EPM (Equine Protozoal Myeloencephalitis)

EPM is a neurological disease which is caused by sarcocystis neurona—a parasite that attacks the brain and spinal cord. It is a disease that, if not detected and treated early, can seriously damage a horse’s central nervous system and may even lead to death. EPM occurs throughout North America, affecting male and female horses equally. Through studies, it has been found that thoroughbreds and Standardbreds are affected more frequently. Signs can appear at any age, but most cases occur in horses under 4 years old. There is no evidence of this, but perhaps the occurrences in youthful horses could be more prevalent due to maturing, stressed, or weak immune systems. Opossums are the primary parasite hosts. Horses can become infected with EPM by coming into contact with opossum feces while grazing or by ingesting feed contaminated by an opossum carrying S. neurona. Horses cannot pass the disease to one another, so there’s no need to isolate affected horses out of fear of spreading the disease. EPM cases are also most likely to appear in the spring, summer, or fall. Stressful events and activities, such as traveling or intense training, can also compromise a horse’s immune system and put him or her at greater risk.

Possible signs of EPM:

• Poor coordination, abnormal gait, or lameness • Muscle atrophy • Paralysis of muscles in the eye, face, or mouth • Difficulty swallowing • Seizures or collapse • Abnormal sweating • Loss of sensation along the face • Head tilt with poor balance

If you suspect your horse has EPM, your veterinarian will conduct a full neurological exam and perform certain tests. Testing both the cerebrospinal fluid (CSF) along with a blood serum sample is best. However, spinal taps can be risky and expensive, so in many cases a positive serum IgG test combined with neurological signs and a history consistent with exposure to EPM will serve as a positive diagnosis. For best prevention efforts, manage any possible contamination of horse feed or water from the primary reservoir host, the opossum. Skunks, raccoons, sea otters and even cats can be protozoal sources, also. Lock away all feed containers in varmint-proof containers and rooms. When possible, avoid feeding horses from the ground and clean up spilled feed immediately to deter attracting wildlife and rodents. Frequently clean and freshen water sources.

Treatment options:

Three FDA-approved anti-protozoal drugs are now available to treat EPM:

• Ponazuril (tradename Marquis®; generic name toltrazuril sulfone), an oral paste administered once daily for 28 days. • Pyrimethamine and sulfadiazine (tradename Rebalance®), an oral suspension administered once daily for as long as 120 days. • Diclazuril (tradename Protazil®), a pelleted, alfalfa-based top-dressing fed for 28 days.

These drugs minimize the infection but do not kill the parasite. The use of anti-inflammatory agents such as Banamine®, corticosteroids, or phenylbutazone are often used to help reduce inflammation and limit further damage to the CNS. Antioxidants, such as vitamin E may help promote the restoration of nervous tissue. Response to treatment is often variable, and treatment may be expensive. Recently though, antiprotozoal treatments that kill the parasite and clear the infection have shown promise.