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Horse Disease Focus - EPM

Updated on March 6, 2013

What is EPM?

EPM is Equine Protozoal Myeloencephalitis. It is an infectious disease that causes neurological and brain problems and can have permanent effects on a horse.


EPM can cause a wide variety of symptoms, depending on exactly what part of the brain or central nervous system is affected.

The most common symptoms are listed below:

1. Lack of coordination or balance, especially in the rear feet. This may show up more when walking or standing on a slope, or become obvious when a hoof is lifted. The horse may lean against the stall wall or a tree or fence for balance. Gaits may lose purity or impulsion. The horse may stumble or slip for no apparent reason.

2. Muscle atrophy might be observed over the rump and shoulders, sometimes dramatically affecting the fit of the saddle.

3. Soreness in the back or tail, including carrying the tail at an odd angle or to one side (note that I've seen horse's carry their tail to one side because of back soreness from other causes).

4. Standing oddly or not square, often with a hoof cocked to the side, or dragging a hoof when walking, especially on a circle.

5. Sweating in an unusual pattern, or sweating for no apparent reason.

6. A visible droop to the lip or one ear, or a facial twitch. Carrying the head tilted to one side.

7. 'Quidding' or dropping feed when there are no dental problems, or having difficulty swallowing.

8. Behavioral changes, that can include aggression.

Rarer symptoms are: A gelding 'dropping' when not urinating (bear in mind many geldings still have a fair bit of libido and will 'drop' when there is a mare in heat around or even when just 'happy'), extreme lethargy or diarrhea.

EPM symptoms may not appear for a long time after infection. The incubation period can be as long as two years.


The first line of attack is to kill the protozoa. This requires a drug regimen which kills or retards the reproduction of the disease-causing organism, S. neurons. This does not generally remove all of the threat, but rather weakens it to the point where the horse's immune system can beat it. Because of this, immune-boosting supplements are often also prescribed. The horse may also be given an anti-inflammatory to reduce symptoms and supplemented with vitamin E. Full rest is recommended.

EPM drugs are generally given orally once a day.

Some horses benefit from acupuncture and probiotics, but horses with an active EPM infection should not be given chiropractic treatment.

Once the infection is dealt with, the horse will need to be rehabilitated. This may take some time. Horses that continue to show stumbling and lack of coordination should not be ridden, for the safety of the rider. The horse may have permanent neurological problems and may never perform at the level it was at before. However, most horses make a reasonable recovery with a slow return to work, a training tune-up and possibly physical therapy. A horse that has had EPM should be treated the same as one that is extremely out of shape.

Horses may also have vision changes, jaw changes that require a change of bit, and may need extra support for a while when being shod or trimmed. While the horse recovers, only an experienced rider capable of supporting and balancing a horse should ride it.

Some horses may, unfortunately, not be ridable again.

Causes and Prevention

The way to prevent EPM is to prevent the infection with S. Neurons in the first place.

The most common host of S. Neurons is the opossum - for this reason, EPM infections are far more common in areas where opossums are plentiful. (Horses cannot infect other horses with EPM). Keep all hay and grain secure so that opossums cannot get to it. Do not leave the barn cat's food out at night if there are opossums in the area - it's far too much of a temptation for them. Lock up all food - horse, human and pet, and clean up any spilled grain. Empty the trash can frequently and consider investing in a wildlife-proof trash can if you have one outside the barn.

Horses in good general health and free of stress are less likely to catch any kind of disease, including EPM. Make sure your horse has a balanced diet with all the trace elements it needs.

There is currently no vaccine for EPM, but one is being actively researched.


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