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EPM is a neurologic disease in horses caused by protozoal parasites, primarily Sarcocystis neurona, leading to inflammation of the central nervous system and a wide range of neurological signs.
Causes and Transmission
EPM is caused by
protozoal parasites, most commonly
Sarcocystis neurona and, less frequently,
Neospora hughesi
horseeducationonline.com+2. Horses become infected by ingesting
feed, hay, or water contaminated with opossum feces containing infective sporocysts
horseeducationonline.com+1. Opossums are the definitive host, while intermediate hosts include raccoons, skunks, cats, armadillos, and sea otters
The Vet Expert. Horses are
dead-end hosts and cannot transmit the disease to other horses
umn.edu+1.
Clinical Signs
EPM affects the
brain and spinal cord, producing neurological symptoms that vary depending on the region affected
horseeducationonline.com+1. Common signs include:
- Incoordination and ataxia, often worse on one side of the body
umn.edu+1
- Muscle atrophy, particularly asymmetric along the topline, hindquarters, or face
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- Weakness or stumbling, especially on slopes or when the head is raised
umn.edu
- Facial drooping, difficulty swallowing, head tilt, or abnormal posture
umn.edu+1
- Seizures, abnormal sweating, or loss of sensation in affected areas
umn.edu
Early detection is critical, as subtle changes like toe dragging, uneven gait, or slight asymmetry may indicate the onset of EPM
horseeducationonline.com.
Diagnosis
Diagnosis involves a
thorough neurologic examination and targeted testing to distinguish EPM from other neurologic diseases
UC Davis. Veterinarians may use blood or cerebrospinal fluid tests to detect antibodies against the parasites, combined with clinical signs and observation of progression
horseeducationonline.com+1.
Treatment
Treatment is
veterinarian-directed and typically involves
antiprotozoal medications such as:
- Ponazuril (Marquis), administered daily for 28 days
- Protazil, given with feed for 28 days
- Sulfadiazine and pyrimethamine (ReBalance), administered for 90–270 days
The Vet Expert
Supportive care may include anti-inflammatory drugs, corticosteroids, and vitamin E supplementation. Early intervention improves recovery outcomes
horseeducationonline.com+1.
Prevention
Preventive measures focus on
reducing exposure to opossum feces:
- Store feed in animal-proof containers and protect hay from wildlife
- Maintain clean water sources and stable areas
- Control rodents and discourage opossum visits
- Properly dispose of dead animals and spilled feed
horseeducationonline.com+2
Epidemiology
EPM is most prevalent in
North America, particularly in areas with high opossum populations
UC Davis. While
50–90% of horses may be exposed, fewer than 1% develop clinical disease
UC Davis. Younger horses, males, and certain breeds (Standardbreds, Tennessee Walking Horses, Thoroughbreds, Warmbloods) are at higher risk
madbarn.com.
Key Takeaway
EPM is a
serious neurologic disease that can be life-threatening if untreated. Recognizing early signs, seeking prompt veterinary care, and implementing preventive measures are essential for protecting horses from this debilitating condition
horseeducationonline.com+1.