EPM In Horses: Causes & Prevention

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Equine protozoal myeloenencephalitis (EPM) in horses is a neurologic disease spread by a parasite, Sarcocystis neurona, that attacks a horse’s spinal cord and brain. Sarcocystis neurona can seriously damage the central nervous system, resulting in short- or long-term damage or even death if not caught and treated early.
EPM in Horses

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Horses under five years of age and over 13 tend to be the most likely to contract the disease, and most cases occur in the summer, spring, and fall. This article discusses how horses acquire EPM, how to prevent the disease, and some treatment options if your horse already contracted it.

How A Horse Contracts EPM

Opossums are the primary host for the Sarcocystis neurona parasite, and horses generally become infected after eating food that has come into contact with opossum feces. The parasite’s life cycle calls for a short incubation period, where it reproduces and sheds sporocysts in the opossum’s digestive tract. Sporocysts come out in the opossum’s feces and infect hay, pasture grass, feed, and even water. If a horse consumes contaminated hay, feed, or water, the sporocysts enter the body through the intestines and travel through the blood vessels. Sporocysts travel to the liver, where they reproduce and turn into merozoites. From the liver, the merozoites travel to the spinal cord and the brain. Once the merozoites are in the central nervous system, they infect neural cells and pressure surrounding nervous tissue. The stress kills neural cells, releasing more merozoites and perpetuating a continuous cycle of deterioration inside the central nervous system.

Related: A Complete Guide to Gastric Ulcers in Horses

Symptoms Of EPM

As cells in their nervous system die off, the horse may start to exhibit subtle signs like gait abnormalities, mild depression, and irregular upper airway function. More severe symptoms include muscle wasting, incoordination, inability to stand, stumbling, seizures, and other neurologic signs. It can be challenging to diagnose EPM because the signs can appear similar to symptoms caused by different kinds of diseases, like rabies, wobbler syndrome, equine herpes virus, West Nile virus, and equine motor neuron disease. You should always consult with your veterinarian immediately if you detect anything abnormal about your horse or if you suspect it may be ill. 

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Which Horses Are Most Susceptible?

Horses of any breed, sex, or age can contract EPM. However, research shows that younger horses between the ages of 1 and 5 seem to acquire the condition most often. Horses who are frequently transported also appear to be at greater risk of contracting EPM. Risk is also considered to be higher in the fall months than any other time of the year. Fall may be a higher risk season since opossums are on the hunt for warm winter nests in and around stables as cooler weather starts moving in. However, the weather itself doesn’t directly affect EPM, and a horse can contract the condition any time of the year. 

Related: How Long Do Horses Live? [Complete Guide]

Preventing EPM

Horses Running On A Farm

Currently, there are no vaccines for EPM, which is why pest and feed management are the best ways a stable owner can prevent their horses from contracting the disease. Luckily, a horse who has EPM can’t spread it to others, so you don’t need to keep an infected horse quarantine. The biggest thing horse owners and caretakers can do to prevent EPM is to keep opossums away from horses and the areas they frequent. source


Opossums are nocturnal animals that hunt for food in the nighttime, but they can commonly be spotted during daylight hours if the weather is cold or if their food supply runs low. Opossums are attracted to garages and bars since they often have food that is easy for them to get into, like cat food, dog food, garbage cans, and open storage bins of horse feed. If opossums discover that your barn is a reliable source of food, they may start to utilize your hay storage for their nest. All of this raises the chance of infection for your pasture, hay, or grain.

Related: Laminitis In Horses

The most effective way you can prevent your horses from contracting EPM is to properly stow their feed and make sure not to leave any food or rubbish out. Don’t leave open bags of horse feed in the barn where opossums can reach them; make sure to lock up feed storage containers, and never leave cat and dog food out (they make easy targets). Moreover, keep garbage cans sealed tightly, make sure your property is clear of animal carcasses, and regularly change the tank water. 

The best way to protect your horse from disease is to keep it healthy. Horses who have a weakened immune system are more likely to contract EPM. Intense training and travel-related stress can be the straw that broke the camel’s back to a weakened immune system. Research shows that a daily supplement of oral plasma may prevent a horse from contracting EPM, since the biologically active protein in plasma supports and strengthens the immune system

Treatment Options For EPM

Since EPM can resemble many other types of neurological diseases, your vet must examine your horse to make an official diagnosis thoroughly. If the condition is caught early on and proper medication is administered, most horses will make some kind of recovery. Unfortunately, there may be some permanent damage, mainly if the disease wasn’t treated early enough.

To diagnose the disease, your veterinarian will closely examine the horse’s gait and movement, watch for any lameness issues, draw blood, and in certain situations, draw spinal fluid samples (if suspected and warranted). These samples will be sent out to labs, and technicians will look for protozoa under a microscope. 

A horse grazing in a field

Veterinary treatment options typically consist of antimicrobial, antiprotozoal, and anti-inflammatory drugs, which your veterinarian will need to administer in some instances. Treatment may be expensive and lengthy and may not always be successful if the protozoa have severely damaged the brainstem and spinal cord. It can help to supplement your horse’s diet with omega-3 fatty acids and antioxidants, supporting damaged muscle and nervous tissue. 

Can EPM Be Contracted From My Feed Manufacturer?

Reputable feed manufacturers like Rogue Pet Science go to extreme lengths to control pests and rodents. Mills that use upright storage (like grain bins and silos) are at lower risk of opossums contacting raw ingredients versus mills that utilize flat storage (like commodity barns and open bays). Exposure to heat over a specific temperature also kills the EPM-causing protozoa, which means pelleted feeds may be a safer option than oats, textured feeds, and sweet feeds. However, it is ultimately the horse owner’s responsibility to prevent opossums from coming into contact with open bags once the feed is in their barn.

The best way to prevent your horse from contracting EPM, or any other disease, is to take good care of it and keep it healthy. Origins Equine 5in1 from Rogue Pet Science contains equine-specific support from prebiotics, probiotics, digestive enzymes, and butyric acid to optimize the digestive and gut system beyond what any diet on its own could achieve. Keep your horse healthy and happy with Rogue Pet Science!

Looking for high-quality equine supplements? Call Rogue Pet Science and order today!

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