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Understanding and Treating EPM

Posted in Horse Racing News

EPM (Equine Protozoal Myeloencephalitis) is a master of disguise. This very serious disease can be difficult to diagnose because its signs often act like other health and lameness problems in horses and the signs can range from mild to extremely severe.

More than fifty percent (and in some areas of the U.S. and Canada can be as high as 90%) of horses may have been exposed to the organism that causes EPM. The primary organism that causes EPM is a protozoal parasite called Sarcocystis Neurona, and also another protozoal parasite Neospora Hughesi can also cause EPM.  Fortunately for us, only a very small percentage of horses which have been exposed to either of these organisms will develop EPM.  This disease is not transmitted from horse to horse.  The organism Sarcocystis Neurona is spread by the definitive host the opossum, which acquires this organism from raccoons, skunks, cats, armadillos and sea otters. The infectious stage of the organism, the sporocysts, is passed through the opossum’s feces. The horse then comes into contact with the infective sporocysts either while grazing, eating contaminated feed and/or drinking water. The definitive or intermediate hosts for Neospora Hughesi have still not yet been identified.

Once ingested by the horse, the sporocysts migrate from the intestinal tract into the bloodstream and then they cross into the blood/brain barrier.  Once there, they begin to attack the horse’s central nervous system. The initial onset of the disease may be slow or sudden. If it is left untreated, EPM can cause devastating and long-lasting neurological damage.


The clinical signs of EPM can vary.  Clinical signs are usually asymmetrical – not the same on both sides of the horse.  Actual signs may depend on the location and severity of lesions that develop in the horse’s brain, brain stem or spinal cord.  These signs may include:

  • Incoordination (Ataxia) and weakness, which worsens when going up or down hills or when their head is elevated
  • Spasticity (stiff movements), abnormal gait or lameness
  • Muscle atrophy, which is most noticeable in the large muscles of the hind end, but can also involve the muscles of the face or front limbs
  • Paralysis of muscles of the eyes, mouth or face, which is evident by drooping eyes, lips or ears
  • Difficulty with swallowing
  • Abnormal sweating
  • Seizures or collapse – which is extremely rare
  • Loss of sensation along the face, body, and neck
  • Odd head tilt along with poor balance – the horse may also lean against stall walls for support.

There are several factors that may influence the progression EPM, however, these four issues appear to be most important:

  • How long does the horse harbor the parasite prior to treatment?
  • What is the extent of the infection – the number of organisms ingested
  • Stressful events following initial infection or stressful events, such as racing or eventing, while infected.
  • The point or points in the brain or spinal cord where the organism settles in and the damage occurs


EPM causes clinical disease in approximately one percent, or less, of exposed horses. Almost every part of the country has reported cases of EPM. However, the incidence of disease is much lower in the western United States, especially in regions with small opossum populations. Due to the transport of horses and feedstuffs from one part of the country to another, almost all horses are at risk.

Not all horses exposed to the protozoa Sarcocystis neurona or Neospora hughesi will develop the disease and show clinical signs of EPM. Some horses seem to mount an effective immune response and are able to combat the disease before it gains a foothold. Other horses, especially those under stress, can succumb rapidly to the debilitating effects of EPM. Still others may harbor the organisms for months or years and then slowly or suddenly develop symptoms.


Diagnosis of EPM is difficult to diagnose because there is no specific diagnosis for this disease.  The clinical signs of EPM mimic other neurological diseases. Your vet will first conduct a very thorough physical exam to assess your horse’s general health and to also identify any suspicious signs.  One notable clue is that the disease often tends to affect only one side or a specific part of the horse more than the other.

If your vet suspects EPM, they may order a blood sample along with a Cerebrospinal fluid (CSF) analysis.  Cerebrospinal fluid may be collected by injecting a needle into the spinal canal either in a site on the lower back or the poll – A Spinal Tap.   There are potential risks involved with this procedure that should be discussed with your vet.  A blood test that comes back positive will only mean that the horse has been exposed to the parasite… not that it has or will develop clinical aspects of the disease.  A quick and accurate diagnosis is essential and the treatment should begin immediately.


The sooner the treatment begins, the better the chances are for recovery.  Sixty to seventy percent of cases aggressively diagnosed and treated show significant or complete reversal of symptoms. Most horses are able to return to normal activity.  Below is a list of steps treating EPM:

  • Toltrazuril is one of the best products to treat EPM.  Consult with your vet for more detailed information.
  • Anti-inflammatory drugs (Bute, Flunixin) may be prescribed to alleviate symptoms and prevent reactions to parasite die-off during treatment.
  • Supplementation with vitamin E (which is an antioxidant) is often recommended as an aid to healing nervous tissue.
  • The average duration of treatment with Toltrazuril is one month, although some horses may require a longer-term treatment (up to 3-9 months) plus, depending on the area, may require maintenance doses.
  • While success rates are very high, not all horses will respond positively to therapy.  Approximately ten to twenty percent of treated horses may experience a relapse… again, location and re-exposure to the parasite can also be an issue.  This is why a monthly maintenance dose of toltrazuril may be required.
  • While a horse is being treated, you should take monthly blood samples to monitor any potential side effects such as low platelet count, anemia and low white blood cell count.
  • Be sure to report all changes in your horse’s condition to your vet.


There are several things horse trainers can do to reduce the risk of contracting EPM.  A clean and well-maintained barn is the first step to keep out such carriers such as opossums and other rodents from contaminating the hay, grain and bedding.  Also, do not feed grain on the ground in paddocks.  You need to do all possible steps to keep these carriers away from where your horse eats.

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