Know Thy Enemy

EPM is one of the most devastating diseases a horse can acquire.

It is the most common neurological disease in horses today. Equine protozoal myeloencephalitis attacks the horse’s central nervous system, which left untreated can cause seizures and death.

The disease’s symptoms were first reported about 40 years ago, but it wasn’t until the past decade that the cause was found and treatments created.

EPM is caused by the protozoa Sarcocystis neurona, a single-celled parasitic organism. The protozoa is not transferred from horse to horse, but rather spread by a definitive host, the opossum. Intermediate hosts such as cats, raccoons, skunks and armadillos carry the protozoa, and the opossum picks it up by feeding on carcasses. The organism is passed through the host’s body or waste products as a sporocyst, and horses pick it up through eating contaminated feed, grazing or drinking contaminated water.

Once ingested, the sporocyst goes through another maturation stage and becomes a merozoite. Where this maturation happens is uncertain, but is thought to be in endothelial cells. The merozoite then gains access to the central nervous system by crossing the blood/brain barrier to the brain.

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“We don’t know for sure, but we think that perhaps the parasite catches a ride inside certain white blood cells that cross over into the central nervous system,” says Dr. Kenton Morgan with Fort Dodge Animal Health.

Once inside the nervous tissue, the merozoite begins to replicate itself, forming a structure called a schizont. A small sack or pouch of multiple merozoites, the schizont eventually ruptures. This destroys the host cell, damages nearby nerve tissue and releases more merozoites to infect other cells. The tissue death eventually causes the neural distress seen in symptoms.

The disease can be contracted all over North and South America, but is less common in the western United States where opossum populations are smaller. Dr. Morgan estimates that more than 50 percent of all horses in the United States have been exposed to the organism, but only a small percentage (approximately 0.14 percent) develop the disease.

“Every presentation is a little different,” Dr. Morgan says. (It can vary from a horse that’s just not doing right, so to speak, to one that’s got profound neurological disease that is stumbling all around and can hardly right itself.”

Signs of EPM

Signs of EPM vary from horse to horse, depending on where the protozoa attack the nervous system. Onset of the disease is most common in summer and fall months, and symptoms can develop slowly or present themselves within several days. They can range from mild to severe. Often the first indications are stiffness, asymmetrical gaits and cranial nerve deficits. Symptoms include ataxia (incoordination), spasticity (stiffness, abnormal gaits or lameness, muscle atrophy, paralysis, difficulty swallowing, head tilt, seizures and collapse, abnormal sweating, loss of sensation and poor balance). Incoordination and weakness are often exacerbated by going up or down slopes or movement when the head is elevated. Cranial nerves control function of the head, so if affected, the horse might have paralysis of the face, problems coordinating actions (chewing, dropping feed, etc.), swallowing or vocalizing.

“I’ve had people report that they noticed when the horse whinnied, it sounded different,” Dr. Morgan says.

A horse suspected of having the disease should be inspected by a veterinarian as soon as possible. The first step is a general examination, followed by a neurological exam. The neurological exam includes assessing the cranial nerve function and working down the body to make sure the sensations are normal. Neurological deficiencies are scored from 0 (none) to 4 (obvious abnormalities). Cranial nerve checks include making sure the nerves that control eye dilation, blinking and the gag reflex work properly. Problems the vet might watch for in the neck include lack of flexibility or abnormal muscling. The vet will perform a panniculus reflex test, using a blunt object (such as a ballpoint pen) to press on the skin all the way down the backbone. A normal horse will twitch the skin, as if trying to rid himself of a fly. There are also gait assessments (known as proprioceptive tests) that include watching the horse back, circle and move on an incline. The examiner might also test the horse’s balance by pulling his tail while walking. If the horse loses its balance or is unable to resist the pull, this shows neurological damage. At a halt, the vet will also move the horse’s leg across its other leg to check its awareness of where its feet are (the horse should return the moved leg to it normal position within 30 seconds).

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If there are signs of EPM, an owner might choose to start treatment without knowing for sure what it is. However, to confirm a diagnosis, a blood and cerebrospinal fluid analysis can be done on the horse. The blood test will only tell if the horse has been exposed to the protozoa, not if he has the disease. A spinal tap, however, will reveal if the protozoa has accessed the nervous system.

Studies estimate that only 40 percent of horses affected by the disease are able to recover fully, although according to Dr. Morgan, up to 60 percent are able to improve to some degree following treatment.


The best way to treat EPM is to prevent it. Because the disease is transferred via the definitive host, the opossum, prevention must be centered on keeping these critters as far from horses as possible. Discouraging the proximity of the hosts can be done by good horsekeeping practices. Keep feed rooms and containers closed, and make sure they are rodent-proof.

“Opossums are just nasty,” Dr. Morgan says. “They’re kind of like big rats, so if they can get into the feed source, they poop in it and everything else. So keeping your feed contained and keeping animals out is very beneficial.”

Clean up dropped feed immediately to discourage scavenging. Keep water tanks clean and cover stored hay to prevent the opossums nesting. Do not feed horses on the ground, instead place feed in a rack or buckets. Also clean up roadkill or dead intermediate host animals, as this is what opossums feed on.