Equine protozoal myeloencephalitis (EPM)

What is EPM?

EPM is a disease that affects the brain and spinal cord. It is often caused by the parasite, Sarcocystis neurona, commonly found in the opossum. Research in other states suggests that about 10 to 33 percent of opossums are infected with the parasite in Michigan and Missouri. Horses that come in contact with infected opossum feces can develop neurologic disease. It is not fully understood why some horses show neurologic signs and some do not after exposure to the parasite. While EPM is considered rare, it is a serious disease that some horses may not survive even with treatment. Horses cannot pass EPM to other horses.

Other hosts of this parasite include:

• Armadillos
• Skunks
• Domestic cats

Unlike the opossum, these hosts likely do not directly pass the disease to horses. 

Signs of illness

Many EPM signs mimic other neurologic diseases or may come and go.

• Incoordination; stiff, stilted movements; abnormal gait or lameness.
• Incoordination and weakness.
  • This worsens when going up or down slopes or when the head is up.
• Muscle atrophy:
  • Most noticeable along the topline or in the hindquarters.
  • Sometimes involves the face or front limb muscles.
  • A common feature of this muscle atrophy is that it is asymmetric.
• Drooping eyes, ears or lips.
• Difficulty swallowing.
• Seizures or collapse.
• Abnormal sweating.
• Loss of feeling along the face, neck or body.
• Head tilt with poor balance.
  • May stand splay-footed or lean against stall walls for support.
• Vague lameness is sometimes thought to be caused by EPM but you should thoroughly rule out more common causes of lameness first.

Diagnosis

Equine protozoal myeloencephalitis is hard to diagnose. Your veterinarian will likely perform a physical exam first. Then, they may take blood or spinal fluid samples for testing, which can help support a diagnosis and rule out other diseases.

Blood tests

This test allows your veterinarian to look for S. neurona antibodies in the blood. Horses with recent or no exposure to S. neurona may not have antibodies present. As a result, retesting 10 to 14 days after signs of EPM occur can help rule out the disease. 

While antibodies confirm exposure to S. neurona sometime in your horse’s life, they do not confirm disease. About 50 to 60 percent of horses have been exposed to S. neurona but only a few of them develop EPM. The presence of antibodies can help support a diagnosis of EPM when horses display signs of illness but, alone, a positive blood test does not confirm a diagnosis.

Spinal tap

A spinal tap allows your veterinarian to collect and analyze your horse’s cerebrospinal fluid (CSF) for S. neurona antibodies. Testing your horse’s blood and CSF in unison to compare antibody levels between the two samples is the current gold-standard diagnostic approach and helps rule out other diseases. The procedure is typically done standing under heavy sedation. 

Treatment

Always work with your veterinarian when developing a treatment plan. Three treatments are currently approved by the Food and Drug Administration for EPM.

• A 28-day course of ponazuril, which is given as a paste. Horses may need a second or third round of ponazuril in some cases.
• A 28-day course of diclazuril, available as a pelleted product that you can add to feed. Multiple rounds of treatment may also be recommended.
• A 90 to 270-day course of sulfadiazine and pyrimethamine is given as a liquid suspension by mouth. This treatment may not be advised for pregnant mares. 

Depending on the severity of your horse’s condition, your horse may need supportive nursing care or hospitalization.

The horse’s symptoms may appear to worsen after initial treatment since the medication is killing the parasites causing the illness.

Outcome

About 60 to 70 percent of horses treated for EPM will improve, but it is estimated that only 15 to 25 percent will recover completely. Starting treatment early will lead to the best results. The greatest amount of improvement is seen within the first four weeks. Relapses occur within two years in about 10 to 20 percent of these horses.

About 80 percent of horses will remain positive on the spinal tap despite treatment. They may not even show signs of illness.

Prevention

Opossums are important members of our wildlife ecosystem. Prevention methods should focus on making your farm less attractive to opossums such as:

• Keep feed in enclosed containers to prevent access by opossums and other wildlife.
• Sanitation is key. Clutter tends to invite rodents.
• Regularly check your facility, especially feed rooms, for signs of wildlife presence including denning, feces, and evidence of chewed material.
• Eliminate external entrances for rodents. Steel wool can be used as a temporary plug to deter rodents.
• Stack firewood off the ground (in racks or on pallets) and frequently inspect piles for signs of denning. It’s best to keep wood piles away from house or barn areas.
• Farm garbage can also attract wildlife and should be secured.
• Food for barn cats and dogs will attract unwanted visitors. It is best to feed pets in areas away from where feed is stored and horses are fed.