Exploring the Subtle Signs of EPM in Horses & What to Do if You Notice Them

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.

What is EPM in Horses?

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.

How to Identify Subtle Signs of EPM in 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.

Signs of S. neurona EPM Infection

Spinal Cord Involvement:

• Gait abnormalities and incoordination
• Ataxia (difficulty controlling voluntary movement)
• Muscle stiffness or spasticity, especially in all four limbs
• Muscle wasting or atrophy

Brainstem Involvement:

• Lethargy and depression
• Personality or behavioral changes
• Cranial nerve dysfunction, such as:
  • Facial paralysis
  • Tongue weakness or paralysis
  • Difficulty swallowing

• Abnormal stances, like standing with feet wide apart or leaning on walls or fences

Signs of N. hughesi EPM Infection

• Incoordination or ataxia, especially in the hind limbs
• Weakness in the hindquarters
• Generalized weakness, possibly leading to recumbency (lying down)
• Irregular or uneven gaits

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.

How Veterinarians Diagnose EPM in Horses

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.

Evaluation for EPM in Horses

• Neurologic Evaluation: Your veterinarian will ask about or observe your horse for signs of brain disease we discussed previously, such as facial nerve abnormalities or difficulty swallowing.
• Exclusion of Other Diseases: Your veterinarian may want to rule out other conditions with similar symptoms, such as bacterial meningitis, rabies or equine herpesvirus type 1 (EHV-1) infection.
• Serum and Cerebrospinal Fluid (CSF) Antibody Testing: These tests allow your veterinarian to compare antibody levels in your horse’s blood and CSF to identify active infection.

Testing for EPM in Horses

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:

Can EPM in Horses Be Cured?

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.

Exploring Treatment for EPM in Horses

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.

Anticoccidial 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:

• Diclazuril 8.3% Oral Suspension
• Diclazuril 8.3% + Vitamin E Succinate (10,000 IU) Oral Suspension
• Diclazuril 8.3% + Levamisole 3% + Vitamin E (10,000 IU) Oral Suspension
• Diclazuril 8.3% + Levamisole 3% + Vitamin E (656 IU) Paste (60 mL)

Ponazuril:

• Ponazuril 150 mg/mL + Vitamin E (10,000 IU) Oral Paste (60 mL)

Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

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:

• Phenylbutazone:

• Phenylbutazone 1 gm/scoop + Cimetidine 500 mg/scoop Oral Powder
• Phenylbutazone 200 mg/mL + Cimetidine 100 mg/mL Oral Paste
• Phenylbutazone 500 mg/mL Oral Paste

• Flunixin meglumine (Banamine):

• Banamine (Flunixin Meglumine) 100 mL Injectable Solution

Corticosteroids

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.

Immunomodulators

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.

Vitamin E Supplements

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.

Preventive Measures

There is no vaccine to prevent EPM, but minimizing environmental risk factors can lessen your horse’s chance of exposure.

• Avoid feeding directly from the ground
• Use covered feeders and keep water sources clean
• Secure all feed storage areas
• Keep garbage and open food bags in galvanized metal containers
• Get rid of bird feeders on the property
• Promptly clean up spilled grain or fallen fruit
• Deter wildlife from barns and pastures
• Consider trapping and relocating opossums
• Carefully and quickly dispose of any wildlife carcasses

Immune System Support

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.

• Feed a forage-based, low-grain diet
• Provide vitamin and mineral supplements under veterinary guidance
• Incorporate immune-supportive herbs under veterinary guidance
• Integrate holistic care, such as acupuncture or massage therapy
• Minimize stress, such as during travel or weather events
• Avoid overuse of dewormers and antibiotics

Your Proactive Partner: Fast Treatment for EPM in Horses

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.