Equine protozoal myeloencephalitis (EPM)
EPM is a neurologic disease that horses get from eating infected opossum feces. Incoordination, muscle atrophy and loss of feeling around the body are a few signs of illness. Keep your horses healthy by storing grain in sealed bins and controlling opossum populations around your barn.
What is EPM?
EPM is a disease that affects the brain and spinal cord. It’s caused by the microbe, Sarcocystis neurona, commonly found in the opossum. Horses that come in contact with infected opossum feces can develop neurologic disease.
Other hosts of this microbe include
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Armadillos
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Skunks
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Domestic cats
Unlike the opossum, these hosts can’t directly pass the disease to horses.
Signs of illness
Many of the EPM signs mimic other neurologic disease or may come and go.
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Incoordination; stiff, stilted movements; abnormal gait or lameness
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Incoordination and weakness
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This worsens when going up or down slopes or when the head is up
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Muscle atrophy
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Most noticeable along the topline or in the hindquarters
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Sometimes involves the face or front limb muscles
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Drooping eyes, ears or lips
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Difficulty swallowing
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Seizures or collapse
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Abnormal sweating
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Loss of feeling along the face, neck or body
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Head tilt with poor balance
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May stand splay-footed or lean against stall walls for support
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Diagnosis
Three EPM tests are available for diagnosis. Each of them have pros and cons, which you must take into consideration when deciding which to perform.
Serum antibody test
This blood test allows your veterinarian to look for any S. neurona antibodies. If none are present, your horse doesn’t have the disease. If these antibodies are present, it means your horse was exposed to the microbe sometime in its life. It doesn’t mean your horse has the disease.
About 50 to 60 percent of horses have been exposed to S. neurona but only 0.14 percent develop the disease.
IgM capture ELISA
This blood test looks for antibodies found during an active S. neurona infection. Thus, if the test is positive, the horse does have the disease. This test isn’t widely used yet. Only the University of California-Davis runs these tests so you must ship your samples. The test costs $61 per sample and the results usually come back within a week.
Spinal tapping allows your veterinarian to look at your horse’s cerebrospinal fluid (CSF). A spinal tap better detects infection but it’s more invasive than a blood test. Blood contamination of the CSF sample can result in false positive tests.
Treatment
There are two treatment options for EPM.
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A six-month course of antibiotic (trimethoprim-sulfonamide) and antiprotozoal agent (pyrimethamine).
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A 28-day course of antiprotozoal (ponazuril). Horses may need a second round of ponazuril in some cases. This is the only FDA-approved treatment for EPM.
Depending on your horse’s condition, your horse may need general supportive care.
Outcome
About 60 to 70 percent of horses treated for EPM will improve, and 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.
About 80 percent of horses will remain positive on the spinal tap despite treatment. They may not even show signs of illness. Relapses occur within two years in about 10 to 20 percent of these horses.
Prevention
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Keep grain in covered bins
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Control opossum populations around your barn
Reviewed in 2018