EPM: The Master of Disguise

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Stumbling. Weakness and muscle atrophy in the hind end. Behavioral changes. Any of these could be a sign your horse has equine protozoal myeloencephalitis (EPM). It can have devastating effects on a horse’s ability to perform, which is why few diseases are feared more by performance horse owners than EPM.

A progressive and potentially fatal infection of the central nervous system, EPM is caused by a single-celled protozoal microorganism, most commonly Sarcocystis neurona (S. neurona), and is primarily transmitted by opossums through their feces.

EPM is one of the most common neurological diseases of horses in North America. Researchers estimate that approximately 30 to 80% of the U.S. horse population has encountered the parasite and produced antibodies. However, not all horses exposed become infected or develop clinical signs.

Often referred to as the “master of disguise” because it can look like many other neurologic diseases or even obscure lameness. West Nile virus, rabies, tetanus, wobbler syndrome, eastern and western equine encephalitis and equine herpes myeloencephalopathy (EHM), and even trauma, can all cause neurologic deficits in horses. Without thoroughly evaluating the horse and performing appropriate diagnostic tests, it is difficult to effectively treat them. It is important to have your veterinarian conduct a thorough physical and neurological exam and appropriate diagnostic work up to confirm EPM. Treating without an operating diagnosis can be frustrating.

Clinical signs of EPM may include:

  • Ataxia (unsteady gait)
  • Weakness
  • Muscle atrophy (typically asymmetrical)
  • Cranial nerve deficits (including inability to swallow)
  • Decreased tongue tone
  • Ear or eyelid droop
  • Head tilt
  • Blindness
  • Seizures (rarely)

Disease prevention

Although the veterinary community is constantly working toward a proven way to prevent EPM, for now, the best way to keep your horse from getting the disease is to avoid exposing him to opossum feces. Keeping a clean barn and property to avoid attracting opossums into feed sources is important.

If you’re concerned about EPM, talk to your veterinarian and keep the following risk factors in mind:

  • Mature horses are more commonly affected than very young horses
  • There is a higher incidence among horses used for western performance, racing and other strenuous activities
  • Events such as long-distance transportation, concurrent illness, or strenuous exercise increase the risk because they can suppress the horse’s immune system
  • Increased numbers of cases are identified during late summer and fall, but EPM can appear any time of the year
  • Risk is increased on premises where horse feed is not protected from opossums and/or pet food is left in the barn, which can attract them
  • Barns and pastures near wooded terrain are more likely to attract opossums
  • Horses residing on farms with a history of EPM cases are at increased risk

Treatment Options

If your veterinarian confirms your horse has EPM, there are fortunately FDA-approved EPM treatment products available. One option is PROTAZIL (1.56% diclazuril). PROTAZIL is the only FDA-approved product that comes in a convenient top-dress, pelleted formula.

Having your horse diagnosed with EPM is stressful and having to treat them for 28 days or more with an oral paste is not always easy for the owner or the horse. The alfalfa-based PROTAZIL pellets are well-accepted by horses and consumed without the mess, fuss and stress of a paste.

“Our clients find the pellets much easier to administer than a paste,” says Greggory S. Bell, D.V.M., of Centennial Equine Sports Medicine in Colorado. “And, the horses like it better as well. We’ve even seen some horses eat the PROTAZIL pellets before consuming their grain concentrate. As a result, we’ve been able to achieve better treatment results against this devastating disease.”

Equally as important as the product’s palatability and easy administration is its ability to quickly reach therapeutic levels without the need for a loading dose at the start of treatment.[1] When it comes to treating horses with EPM, using a product that is rapidly absorbed means the damaging parasite has less time to attack the horse’s nervous system. PROTAZIL attains effective drug levels within 12 hours against a disease where time matters.

Early diagnosis and treatment critical to recovery

From diagnosis to treatment and prevention, your veterinarian is your best EPM resource. If you are concerned about EPM or think your horse might be showing neurological signs, call your veterinarian immediately. Horses that are diagnosed early and treated aggressively have the best chance for recovery.

Important Safety Information

Use of Protazil® (1.56% dicazuril) is contraindicated in horses with known hypersensitivity to diclazuril. Safe use in horses used for breeding purposes, during pregnancy, or in lactating mares has not been evaluated. The safety of Protazil® (1.56% dicazuril) with concomitant therapies in horses has not been evaluated. See related page in this issue for details. For use in horses only. Do not use in horses intended for human consumption. Not for human use. Keep out of reach of children.

Copyright © 2017 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.


[1] Hunyadi L, Papich MG, Pusterla N. Pharmacokinetics of a low-dose and FDA-labeled dose of diclazuril administered orally as a pelleted top dressing in adult horses. J of Vet Pharmacology and Therapeutics (accepted) 2014, doi: 10.111/jvp.12176