top of page

Unraveling the Mystery of Equine Encephalitis: Exploring EEE and WEE

Updated: Dec 15, 2023


horses outside swishing their tails to swat mosquitoes and flies off

Welcome to our blog, where we delve into the fascinating world of equine health. Today, we shine a spotlight on two significant diseases that have a profound impact on horses and other equines: Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE). These viral infections, transmitted through the bite of infected mosquitoes, pose a serious threat to the equine community and demand our attention. Join us on a journey as we unravel the mysteries surrounding EEE and WEE, exploring their symptoms, prevention strategies, and the importance of early detection and intervention. Get ready to gain valuable insights and equip yourself with the knowledge needed to protect our beloved equine companions.


Understanding Equine Encephalitis

mosquito biting horse could be carrying equine encephalitis
Photo credit: Horse Health Products

Equine encephalitis, or Equine Sleeping Sickness, is a term used to describe a group of viral diseases that affect horses and other equines. It refers to several types of encephalitis, which is the inflammation of the brain. The two main types of equine encephalitis are Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE).


Both EEE and WEE are considered zoonotic diseases, meaning they can be transmitted from animals to humans. However, human infections are relatively rare and typically occur through mosquito bites.


Eastern Equine Encephalitis (EEE)

EEE is caused by the Eastern Equine Encephalitis virus, which is a member of the Togaviridae family. It is primarily transmitted through the bite of infected mosquitoes, with birds acting as the main reservoir for the virus. While horses are considered dead-end hosts for the virus (meaning they do not contribute to its transmission cycle), they can still become infected and develop severe disease.


EEE is characterized by a sudden onset of symptoms, including fever, depression, loss of appetite, and neurological signs such as stumbling, weakness, and altered behavior. The disease can progress rapidly, leading to severe neurological impairment, seizures, coma, and even death in some cases. EEE has a high mortality rate in horses, with few surviving the infection.


Western Equine Encephalitis (WEE)

WEE is caused by the Western Equine Encephalitis virus, also a member of the Togaviridae family. Like EEE, it is primarily transmitted by infected mosquitoes, with birds serving as the reservoir. Horses and other equines can become infected when bitten by infected mosquitoes.


The symptoms of WEE are similar to those of EEE and include fever, depression, anorexia, and neurological signs such as incoordination, muscle tremors, and weakness. While WEE is generally less severe than EEE, it can still cause significant illness and occasionally result in death.

diagram showing mosquito vector of Equine Encephalitis and how it transmits to horses and humans
Photo credit: WBUR

It's important to note that there are other types of encephalitis that can affect horses, such as Venezuelan Equine Encephalitis (VEE) and West Nile Virus (WNV), which also have similar symptoms and transmission patterns.


Eastern Equine Encephalitis (EEE) Symptoms

Eastern Equine Encephalitis (EEE) is a viral disease that affects horses and humans. The symptoms of EEE can vary in severity, and some infected individuals may not display any symptoms at all. However, when symptoms do appear, they often develop suddenly and can be severe. Here are some common symptoms associated with EEE:

  • Fever: Infected horses may experience a high fever, which is often the first noticeable sign of the disease. The fever may persist for several days.

  • Depression and Lethargy: Horses with EEE may exhibit signs of depression, appear weak, and lack energy. They may become lethargic, unwilling to move, and display a general disinterest in their surroundings.

  • Neurological Signs: EEE primarily affects the central nervous system, leading to neurological symptoms. These may include:

    • Stumbling and Incoordination: Horses may have difficulty walking or standing, displaying a lack of coordination. They may stumble, appear unsteady on their feet, or exhibit a wobbly gait.

    • Muscle Twitching and Fasciculations: Involuntary muscle twitches and fasciculations can occur, affecting various parts of the body.

    • Head Pressing: Affected horses may press their heads against walls, fences, or other objects. This behavior is often a result of neurological discomfort or confusion.

    • Altered Behavior and Personality: Horses may display changes in behavior, becoming more irritable, aggressive, or unusually calm and docile.

    • Seizures: In severe cases, EEE can cause seizures characterized by uncontrolled muscle contractions and convulsions.

    • Impaired Vision: Some horses with EEE may experience vision problems, such as blindness or partial loss of sight.

  • Difficulty Swallowing: EEE can affect the horse's ability to swallow properly, leading to difficulties in eating and drinking. This can result in weight loss and dehydration.


It's important to note that EEE can progress rapidly and be fatal. If you suspect your horse may have EEE based on the symptoms described above, it is crucial to seek immediate veterinary attention for diagnosis and treatment. Early intervention can greatly improve the horse's chances of recovery. Additionally, practicing effective mosquito control measures can help reduce the risk of EEE transmission to horses.


Western Equine Encephalitis (WEE) Symptoms

Western Equine Encephalitis (WEE) is a viral disease that affects horses and other equines. The symptoms of WEE can vary in severity, and not all infected animals will display symptoms. However, when symptoms do occur, they typically emerge suddenly. Here are some common symptoms associated with WEE:

  • Fever: Infected horses may develop a fever, which is often one of the initial signs of the disease. The fever may persist for several days.

  • Depression and Lethargy: Horses with WEE may exhibit signs of depression, appearing lethargic and lacking energy. They may be reluctant to move or engage in normal activities.

  • Neurological Signs: WEE primarily affects the central nervous system, leading to neurological symptoms. These may include:

    • Incoordination and Weakness: Horses may have difficulty walking or standing, displaying a lack of coordination and balance. They may stumble or show a wobbly gait.

    • Muscle Tremors and Twitching: Involuntary muscle tremors and twitching may be present, affecting various parts of the body.

    • Altered Behavior: Infected horses may exhibit changes in behavior, such as irritability, restlessness, or a docile and subdued demeanor.

    • Impaired Vision: Some horses with WEE may experience vision problems, ranging from partial loss of sight to complete blindness.

    • Seizures: In severe cases, WEE can cause seizures characterized by uncontrolled muscle contractions and convulsions.

  • Difficulty Swallowing: WEE can interfere with the horse's ability to swallow normally, resulting in difficulties in eating and drinking. This can lead to weight loss and dehydration.


It's important to note that WEE can progress rapidly and may have a significant impact on the affected horse's health. If you suspect your horse may have WEE based on the symptoms described above, it is essential to seek veterinary attention promptly for diagnosis and treatment. Early intervention can increase the chances of a positive outcome. Additionally, practicing mosquito control measures can help reduce the risk of WEE transmission to horses by minimizing exposure to infected mosquitoes.


Diagnosing EEE and WEE in Equines

Diagnosing Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) typically involves a combination of clinical evaluation, laboratory tests, and ruling out other possible causes of the symptoms. Here are some common methods used for diagnosing EEE and WEE:


Clinical Evaluation: A veterinarian will assess the horse's symptoms and medical history, including any known exposure to mosquitoes or affected areas. The clinical signs, such as fever, neurological abnormalities, and behavioral changes, can provide important clues for diagnosis.


Equine encephalitis under a microscope
Photo credit: CDC

Blood Tests: Blood samples may be collected to test for the presence of specific antibodies against the EEE or WEE viruses. This can be done through various laboratory techniques, including enzyme-linked immunosorbent assay (ELISA) or virus neutralization tests. These tests help confirm the presence of the virus and differentiate between EEE and WEE.


Cerebrospinal Fluid Analysis: In cases where neurological symptoms are prominent, a sample of cerebrospinal fluid (CSF) may be collected through a procedure called a spinal tap or lumbar puncture. CSF analysis can help detect abnormalities and provide further evidence of viral infection.


PCR Testing: Polymerase chain reaction (PCR) testing can be used to detect the genetic material (RNA) of the EEE or WEE virus in blood or CSF samples. PCR is a highly sensitive and specific technique that helps identify the presence of the virus, especially in the early stages of infection.


Differential Diagnosis: Since the symptoms of EEE and WEE can overlap with other diseases, it's important to rule out alternative causes. The veterinarian may consider other conditions such as West Nile Virus (WNV), rabies, equine herpesvirus, or other neurological disorders.


It's crucial to involve a veterinarian in the diagnostic process, as they have the expertise and resources to accurately diagnose EEE and WEE. Early diagnosis is vital for initiating appropriate treatment and supportive care to improve the horse's chances of recovery. Remember, prevention through vaccination and mosquito control is also essential in reducing the risk of these diseases in horses.


map showing reported cases of eastern equine encephalitis from 2019
2019 reported cases of Eastern Equine Encephalitis in the US. Photo credit: USDA

Treatment for EEE and WEE

The treatment for Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) in horses primarily focuses on supportive care and managing the symptoms. Since there are no specific antiviral treatments available for these diseases, the goal is to provide comfort, alleviate symptoms, and facilitate the horse's recovery. Here are some common approaches used in the treatment of EEE and WEE:

equine veterinarian giving fluids to a horse with EEE
Photo credit: The Ohio State University
  • Veterinary Care It is crucial to involve a veterinarian in the treatment of EEE or WEE. They will evaluate the horse's condition, monitor vital signs, and provide appropriate medical care throughout the course of the disease.

  • Symptom Management: Various medications and therapies can be used to manage the symptoms associated with EEE and WEE, including:

    • Anti-inflammatory Medications: Nonsteroidal anti-inflammatory drugs (NSAIDs) may be prescribed to reduce fever, inflammation, and pain.

    • Fluid Therapy: Intravenous fluid administration helps maintain hydration, electrolyte balance, and support the horse's overall well-being.

    • Nutritional Support: Horses with EEE or WEE may experience reduced appetite or difficulty eating due to neurological symptoms. Specialized feeds or assisted feeding methods may be necessary to ensure adequate nutrition.

    • Seizure Control: Anticonvulsant medications can be prescribed to manage seizures and control convulsions if they occur.

    • Eye Care: If the horse experiences vision problems, ophthalmic treatments or supportive measures may be employed to protect the eyes and promote healing.

  • Nursing Care: Providing a quiet and calm environment is essential for horses with EEE or WEE, as they may be sensitive to external stimuli. Proper bedding, regular cleaning of the stall, and minimizing stressors can contribute to the horse's comfort and recovery.

  • Wound Management: In some cases, horses with EEE or WEE may develop secondary complications such as skin lesions or pressure sores due to difficulty moving or lying down. Proper wound care and prevention of infection are crucial in such cases.


It's important to note that the prognosis for horses affected by EEE or WEE can vary depending on the severity of the disease and the horse's overall health. Unfortunately, some cases may be fatal despite the best supportive care. Prevention through vaccination and mosquito control measures remains the most effective approach to protect horses from EEE and WEE. Consulting with a veterinarian is essential to determine the most appropriate treatment plan based on the individual horse's condition.


Effective Prevention Strategies for EEE and WEE

Prevention plays a crucial role in minimizing the risk of Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) in horses. Since these diseases are primarily transmitted through mosquito bites, effective mosquito control measures are key. Here are some prevention strategies to consider:


Vaccination

Vaccination is an essential preventive measure for EEE and WEE. There are vaccines available that provide protection against these diseases. Consult with your veterinarian to determine the appropriate vaccination schedule for your horses. Keep in mind that vaccination may need to be repeated annually or as recommended by the vaccine manufacturer.


Mosquito Control

horse owner using fly spray on their horse to prevent mosquitoes
  • Eliminate Breeding Sites: Regularly inspect your property for standing water, which serves as breeding grounds for mosquitoes. Empty or treat containers, such as buckets, troughs, and birdbaths, to prevent water accumulation.

  • Reduce Exposure to Mosquitoes: Minimize the time horses spend outdoors during peak mosquito activity periods, which are typically dawn and dusk. Use fans or protective screens in stable areas to reduce mosquito presence.

  • Use Insect Repellents: Apply equine-approved mosquito repellents or insecticides on horses to help repel mosquitoes. Follow the product instructions carefully, and use products specifically formulated for use on horses.

  • Protective Measures: Use lightweight and breathable horse blankets or fly sheets to cover horses during peak mosquito activity times. Additionally, consider using mesh fly masks to protect their faces.


Environmental Management

  • Mow Grass and Vegetation: Regularly mow grass and trim vegetation in and around horse paddocks and stables to reduce mosquito resting areas.

  • Remove standing water: Removing standing water will reduce the areas for mosquitos to breed and remove any current larva growing in it.

  • Fence and Secure Perimeter: Install fine-mesh screens or netting around stables or create screened-in areas to prevent mosquitoes from entering horse living spaces.

  • Fans and Ventilation: Promote good air circulation and ventilation in stables and barns using fans, as mosquitoes are less likely to settle and bite in windy conditions.

diagram showing the life cycle of a mosquito. mosquitos carry equine encephalitis
Photo credit: Community Impact

Surveillance and Early Detection

Stay informed about EEE and WEE cases in your area. Be aware of any local disease outbreaks or increased mosquito activity. Report any suspected cases promptly to local animal health authorities and seek veterinary attention for affected horses.


Remember, prevention is essential not only for the health of individual horses but also for reducing the overall transmission and spread of EEE and WEE in equine populations. Implementing a comprehensive approach that combines vaccination, mosquito control, and environmental management can significantly decrease the risk of these diseases in horses. Consult with your veterinarian for specific prevention recommendations based on your location and horse's individual needs.


Concluding Thoughts on EEE and WEE

As we conclude our exploration of Eastern Equine Encephalitis and Western Equine Encephalitis, it becomes clear that these diseases pose a significant threat to the well-being of our beloved equine companions. The sudden onset of symptoms, the potential for severe neurological impairment, and the absence of specific antiviral treatments highlight the importance of prevention and early intervention.


By prioritizing vaccination and implementing effective mosquito control measures, we can significantly reduce the risk of EEE and WEE in horses. Regular veterinary care, prompt reporting of suspected cases, and a vigilant approach to environmental management all play crucial roles in safeguarding equine health.


Let us remain vigilant and informed about the prevalence of EEE and WEE in our regions. Together, we can protect our horses from these potentially devastating diseases. Remember, your local veterinarian is your greatest ally in devising a comprehensive prevention plan tailored to your horse's unique needs.


By staying proactive and committed to responsible horse care, we can ensure that our equine companions enjoy a healthy and thriving life, free from the grasp of EEE and WEE. Let us join hands in preserving their well-being and celebrating the deep bond we share with these majestic creatures.

horse laying down in the pasture is vaccinated from EEE
66 views0 comments
bottom of page