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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]
What's your top priority with your horse's health?
What's your top priority with your horse's health?
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 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]
Prevention
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
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]
Diagnosis
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 initial 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]
- Rabies, EHV-1, WEE virus, WNV and other infections that cause brain and spinal cord swelling
- Equine herpesvirus-1 myeloencephalopathy (EHM)
- Bacterial meningitis
- Equine protozoal myeloencephalitis (EPM)
- Botulism
- Lead poisoning
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. [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, the veterinarian will rule out differential diagnoses one by one until the cause is determined. [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.
Treatment
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]
Medication
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.
Prognosis
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.
Frequently Asked Questions
Here are some frequently asked questions about Eastern Equine Encephalitis (EEE) in horses:
Eastern Equine Encephalitis, or EEE, is a rare but highly lethal mosquito-borne viral disease that causes inflammation of the brain and central nervous system. It is also called Triple E or sleeping sickness. Affected horses can develop severe neurological signs that progress rapidly. Because there is no cure, vaccination and mosquito control are the most important ways to protect horses. [1][2]
The terms encephalitis and encephalomyelitis are often used when discussing Eastern Equine Encephalitis. Encephalitis means inflammation of the brain, while encephalomyelitis means inflammation of both the brain and spinal cord. EEE can cause serious nervous system dysfunction, similar to other insect-borne equine viruses such as Western Equine Encephalitis, Venezuelan Equine Encephalitis, and West Nile virus. EEE and Venezuelan Equine Encephalitis typically cause more severe clinical signs than some other viral encephalitides. [3][4]
Horses become infected with Eastern Equine Encephalitis when bitten by a mosquito carrying the virus. The virus normally circulates between birds and mosquitoes, and horses are considered dead-end hosts because they do not develop enough virus in their blood to spread infection to other animals or insects. The mosquito species Culiseta melanura is an important vector in North America, although more than 20 mosquito species may transmit the virus. Outbreaks are more likely in areas with heavy mosquito activity, especially near freshwater swamps or standing water. [6][8][9]
Eastern Equine Encephalitis is most commonly found in eastern regions of North America, particularly around the Great Lakes and Atlantic coast states. Cases are reported in parts of the United States such as Florida, Georgia, New Jersey, New Hampshire, Maine, and Massachusetts. In Canada, EEE-related equine fatalities are more common in Ontario and eastern provinces, although cases have also been reported elsewhere. Transmission is especially associated with regions that have freshwater swamps and persistent mosquito populations. [6][8]
Signs of Eastern Equine Encephalitis in horses often include fever, dullness, depression, tremors, poor coordination, abnormal gait, impaired vision, wandering, head pressing, seizures, paralysis, inability to urinate or defecate, and death. Symptoms usually appear after an incubation period of about two to three weeks. Once neurological signs develop, the disease often progresses quickly and can be fatal within a few days. Any horse showing neurological signs should be examined by a veterinarian immediately. [4][5][6][7]
Horses cannot spread EEE to people, other horses, or mosquitoes because they are dead-end hosts. Humans are also dead-end hosts and become infected through mosquito bites rather than contact with horses. However, caution is still important around symptomatic horses because neurological disease can cause abnormal, unpredictable, or unsafe behaviour. Handling should be done carefully and under veterinary guidance. [6][8][10]
People can become infected with Eastern Equine Encephalitis through the bite of an infected mosquito, but human cases are rare. Infection in humans may cause flu-like symptoms such as fever, chills, muscle aches, and vomiting. Severe cases can be fatal, and there is no licensed human vaccine. EEE is considered one of the most lethal mosquito-borne diseases in North America for both horses and humans. [6][8]
EEE is diagnosed using the horse's history, clinical signs, geographic location, physical examination, neurological examination, and laboratory testing. Bloodwork and cerebrospinal fluid analysis may be used to detect IgM antibodies that indicate EEE virus infection. Because many neurological diseases can look similar, testing is important to distinguish EEE from conditions such as rabies, equine herpesvirus-1 myeloencephalopathy, West Nile virus, equine protozoal myeloencephalitis, botulism, bacterial meningitis, and lead poisoning. [4][9]
A neurological exam helps assess how the horse's brain, spinal cord, and nerves are functioning. A veterinarian may observe the horse at rest, then evaluate posture, behaviour, reflexes, walking in a straight line, circling, and backing. Horses with encephalitis may show depression, poor responsiveness, compulsive circling, head pressing, or severe incoordination. Horses with significant ataxia should not be ridden because they can be unsafe for both horse and rider. [14]
There is no cure for Eastern Equine Encephalitis in horses. Treatment is supportive and focuses on reducing pain, controlling inflammation, maintaining hydration and nutrition, managing seizures if they occur, and keeping the horse as safe and comfortable as possible. Horses with neurological signs may need a quiet, dark stall with deep bedding to reduce stimulation and prevent injury. Severe cases may require intravenous fluids, assisted feeding, a sling, anti-inflammatory medication, antipyretics, anti-seizure medication, or other intensive supportive care. [4][5][10]
The prognosis for horses with Eastern Equine Encephalitis is very poor, especially once severe neurological signs develop or the horse becomes unable to stand. The mortality rate is approximately 90%, and many affected horses die within two to three days after severe signs appear. Horses that survive may have permanent brain damage. Because the disease progresses quickly, immediate veterinary care is critical if EEE is suspected. [2][4]
Horse owners can help prevent Eastern Equine Encephalitis through vaccination and mosquito control. EEE vaccination is considered a core vaccine for horses in the United States and Canada, along with Western Equine Encephalomyelitis, tetanus, West Nile virus, and rabies. Mosquito control measures include eliminating standing water, cleaning and refilling water sources daily, using fly sheets, masks, sprays, and fans, and turning horses out during daylight hours when mosquitoes are less active. These steps reduce mosquito exposure and lower the risk of infection. [11][13]
Adult horses should generally be vaccinated yearly at the start of mosquito season. Horses in high-risk areas, swampy regions, or locations with long mosquito seasons may need more frequent boosters at a veterinarian's discretion. The EEE vaccine may provide immunity for only about four to six months, which is why extra boosters may be recommended during wet or mild years when mosquitoes remain active for longer. Horses with unknown vaccination history should be treated as unvaccinated and receive the initial two-dose series. [11][12]
Summary
Eastern equine encephalitis (EEE), also known as sleeping sickness, is a rare but highly fatal mosquito-borne virus that causes brain inflammation in horses. Once neurological symptoms appear, prognosis is poor, and treatment focuses on supportive care to reduce pain and inflammation. Preventative vaccination and mosquito control are essential to protect horses from infection.
- EEE is transmitted by mosquitoes and affects the central nervous system, causing severe neurological dysfunction
- Clinical signs include fever, ataxia, seizures, paralysis, and rapid progression to death within days
- Vaccination is the most effective preventive measure, with immunity lasting 4–6 months before boosters are needed
- Reducing mosquito exposure through environmental control and turnout management helps lower infection risk
References
- Guidelines for the Vaccination of Horses. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Accessed May 28, 2023.
- Young, A. Eastern Equine Encephalitis (EEE). UC Davis Veterinary Medicine. 2020.
- Callanan, J. J. Meningitis, Encephalitis, and Encephalomyelitis in Animals. Merck Veterinary Manual. 2021.
- Long, M. T. Equine Viral Encephalomyelitis (Encephalitis). Merck Veterinary Manual. 2019.
- Eastern Equine Encephalomyelitis. VetLexicon. Accessed June 2, 2023.
- Aréchiga-Ceballos, N. & Aguilar-Setién, A. Alphaviral equine encephalomyelitis (Eastern, Western and Venezuelan). Rev Sci Tech. 2015. View Summary
- Barba, M. et al. Equine viral encephalitis: prevalence, impact, and management strategies. Vet Med (Auckl). 2019. View Summary
- Corrin, T. Eastern Equine Encephalitis Virus: A Scoping Review of the Global Evidence. Vector Borne Zoonotic Dis. 2021. View Summary
- Franklin, R. P. Eastern Equine Encephalomyelitis Virus Infection in a Horse from California. Emerg Infect Dis. 2002. View Summary
- Lecollinet, S. et al. Viral Equine Encephalitis, a Growing Threat to the Horse Population in Europe? Viruses. 2019. View Summary
- Vaccine Guidelines. American Association of Equine Practitioners (AAEP). Accessed May 28, 2023.
- Del Piero, F. et al. Clinical, pathologic, immunohistochemical, and virologic findings of eastern equine encephalomyelitis in two horses. Vet Pathol. 2001. View Summary
- Kramer, L.D. and Tavakoli, N.P. Eastern Equine Encephalitis Virus. IN: Manual of Security Sensitive Microbes and Toxins. CRC Press. 2014.
- Johnson, A. L. How to Perform a Complete Neurologic Examination in the Field and Identify Abnormalities. American Association of Equine Practitioners (AAEP). 2010.
