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Understanding the EPM Disease

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

More than 50 percent of all horses in the United States may have been exposed to the organism that causes EPM. The causative organism is a protozoal parasite called Sarcocysticneurona. The disease is not transmitted from horse to horse. Rather, the protozoa are spread by the definitive host, the opossum, which acquires the organism from cats, raccoons, skunks and armadillos, and possibly even from harbor seals and sea otters. The infective stage of the organism (the sporocysts) is passed in the opossum’s feces. The horse comes into contact with the infective sporocysts while grazing or eating contaminated feed or drinking water.

EPM Disease - Equine Protozoal MyeloencephalitisOnce ingested, the sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There they begin to attack the horse’s central nervous system. The onset of the disease may be slow or sudden. If left undiagnosed and untreated, EPM can cause devastating and lasting neurological damage.


The clinical signs of EPM can be quite varied. Clinical signs are usually asymmetrical (not the same on both sides of the horse.) Actual signs may depend on the severity and location of the lesions that develop in the brain, brain stem or spinal cord. Signs may include:

  • Ataxia (incoordination), spasticity (stiff, stilted movements), abnormal gait or lameness
  • Incoordination and weakness, which worsens when going up or down slopes or when the head is elevated
  • Muscle atrophy, most noticeable along the topline or in the large muscles of the hindquarters, but can sometimes involve the muscles of the face or front limbs
  • Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips
  • Difficulty swallowing
  • Seizures or collapse
  • Abnormal sweating
  • Loss of sensation along the face, neck or body
  • Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support

Several factors may influence the progression of the disease; however, these four things appear to be important:

  1. The extent of the infection (i.e. , the number of organisms ingested)
  2. How long the horse harbors the parasite prior to treatment
  3. The point(s) in the brain or spinal cord where the organism localizes and damage occurs
  4. Stressful events following infection or stressful events while infected

Horses at Risk

Epm Disease United States

EPM is considered the number-one cause of neurologic problems in horses today. 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. But, 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 protozoan Sarcosysticneurona 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.


Your veterinarian will first conduct a thorough physical and neurological examination to assess your horse’s general health and identify any suspicious signs. One notable clue is the disease often tends to affect one side of part of the horse more than another.

If your equine practitioner suspects EPM, he or she may order blood and cerebrospinal fluid (CSF) analysis. Cerebrospinal fluid may be collected by way of a special needle inserted into the spinal cord canal either in a site on the lower back or at the poll. Potential risks are involved with the procedure that should be discussed with your veterinarian.A positive blood test only  means that horse has been exposed to the parasite, not that it has or will develop clinical disease. Currently three laboratories are analyzing blood and spinal fluid for the prescence of S. neurona antibodies. Prompt, accurate diagnosis is essential and treatment should begin immediately.


The sooner treatment begins; the better the horse’s chances are for recovery. Sixty to seventy percent of EPM cases aggressively treated show significant or complete reversal of symptoms. Many horses are able to return to normal activity. Here are some things you should know about treating EPM:

  • At present, there are two labeled anti-protozoal drugs approved by the FDA to treat EPM. There are also combination therapies that may include sulfa antibiotics and anti-folate medications. Consult your veterinarian.
  • Anti-inflammatory drugs may be prescribed to alleviate symptoms and prevent reactions to parasite die-off during treatment.
  • Supplementation with vitamin E, an antioxidant, is often recommended to aid healing of nervous tissue.
  • Average duration of treatment is one month with the proprietary products, although depending on which medication is used, can sometimes be longer (up to 3-6 months with combinations).
  • Treatment can be expensive.
  • Although complications are rare, treatments may affect stallion fertility and may pose certain health risk to unborn foals.
  • While success rates are high, not all horses respond positively to therapy. Approximately 10-20% of horses may experience a relapse.
  • While a horse is being treated, taking intermittent blood samples may be recommended to monitor potential side effects such as anemia, low platelet count and low while blood cell count.
  • Some drugs used to treat EPM are anti-folate drugs. Therefore, periodic examination for anemia is indicated during treatment.
  • Horses undergoing treatment should be closely observed for signs of improvement or decline, especially negative side effects to the drugs, such as acute diarrhea.
  • Be sure to report any changes in the horse’s condition to your veterinarian.

Methods of Prevention

Based on published research, there are several things horse owners can do to protect their horses from infection with EPM. There is currently a vaccine to immunize against Sarcocystisneurona; however, the efficacy is unknown at this time. At best, good horse-keeping practices will discourage unwanted visitors such as opossums and other rodents from contamination hay, grain and bedding.

Here are a few suggestions:

  • Keep feed rooms and containers closed and sealed.
  • Use feeders, which minimize spillage and are difficult to wild animals to access.
  • Clean up any dropped grain immediately to discourage scavengers.
  • Feed heat-treated cereal grains and extruded feeds since these process seem to kill the infective sporocysts.
  • Keep water tanks clean and filled with clean, fresh water.
  • Maximize your horse’s health and fitness through proper nutrition, regular exercise and routine deworming and vaccinations.
  • Schedule regular appointments with your equine veterinarian.

Ongoing Research

EPM was initially identified in 1964. In recent years, awareness among veterinarians and horse owners has grown considerably. Research at the University of Kentucky, the University of Florida, Ohio State University, the University of California at Davis, University of Missouri, Virginia Tech, University of Maryland and Michigan State University, as well as other institutions, is leading to advancements in EPM diagnosis, treatment and a better understanding of the lifecycle ofSarcocystisneurona.

**This information provided courtesy of the AAEP and Bayer Animal Health.  For more information please reference www.yourhorseshealth.com

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