What is EPM?

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Equine Protozoal Myeloencephalitis (EPM)

By: Excel Supplements

What is EPM?

Equine Protozoal Myeloencephalitis, commonly known as EPM, is a disease that affects the central nervous system of a horse.

What causes EPM in horses?

EPM is caused by the single-celled, parasitic, protozoan organism, Sarcocystis neurona (shown to be the primary causative agent for EPM). Through a complicated life cycle this protozoa eventually resides in the horse’s central nervous system causing inflammation in the brain stem or spinal cord, creating the condition referred to as myeloencephalitis.

Sarcocystis neurona breaks down into a two-part cycle which includes a definitive host and several possible intermediate hosts. The definitive host is the opossum who then sheds in its feces an infective form of the parasite, S. neurona. The ineffective form of the parasite, defined as the “sporocyst”, is then ingested by the horse, allowing the parasite to go through a maturation or reproductive phase and eventually produce a form of the parasite that we call “merozoites”. Eventually, the merozoites reach and damage the central nervous system of the horse leading to neurological symptoms in some horses.

The intermediate host is an animal that does not spread an infective form of S. neurona but can be inhabited by the parasite so that it can complete its life cycle. Intermediate hosts include; skunks, armadillos, raccoons, cats and even sea otters.

Chestnut Horse Eating Fresh Grass

A weak immune system appears to be one of the greatest risk factors when considering disease progression in horses.

What are EPM symptoms in horses?

Being a disease of the central nervous system, EPM can affect multiple locations within the brain and the spinal cord leaving the signs and severity of this disease to vary. It is more common for the signs of disease to be associated with spinal cord damage, but we also see evidence of damage in the brain.

Dr. Rob MacKay and Kenton Morgan have defined three components of this disease referred to as the 3 “A’s”.

  1. Asymmetry is used to describe a symptom that is worse on one side of the body than on the opposite side (most commonly seen in muscle development).
  2. Ataxiais used to describe the horse’s inability to coordination its movement.
  3. Atrophy describes muscle deterioration.

Other symptoms include:

  • Abnormal gaits
  • Intermittent lameness
  • Loss of sensation in the face, neck, or body
  • Paralysis of the muscles of the eyes, face or mouth
  • Poor balance
  • Major changes in mood

The most severe symptoms include seizures and collapsing, and the inability to stand.
EPM can either be fast or slow in the progression of the disease with no two horses presenting the same. These symptoms can also be part of a multiple neurological disorder. If any of these symptoms, including the American Association of Equine Practitioners’ “3 A’s”, are present in your horse, discuss with your vet the next steps in EPM diagnosis.


Excel Supplements is a producer of unique nutritional supplements for horses and dogs. Our products provide an extraordinary combination of omega 3, 6, 9 and other nutritional components. They are particularly rich in omega 3 essential fatty acid content as well as in vitamin E, phytosterols, polyphenols and triterpenes. We provide the best all-natural supplements you can give your animal, period.

How do horses get EPM?

There are multiple different risk factors that can directly affect the development and course of this disease. A weak immune system appears to be one of the greatest risk factors when considering disease progression in horses. Due to the fact that the immune system can be seriously compromised during times of stress, horses who are in heavy training, traveling long distances, experiencing environmental changes, older horses with a weakened immune system and younger horses with developing immune systems are at a high risk for developing EPM. Another significant risk factor is geography and relates to horses that are residing within an exposed area where EPM is heavily prevalant.


How do you diagnose EPM in horses?

EPM found early and treated quickly has a better prognosis. Your vet can run tests on two different sample types: blood and cerebrospinal fluid. Even though there is not a definitive test, blood titers will give you a good idea of exposure with possible infection. A positive titer in the cerebrospinal fluid will be even more indicative infection versus exposure.


How do you treat EPM in horses?

If your horse is diagnosed with EPM, your vet can prescribe medications to help combat it, however treatments can be extremely expensive and taxing on your horse. Treatment times are long and can last from one month to several months. Supportive medications like NSAIDs can be given to help decrease the inflammation created during protozoa die off.
At Excel Supplements we are happy to be able to compliment medical treatment for EPM symptoms with our ExcelEQ ProElite™. Our ProElite™ formula filled with natural vitamin E antioxidants, polyphenols and a blend of plant-based Omegas, help keep an infected horse’s hind-gut absorbing vitamins and nutrients at full capacity. Protection against free radicals released during protozoa kill-off is of high importance so omega-3 and vitamin E supplementation is a great compliment during treatment and beyond. It is imperative that these horses receive the best possible medical attention and nutritional support to lead to the best outcome.


How do you prevent EPM in horses?

The best way to prevent exposure to the protozoa that cause EPM is by keeping food and water areas clean of feces and other debris from high traffic areas. It is imperative that your horses are on a well-balanced diet and have check-ups with your veterinarian on a regular basis.

1. Reed S. Neurology is Not a Euphemism for Necropsy: A Review of Selected Neurologic Diseases Affecting Horses, Proceedings of the 54th Annual Convention of the AAEP, San Diego, CA December 2008;78-109
2. Saville WJ, Reed SM, Granstrom DE, et al, Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio. J Am Vet Med Assoc1997;210(4):519-524
3. West, C. Equine Protozoal Myeloencephalitis: Less Common Than We Thought?, TheHorse.com, March 22, 2005, Article #5608. ©2005 TheHorse.com
4. Lindsay DS, Dubey JP, Kennedy TJ Determination of the Activity of Ponazuril against Sarcocystis neurona in Cell Cultures. Vet Para 2001;100(3-4):131-134.
5. Dubey, J.P., Lindsay, D.S., Saville, W.J.A., Reed, S.M., Granstom, D.E., Speer, C.A. A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM), Vet Parasitol 95 2001:89-131


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