Equine Protozoal Myeloencephalitis (EPM) is a neurological disease affecting horses and other equids, including mules and donkeys. It’s caused by protozoan parasites and, though relatively uncommon, it can have serious consequences if left untreated. Here’s what you need to know about EPM, including causes, symptoms, diagnosis, and treatment.

What Causes EPM in Horses? Two Types of Protozoa

EPM is primarily caused by the protozoan Sarcocystis neurona, responsible for the vast majority of cases. A smaller number of cases are attributed to Neospora hughesi. These protozoa invade the horse’s central nervous system (CNS), leading to inflammation and neurological symptoms.

Geographic Range of EPM

EPM has been diagnosed throughout the 48 contiguous U.S. states, southern Canada, Mexico, and parts of Central and South America. While exposure is common, the disease itself is rare, thanks to the horse’s immune system.

How Horses Get EPM

The infective form of the protozoa—sporocysts—are shed in opossum feces and can contaminate feed or water. Horses ingest these sporocysts, which once inside your horse’s intestinal tract, the protozoa begin their journey within one to two days. They penetrate the intestinal wall and spread through the body via the lymphatic system and bloodstream, often reaching the liver and muscles. In most healthy horses, the immune system successfully eliminates the protozoa at this stage. However, in horses with a weakened or compromised immune response, the protozoa may continue to migrate to the central nervous system—particularly the spinal cord—where they damage nerve tissue and trigger inflammation. This is what leads to the neurological signs seen in EPM.

Understanding the Two Possible Outcomes After EPM Exposure

When a horse is exposed to EPM-causing protozoa, one of two outcomes can occur:

1. Exposure Without Disease (Most Common – ~99%)
   The horse’s immune system successfully eliminates the protozoa before they reach the central nervous system.

• Blood test: Positive (due to exposure)
• Cerebrospinal fluid (CSF) test: Negative
• Clinical signs: None—no neurological symptoms develop.

2. Exposure With Active Disease (~1%)
   The immune system fails to clear the protozoa, allowing them to migrate into the spinal cord and cause damage.

• Blood test: Positive
• Cerebrospinal fluid (CSF) test: Positive
• Clinical signs: Present—neurological symptoms such as ataxia, muscle loss, or behavior changes.

Are All Horses at Risk of Developing EPM?

Interestingly, around 90% of horses show exposure to EPM protozoa based on blood tests, but only about 1% develop clinical disease. This means that most horses’ immune systems successfully prevent the parasite from causing harm. A positive blood test simply indicates exposure—not active disease. Currently, there is no vaccine available to prevent EPM.

Is EPM Contagious Between Horses?

No. Horses are dead-end hosts for the EPM protozoa. They do not shed the infective sporocysts, and the disease is not spread through manure, saliva, or contact. A horse with EPM is safe to be around other horses in the barn.

Signs and Symptoms of EPM in Horses

EPM presents with neurological signs, but they are not specific to this disease, so other causes must be ruled out. Notably, EPM does not usually cause fever or pain, which helps distinguish it from some other neurological disorders.

Common EPM Symptoms Include:

• Asymmetric muscle loss, especially in the gluteals
• Incoordination or ataxia, often worse on one side
• Wobbling or stumbling when turning in circles
• Spooking or ducking at imaginary obstacles
• Dropping the head suddenly when entering stalls

Diagnosing EPM in Horses

1. Rule Out Other Causes

Since EPM mimics other neurological diseases, a full neurological exam is essential to eliminate other possibilities.

2. Testing Options

• A single blood test for EPM is not considered reliable on its own, as approximately 90% of horses—including healthy ones—may test positive due to past exposure. This means the test cannot distinguish between horses that have simply been exposed and those with active neurological disease, leading to potential false positives.
• The most accurate diagnostic approach involves comparing antibody levels in both blood and cerebrospinal fluid (CSF). This helps determine whether the protozoa have crossed into the central nervous system.
• In many cases, veterinarians will use atherapeutic trial—starting treatment for EPM and monitoring for improvement—as part of a diagnostic strategy, especially when clinical signs are strongly suggestive of the disease.

Treatment for EPM: More Than Just Medications

It’s important for horse owners to understand that only about 60% of horses show noticeable improvement within 90 days of starting anti-protozoal treatment. This highlights the need for a comprehensive approach—anti-protozoal medications alone are not a guaranteed cure. Incorporating easy, cost-effective supportive therapies can greatly enhance recovery and improve long-term outcomes. Treating EPM involves anti-protozoal medications and support therapies to enhance recovery.

1. Anti-Protozoal Medications

These drugs require a veterinary prescription and are FDA-approved to treat Sarcocystis neurona.

• Marquis® (ponazuril) – Oral gel given for 28 days
• Protazil® (diclazuril) – Oral pellets or compounded powder

Tip: Bioavailability improves when given with corn oil or omeprazole.

2. Supportive Therapies

Support therapies are crucial for recovery and are cost-effective and easy to administer.

• Thyro-L® (levothyroxine) – Supports circulation and immune function
• Vitamin E (e.g., Health-E®) – Antioxidant, nerve, and muscle support
• Equioxx® – Anti-inflammatory to reduce CNS inflammation as protozoa die
• Good Equine Deworming Program – Reduces parasite burden and strengthens immunity
  • Use Equimax in spring/fall, Quest in winter

Controlling Exposure to Opossum Feces

While horses don’t spread EPM, opossums do. Their feces can contaminate hay and water with EPM-causing sporocysts. Reduce risk by:

• Securing trash bins with tight-fitting lids
• Avoiding bird feeders, which attract opossums
• Using humane traps to relocate opossums
• Recognizing limitations – Even with precautions, wind and rain can carry contamination from nearby fields

Final Thoughts on EPM in Horses

While EPM can be a serious neurological disease, it’s relatively rare and often preventable through immune support and environmental control. Understanding the disease, recognizing early signs, and working with your veterinarian for diagnosis and treatment can significantly improve outcomes for affected horses. If you need help or have more questions on Equine Protozoal Myeloencephalitis (EPM), call us at 800.786.4751.