EPM: Understanding this Debilitating Disease

Courtesy of AAEP

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 (and in some areas as high as 90%) of horses in the United States may have been exposed to the organism that causes EPM. The primary causative organism is a protozoal parasite called Sarcocystis neurona, and with much less frequency, another protozoal parasite—Neospora hughesi can also cause this condition Fortunately, only a very small percentage of horses which have been exposed to either of these organisms will ever develop the disease we call EPM.The disease is not transmitted from horse to horse. Rather, the organism Sarcocystis neurona is spread by the definitive host, the opossum, which acquires the organism from cats, raccoons, skunks and armadillos 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. The definitive or intermediate hosts for Neospora hughesi have not yet been identified.

Once 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 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 (rarely);
  • 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:

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

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 make because there is no specific assay for this disease and because clinical signs of EPM mimic other neurological diseases. Your veterinarian will first conduct a thorough physical 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 or 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 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 the horse has been exposed to the parasite, not that it has or will develop clinical disease. 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 70 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:

  • There are anti-protozoal drugs specifically labeled by the FDA to treat EPM. Consult with your veterinarian for more detailed information and treatment options. 
  • 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.
  • For most of the FDA approved products, the average duration of treatment is one month, although depending on which medication is used, there is a combination product that may require a longer term treatment—up to 3-9 months.
  • Treatments can be expensive.
  • Although complications are rare, treatments may affect stallion fertility and may pose certain health risks to unborn foals.
  • While success rates are high, not all horses respond positively to therapy. Approximately 10 to 20 percent 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 white blood cell count.
  • Some drugs used to treat EPM are antifolate 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.

Based on published research, there are several things horse owners can do to reduce the risk from infection with EPM.  At best, good horse-keeping practices will discourage unwanted visitors such as opossums and other rodents from contaminating 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 for wild animals to access.
  • Clean up any dropped grain immediately to discourage scavengers.
  • Feed heat-treated cereal grains and extruded feeds since these processes seem to kill the infective sporocysts.
  • Do not feed on the ground
  • Good rodent control
  • Properly dispose of any animal carcasses you may see on or near your property
  • 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.