Equine Protozoal Myeloencephalitis (EPM) is a neurological disease that can affect horses of any age, breed or performance level. It is the most commonly diagnosed cause of neurologic disease in horses throughout the Americas. If not identified and treated promptly, EPM can result in significant and sometimes irreversible neurological damage. Recognizing the early, subtle signs of EPM in horses can help you safeguard your horse’s health and ability to perform.
EPM is a neurologic condition caused by small parasites, most commonly Sarcocystis neurona, and less frequently, Neospora hughesi. These microscopic organisms invade a horse’s central nervous system, potentially affecting their spinal cord and brainstem. The disease occurs when these parasites cross the blood-brain barrier and begin damaging neural tissues, a process that scientists still don’t fully understand.
The opossum is the main carrier of S. neurona. Horses typically become infected by eating or drinking something contaminated with opossum feces that contains the parasite. While about half of horses in regions with opossum populations test positive for antibodies against S. neurona—meaning they’ve been exposed—fewer than 1% actually develop clinical EPM.
Other species like skunks, armadillos, raccoons, sea otters and even domestic cats can act as intermediate hosts, helping spread these parasites throughout the environment. Horses cannot transmit the disease to one another, but there have been rare instances of vertical transmission from infected mares to foals.
EPM cases don’t follow a clear pattern. It can affect any horse, regardless of age, breed, or season, which means vigilance is needed year-round for all horses.
The clinical signs of EPM vary significantly depending on which part of the nervous system is affected. Some horses may develop symptoms suddenly, while others experience a slow progression. One of the most telling clues is that the signs are often asymmetrical, affecting one side of the body more than the other. Unfortunately, this variability and the subtle signs of EPM in horses makes early detection difficult, especially since symptoms can mimic other conditions.
Researchers observed that horses affected by N. hughesi are more likely to have other health problems at the same time, which can further complicate diagnosis.
Diagnosing EPM is not as straightforward as running a single test. This can make it challenging for veterinarians to identify the disease. They must take a comprehensive, multi-faceted approach.
Several antibody-based tests are available to help your veterinarian identify whether your horse’s neurologic symptoms are linked to Sarcocystis neurona or Neospora hughesi. Understanding how these tools work can help you understand the path to diagnosis:
|
Test Type |
Antibody Detection |
How It Works |
Pros |
Cons |
|
Western Immunoblot (WB) |
S. neurona |
Detects presence of antibodies in blood or CSF |
Can rule out EPM in areas where the parasite isn't common |
Healthy horses can carry antibodies without having the disease |
|
Indirect Fluorescent Antibody Test (IFAT) |
SarcoFluor® test for S. neurona; NeoFluor® test for N. hughesi |
Provides antibody levels to determine likelihood of disease |
More sensitive and specific, cost-effective, and offers a lower chance of false positives than the WB |
Requires interpretation of antibody levels to determine likelihood of active disease |
|
Enzyme-linked Immunosorbent Assays (ELISAs) |
S. neurona; N. hughesi is available but validation is limited |
Identifies surface antigens, proteins expressed by the parasite |
More valuable when combined with CSF testing or serum-to-CSF antibody ratio |
Not all strains of S. neurona express the same antigens, limiting reliability |
Yes, EPM is treatable and can be reversible in some cases, especially when caught early. Subtle signs of EPM in horses can make it difficult to identify, but the longer the disease progresses untreated, the more likely it is to cause lasting damage. After treatment, about 60% of horses show noticeable clinical improvement. Approximately 25% of horses with EPM fully recover.
Unfortunately, some horses relapse after successful treatment, sometimes even months or up to a few years later. Your horse should receive ongoing monitoring and supportive care even after they’ve been treated or cured of EPM.
The goals of treatment for EPM in horses include eliminating the parasite, minimizing neurological damage and reducing inflammation. Your veterinarian’s treatment plan may vary depending on the severity and progression of your horse’s symptoms and often includes a combination of medications.
These medications are the cornerstone of EPM treatment and target the protozoa. Courses generally last 28-60 days, but some horses may require longer treatment depending on their response to medication. Your veterinarian will evaluate the medication’s effectiveness based on your horse’s clinical improvement.
Mixlab offers several options, including:
Diclazuril:
Ponazuril:
During your horse’s first few days on anticoccidial medications, your veterinarian may recommend a short course of NSAIDs to help with symptoms. They can prevent the worsening of neurologic signs during early treatment.
Mixlab offers multiple NSAIDs to help with EPM symptoms, including:
Used cautiously, corticosteroids are often prescribed for short-term use if your horse is experiencing significant brain-related symptoms. In these cases, your horse may risk injuring themselves by falling due to neurological deficits. Corticosteroids are frequently combined with dimethyl sulfoxide to help control inflammation.
Since a weakened immune system may increase a horse’s risk of developing EPM, some veterinarians incorporate immunomodulating agents into their treatment plan. While this approach may be promising, it’s important to note that the effectiveness of immunomodulators in horses with EPM has not yet been scientifically validated through clinical studies.
One of these medications is levamisole, available in Mixlab’s compounded formulations with diclazuril. While diclazuril is an anticoccidial medication, levamisole is considered an immunomodulator on its own.
How to Effectively Support Your Horse with EPM
These measures can work alongside your horse’s medical treatment to promote their long-term health and stability. While there is currently no proven method to prevent EPM, implementing supportive care strategies may also help reduce your horse’s risk of disease relapse.
Not only does Vitamin E support your horse’s immune system, but it can act as an antioxidant to protect nerve function and tissue health during EPM treatment. Keep in mind that its potential benefits are not yet confirmed through controlled scientific studies.
There is no vaccine to prevent EPM, but minimizing environmental risk factors can lessen your horse’s chance of exposure.
Maintaining a healthy immune system supports your horse’s overall well-being. This may play a role in promoting a stronger response to diseases like EPM and recovery.
At Mixlab, we know how overwhelming an EPM diagnosis can be. That’s why we offer free, two-day shipping of EPM medications directly to your doorstep, the barn or your veterinary clinic. Our expert equine pharmacy team is always available to answer questions and support your horse’s recovery.
Whether your veterinarian just made a diagnosis or you’re exploring long-term care, we’re here to help. When your horse is facing EPM, you shouldn’t have to face it alone. Simply submit your prescription online or contact us today with questions about treatment for EPM in horses.