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Understanding EPM

By Daniel M. Grove, DVM - September 1st, 2017 - Ask the Vet

AskTheVetEquine Protozoal Myeloenchephalitis (EPM) is a neurologic disease in horses. It can cause a debilitating disease that can lead a horse down a path of changes that can be difficult to diagnose. In this column, let’s discuss what causes it, how we diagnose it, and how we treat it.

EPM is currently known to be caused by one of two protozoans, Sarcocystis neurona and Neosporoa hughesi. A protozoa is a single-celled organism. S. neurona is the most common cause, and we will focus on this particular organism. In the normal life cycle of the organism, the definitive host — or the one where reproduction of the protozoa occurs — is the opossum. The organism also has an intermediate host which can be a variety of animals, such as certain armadillos, racoons and skunks.

Well, where do horses fit into this? Actually, they are not part of the normal life cycle. They do not shed the protozoa, but they are an “aberrant host” that can be on the receiving end of some serious side-effects that result from coming into contact with the organism. The horses ingest the feces of the opossum infected with the protozoa, and the organism can then go to work on doing damage to your horse.

Once ingested, the organism passes from the intestine into the bloodstream. Here, the horse can mount an immune response and may attack and clear the protozoa from its system, and you will never know the difference. For horses that show clinical signs, it usually does not stop here. The organism can pass into the central nervous system where the brain and spinal cord lie. It is here where the damage is done. The protozoa can enter cells in the nervous system and damage or kill them and can also cause inflammation.

The clinical signs seen vary from horse to horse. What really dictates the signs is where the protozoa sets up shop and causes inflammation. If it is in the brain, signs associated with functions performed by that section of the brain will be evident. If it is in the spinal cord, the signs of damage to the affected nerves will be evident. Some common signs are weakness in the hind end, subtle changes in the gate of the hind limbs, muscle atrophy, changes in behavior, and many others. These clinical signs are common to many other neurologic diseases. This makes diagnosing these diseases somewhat challenging sometimes.

Diagnosing the disease is based from clinical signs and testing. Your veterinarian will usually perform a neurologic test and see some abnormalities. Sometimes, the owner notices some subtle change that the veterinarian would not pick up on in an exam, but — when taking the owner’s statement on the history of the horse — might pick up. Testing involves looking for antibodies against the organism in the blood and/or cerebrospinal fluid.

Now that we know we have an infection with S. neurona, it is time to go to work in order to eliminate it from the body. There are a number of antiprotozoal medications for the elimination of the organism. Some work well for one horse, but another horse may need a different medication. Also, there are some supportive medications that are necessary. Vitamin E can help the nerve tissue in preserving it and in its healing. In some cases, the inflammation is really the worst component of the disease, and strong anti-inflammatory drugs such as steroids are required to minimize the destruction of the nerve tissues.

EPM can be a devastating disease. If you are noticing mild changes in your horse, its gait, musculature, or personality, mention it sooner rather than later to your veterinarian. Damage to nervous tissues in the central nervous system can be permanent. The sooner you curb the damage, the less likely your horse will have long lasting effects from the disease process!

–Dan

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