE infection may experience permanent brain damage. Due to the rapid development of clinical signs and severity of neurological symptoms, euthanasia is often necessary.

If your horse shows any signs of neurological issues, contact your veterinarian immediately for an examination. Prompt administration of supportive care may help horses with Eastern equine encephalitis fight infection and improve their long-term outlook.

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  2. Young, A. Eastern Equine Encephalitis (EEE). UC Davis Veterinary Medicine. 2020.
  3. Callanan, J. J. Meningitis, Encephalitis, and EncephalomyelitisinAnimals.Merck VeterinaryManual. 2021.
  4. Long, M. T. Equine Viral Encephalomyelitis(Encephalitis). Merck VeterinaryManual. 2019.
  5. Eastern Equine Encephalomyelitis. VetLexicon. Accessed at June 2, 2023.
  6. Aréchiga-Ceballos, N. & Aguilar-Setién, A. Alphaviral equine encephalomyelitis (Eastern, Western and Venezuelan). Rev Sci Tech. 2015.View Summary
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