Horse Equine Protozoal Myeloencephalitis: Welfare and Treatment

EPM causes progressive neurological disease in horses and requires early diagnosis and treatment for welfare-positive outcomes.

Key Facts

EPM is caused primarily by Sarcocystis neurona, a protozoan parasite transmitted by opossums in North America
Clinical signs range from mild ataxia and muscle atrophy to severe neurological impairment affecting all four limbs
EPM is estimated to affect 1-2% of horses in North America, with higher exposure rates in opossum-endemic regions
Diagnosis is supported by IFAT or SAG2/4/6 tests on serum and CSF - CSF testing is more specific
Treatment with ponazuril, diclazuril, or sulfonamides combined with pyrimethamine is effective if started early
Many horses treated early achieve significant improvement, though severe cases may have permanent neurological deficits

Welfare Considerations

EPM causes welfare harm through progressive neurological dysfunction - ataxia, muscle weakness, and in severe cases, inability to walk normally. Horses with EPM may show distress from their inability to control their movement, and severe cases experience significant distress from loss of coordination. Early diagnosis and treatment offers the best welfare outcome - horses treated before severe neurological damage develop show greater improvement than those treated after prolonged disease progression. The welfare case for prompt investigation of any neurological signs in horses is strong, as delays in EPM diagnosis allow progressive neurological damage that reduces the likelihood of full recovery.

What You Can Do

Investigate any horse showing new onset ataxia, asymmetric muscle loss, or neurological signs promptly
Request CSF analysis alongside blood testing for more specific EPM diagnosis in horses with clinical signs
Begin treatment as early as possible - protracted diagnostic workups delay treatment that could prevent further damage
Implement opossum exclusion from feed storage areas as possums are the definitive host for S. neurona
Work with a veterinary neurologist for EPM cases that are severe or not responding to standard treatment
Provide appropriate physical support and rehabilitation during treatment to prevent secondary injuries from ataxia