Understanding This Debilitating Disease

Equine Protozoal Myeloencephalitis (EPM) is a master of disguise. This serious disease can be difficult to diagnose because its signs often mimic other health problems in the horse and signs can range from mild to severe. Studies show that in Ohio where the opossum is present, an average of 53% of the horse population has been exposed to the parasite that causes EPM.¹

The causative organism is a protozoal parasite named Sarcocystis neurona. The disease is not transmitted from horse to horse. Rather, the protozoa are spread by the definitive host, the opossum. The infective stage of the organism, the sporocysts, are passed in the opossum's feces. The horse comes into contact with the infective sporocysts while grazing or eating contaminated feed or drinking water.²

Once ingested, the sporocysts migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There they begin to reproduce and cause damage to the horse's central nervous system. The onset of the disease may be slow or sudden. If left undiagnosed and untreated, EPM can cause devastating and lasting neurological damage. It is paramount to treat the horse before further permanent damage to the CNS occurs.

Signs of EPM

The clinical signs of EPM can be quite varied. Clinical signs are almost always asymmetrical (not the same on both sides of the horse). Actual signs may depend on the severity and location of the lesions that develop in the brain, brain stem or spinal cord. Signs may include:

• Ataxia (incoordination), spasticity (stiff, stilted movements), abnormal gait, or lameness
• Incoordination and weakness which worsens when going up or down slopes or when head is elevated
• Muscle atrophy, most noticeable along the topline or in the large muscles of the hindquarters, but can sometimes involve the muscles of the face or front limbs
• Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips
• Difficulty swallowing
• Seizures or collapse
• Abnormal sweating
• Loss of sensation along the face, neck or body
• Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support²

Four things seem to influence progression of the disease:

1. The extent of the infection (i.e. the number of organisms ingested)
2. How long the horse harbors the parasite prior to treatment
3. The point(s) in the brain or spinal cord where the organism localizes and damage occurs
4. Stressful events such as training or shipping prior to or during infection.³

Learn More: Horses at Risk

In field studies, blisters on the nose and mouth, skin rash or hives, loose stools, mild colic and seizure were observed. The association of these reactions to treatment was not established.

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¹ Saville WJ, Reed SM, Granstrom DE, et al, Seroprevalence of antibodies to Sarcocystis neurona in horses residing in Ohio. J Am Vet Med Assoc 1997; 210:519-524.

² Reed S. Neurology is Not a Euphemism for Necropsy: A Review of Selected Neurologic Diseases Affecting Horses, Precedings 54th Annual Conf AAEP, Dec 2008, San Diego CA, pg 78 - 109.

³ Saville WJ, Reed SM, Morley PS, et al. Analysis of risk factors for the development of equine protozoal myeloencephalitis in horses. J Am Vet Med Assoc 2000; 217:1174-1180