Slow, progressive exercises help improve a horse’s strength and stability when recovering from this neurologic disease

Equine protozoal myeloencephalopathy (EPM) is a neurologic disease caused by the protozoa Sarcocystis neurona or Neospora hughesi and can lead to deficits including ataxia (incoordination), gait abnormalities, and muscle weakness or atrophy.

S. neurona is typically found in the Western Hemisphere, and the definitive host is the opossum. Opossums can get it by scavenging on infected cats, raccoons, and other intermediate hosts. Horses become infected by ingesting feed or water contaminated with opossum droppings. N. hughesi rarely causes EPM in horses, but sporadic cases arise from this organism. Researchers do not yet know what natural hosts can transmit N. hughesi.

Risk Factors for EPM

Researchers estimate 15-90% of horses have been exposed to S. neurona and, therefore, have antibodies in their bloodstream. However, exposure does not always lead to infection—the annual incidence of EPM is less than 1% in the United States. Horses diagnosed with EPM infection are often younger than 4 or older than 13 years old.

Veterinarians see the highest number of EPM cases during the fall, while the disease is much less common during the winter months. Stress, including immune system compromise, intense exercise, transportation, injury, surgery, or foaling can increase a horse’s chances of developing clinical (apparent) EPM.

Diagnosing EPM in Horses

When a horse exhibits clinical signs of EPM, veterinarians must distinguish the disease from lameness or other neurologic conditions such as cervical vertebral stenotic myelopathy (wobbler syndrome), equine
herpesvirus-1 myeloencephalopathy (EHM), rabies, and Lyme disease. Currently, the only definitive way for veterinarians to diagnose EPM is by identifying parasites in the brain or spinal cord during a necropsy.

Veterinarians typically diagnose EPM in horses by conducting SAG2, 4/3 ELISA testing, which is performed on serum and cerebrospinal fluid (CSF). They compare the antibody levels in the bloodstream to those in the CSF to see if CSF antibody levels are higher than expected compared to normal diffusion of antibodies from the blood. This would indicate antibody production in the central nervous system. Some practitioners might take cervical radiographs to check for spinal changes that appear more significant than expected with normal aging and could indicate another pathology as the cause of neurologic deficits.

Treating EPM in Horses

Veterinarians have several FDA-approved drugs available to treat horses with EPM: triazines and folate-inhibiting drugs. They also often recommend supplementing with vitamin E.

Researchers have suggested the triazines diclazuril and ponazuril both target the apicoplast organelle in S. neurona, which mammal cells do not contain