EPM: Can your horse get it? | TSLN.com

EPM: Can your horse get it?

Jan Swan Wood
for Tri-State Livestock News
As EPM advances, muscle atrophy can occur and the large muscles on one side of the hind quarters are often involved. Drawing by Jan Swan Wood

The disease equine protozoal myeloencephalitis, otherwise referred to as EPM, is not a commonly known disease in the northern plains. Due to the hauling of horses from state to state, though, it’s one that should be on the radar of horse owners.

According to EPMhorse.org, EPM is a disease of the central nervous system, which means it involves the brain and/or spinal cord. It is caused by the protozoal organism sarcocystis neurona. The main host for the organism is the opossum and it is spread to horses through the feces when horses ingest or come in contact with infective sporocysts in the feces. Other hosts of S. neurona include skunks, raccoons, birds, armadillos and domestic cats, but these animals cannot directly transmit the disease to horses.

The website explains that the opossum is infected when it feeds on the carcass of an infected host animal. Once the opossum has consumed the immature stage of the protozoa from a carcass, it takes about two weeks to a month for the protozoa to reach maturity and then be shed in the feces of the opossum. The horse can ingest the protozoa-infected feces through contaminated feed, hay, water or even pasture. The horse is considered an intermediate host, however, and cannot pass the infection on to other animals.

Though opossums are common in the eastern and southern parts of the country, the northern plains and Rocky Mountain regions are not the normal habitat for them. However, with horses being shipped all over the country, exposure to S. neurona can occur without the owner being aware of it, so knowledge about the symptoms could be the difference between an early diagnosis with hope of recovery, and one that comes too late. Feed being shipped in from out of the region can also pose a risk of contamination and knowing where the feed originated is important, says the website.

The clinical symptoms of EPM are extensive and often mimic symptoms of other conditions and diseases. According to S. Ellison in “Early Signs of Equine Protozoal Myeloencephalitis” at http://www.aaep.org/health_articles, they are as follows:

–Uncoordinated movement of the rear feet, often worse on one side. It will also be worse when moving up or down hill and when the head is elevated.

–Lameness that comes and goes and switches sides

–Change in way of going in any gait.

–Hind end weakness, especially on uneven ground

–Problems standing on three legs when a hoof is lifted.

–Falling, staggering, slipping or circling when walking.

–Leaning on a wall for balance.

–Muscle atrophy, often over the hips or shoulders, sometimes in the face.

–Not standing with feet squared under the body.

–Dragging a foot, especially when turning.

–A sore back when no changes have been made to tack.

–Unusual sweat patterns or sweating without reason.

–Holding the tail at an odd position.

–Lack of sensation or an overreaction to stimulus to skin or hooves.

–Sudden change in pecking order in herd (losing dominance).

Common symptoms of the head:

–Drooping lip or facial twitching.

–Dropping feed or having trouble swallowing.

–Holding head crooked

–Change in vision

–Drooping ear

Less common symptoms:

–Geldings that drop their penis when not urinating.

–Extreme lack of energy


A study of EPM horses has shown the subtle cranial (head) symptoms came first in all of the experimentally infected horses. They commonly dropped their feed, had paralysis of their faces, a drooping ear or lack of tongue control. These symptoms occurred before ataxia began. Close observation would be necessary to see some of these symptoms in their early stages, so really being familiar with what normal is for a given horse is critical for early detection.

The clinical trials of horses shipped to continents without the presence of EPM have shown that it takes from two weeks to as long as two years for symptoms to show in horses carrying the infection. A very quick onset can occur and is thought to occur when a very large amount of the protozoa is ingested, therefore assaulting the horse’s system much harder.

Horses exposed to EPM will show it in blood sampling though not all of them will become ill with it. Stress seems to bring on symptoms, and horses often become symptomatic upon being shipped.

The definitive test for EPM without neurological symptoms is a spinal tap, but it is more difficult and expensive, so some people choose to treat the horse for EPM with the a positive blood test and symptoms.

That is what owner Debbie Pate, Hugoton, Kansas, did when her five year old gelding started stumbling behind. “I had bought the horse out of Nebraska and he was fine for a year or so. Then during his futurity year, his hind feet would kind of tangle up like he was going to fall. This went on for weeks until I made an appointment with my vet and they drew blood,” says Pate. “The blood sample showed the protozoa were there. I went to a vet in Spearman, Texas and he was very familiar with EPM. He put him through a battery of physical tests, such as stepping over a log. The horse couldn’t step over with his rear end, so that pretty well verified it was EPM. I never did see another clinical sign of it though. I chose to go ahead and treat him for it rather than do the more expensive spinal tap test.”

Debbie Pate’s story has a happy ending, due to a fairly quick diagnosis and treatment. “After treatment he was absolutely fine and has never had another problem. He’s been a very successful 1D barrel horse for me and I’ve had him for 12 years now.”

The longer the time before treatment, the worse and more numerous the symptoms become. Due to the long list of symptoms, on top of the symptoms mimicking other diseases, diagnosis without testing can be frustrating and lead to a poorer prognosis in the long run. Some horses become so debilitated that they have to be euthanized.

Trish Pitts, Vale, South Dakota, is a veterinary technician at Northern Hills Veterinary Hospital in Sturgis, South Dakota. She has a horse that had EPM and says “I bought the mare at a dispersal sale in the south and by the time she was hauled to my place, she was symptomatic. I suspect she was before I got her but she just wasn’t very obvious. She never recovered enough to compete on but I can still pleasure ride on her.”

Pitts says, “We just do the blood test here due to it being less invasive and expensive. If the test shows that the horse has the protozoa in the sample, we will then do the spinal tap to verify that’s what it is if they have no neurological symptoms.”

Treatment for EPM can be via a paste called Marquis (ponazuril) which costs about $900 for a month, or a combination of sulfadiazine and trimethoprim, which is a longer treatment but less costly. Treatment protocol is up to the veterinarian and owner to determine, based on the desired outcome and financial situation of the individual.

Besides being aware of what can lead to the infection, knowing the signs of EPM and being very familiar with your horse’s normal behavior is critical to the outcome of the disease. As a horse owner, educating yourself about diseases gives you an idea where to start when something about your horse doesn’t seem right.