Know the Signs of Equine Protozoal Myeloencephalitis (EPM) – A Master of Disguise

Why it’s important to diagnose and treat EPM early.

Equine protozoal myeloencephalitis (EPM) is a debilitating neurological disease that cannot be ignored. More than half of all horses in the United States, and in some areas as high as 90 percent, have been exposed to this disease. Its ability to masquerade as other health issues, such as lameness or other neurologic diseases, makes it difficult to diagnose, and its effects can be heartbreaking for the horse and owner.

While Sarah Reuss, VMD, DACVIM, Merial Veterinary Services, acknowledges that most horses exposed to EPM will not develop the clinical disease, horses under stress are more likely to show signs. In some cases, horses may even be infected without showing signs for months or years or ever. When horses do show signs, it often goes misdiagnosed because it can slowly progress and infect any portion of the central neurologic system, therefore mimicking other conditions.

If EPM is left untreated, it may lead to permanent damage.

Reuss emphasizes the importance of early detection by horse owners and farm managers, diagnosis by a veterinarian and an effective treatment plan. “These are the keys to stopping the progression of the disease. The faster treatment begins, the better the chance for the horse to recover.”

When it’s EPM, Be Ready

MARQUIS (15% w/w ponazuril) Antiprotozoal Oral Paste is the first FDA-approved treatment for EPM. It has been proven to make the difficult transition across the tough blood-brain barrier to reach the central nervous system and successfully kill the parasite responsible.

If treated quickly and properly, horses can recover from EPM. The earlier treatment begins, the better the expected outcome.

While not required, your veterinarian may recommend starting treatment with a loading dose of MARQUIS. This enables the drug to reach steady state more quickly – in as little as 24 hours.*

How Does a Horse Get EPM in the First Place?

It starts with a parasite, specifically the protozoal parasite (Sarcocystis neurona) that is found in the opossum after it ingests contaminated tissue from intermediate hosts such as the armadillo, skunk, or raccoon. In its infective stage, the sporocysts are passed through the opossum’s feces, which the horse comes into contact with as he grazes, eats or drinks contaminated feed or water.

Once consumed, the sporocysts travel from the intestine into the bloodstream and cross the blood/brain barrier where they can cause inflammation and damage the horse’s central nervous system.It is also now known that EPM can be caused by a second organism (Neospora hughesi) whose life cycle is not well understood.

Early Diagnosis is Crucial

“EPM can evolve slowly or present suddenly with varied signs from mild to severe,” Reuss says.

There are several factors that can determine its progression and severity, including:

  • How long the horse has been infected,
  • The points in the brain or spinal cord where the sporocysts have infected,
  • Stressful events during EPM infection.

“Lesions, inflammation and pressure may develop in the brain, brain stem or spinal cord,” Reuss says. “Depending on the location, they can affect how the disease presents itself and can make it tricky to diagnose.”

She notes that the American Association of Equine Practitioners (AAEP) has listed these EPM clinical signs to watch out for:

  • Incoordination, weakness, abnormal gait
  • Muscle loss on one side, usually along the topline or the hindquarters,
  • Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips,
  • Loss of sensation of the face
  • Difficulty swallowing
  • Head tilt with poor balance; horse may assume a splay-footed stand or lean against stall walls for support.

She also notes that loss of coordination of the mouth, tongue, and throat area may make picking up feed, chewing, and swallowing it difficult, which can affect your choice of treatments.

The MARQUIS gel paste provides quick and accurate dosing. It also provides peace of mind knowing the treatment was given and not lost in bedding, especially if the horse has a decreased appetite or difficulty chewing due to the disease.

Take-Home Message

Reuss emphasizes the importance of understanding the risk and knowing the signs.
“With an early EPM diagnosis and immediate, effective treatment, horses can show significant improvement, and they may return to normal activity.”

Learn more about EPM and MARQUIS by consulting your veterinarian and visiting

IMPORTANT SAFETY INFORMATION: The safe use of MARQUIS in horses used for breeding purposes, during pregnancy, or in lactating mares, has not been evaluated. In animal safety studies, loose feces, sporadic inappetence, lost weight, and moderate edema in the uterine epithelium were observed.

About Boehringer Ingelheim Animal Health

On January 1st, 2017, Merial became part of the Boehringer Ingelheim group. As the second largest animal health business in the world, Boehringer Ingelheim is committed to making the industry even better at improving animal health. With more than 10,000 employees worldwide, Boehringer Ingelheim Animal Health has products available in more than 150 markets and a global presence in 99 countries. For more information about Boehringer Ingelheim Animal Health, click here.

About Boehringer Ingelheim

Boehringer Ingelheim is one of the world’s 20 leading pharmaceutical companies. Headquartered in Ingelheim, Germany, Boehringer Ingelheim operates presently with a total of some 50,000 employees worldwide. The focus of the family-owned company, founded in 1885, is on researching, developing, manufacturing and marketing new medications of high therapeutic value for human and veterinary medicine. In 2015, Boehringer Ingelheim achieved net sales of about 14.8 billion euros. R&D expenditure corresponds to 20.3 per cent of net sales. For more information, please visit

Merial is now part of Boehringer Ingelheim.

®MARQUIS is a registered trademark of Merial. ©2017 Merial, Inc., Duluth, GA. All rights reserved. EQUIOMQ1613 (4/17)

*Clinical relevance has not been determined.

American Association of Equine Practitioners. Horse Health, EPM: Understanding this Debilitating Disease. Available at:
Accessed February 24, 2017.
MARQUIS Product Label






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