A Closer Look At: Equine Protozoal Myeloencephalitis – Horse Racing News


In this series, we ask some of the equine health questions you’ve wondered about but were too afraid to ask. Today, Dr. Bryant Craig, associate director for scientific sales affairs at Merck Animal Health, answers common questions about Equine Protozoal Myeloencephalitis (EPM).

-Testing for EPM isn’t as black-and-white as it is for other diseases. What can diagnostic tests tell us?

Dr. Craig: While the results aren’t a definite yes or no, we can learn from certain diagnostic tools when dealing with a suspected case of EPM. Tests run on the blood sample or serology give us an indication of exposure to the organisms responsible for EPM. If we get a negative result on serology, the odds are almost certain that EPM is not the cause of disease.

A positive result is less clear due to a high exposure rate in many areas of the country and the fact that only a small percentage of those exposed end up showing signs of disease. In these cases, a blood sample paired with a sample of cerebrospinal fluid gives us a more certain result when we evaluate the ratio of antibody found in each fluid.

At the end of the day, we must rely on a thorough clinical exam coupled with diagnostic results to make our best-informed decision.

-In recent years it sounds like there may be more than one protozoa associated with EPM; does that change the way the disease presents?

Dr. Craig: Not really; while there are two proven causes of EPM in Sarcocystis neurona spirit Neospora hughesi and some other proposed organisms, what we see clinically is virtually the same. While we can see a wide variety of symptoms, ataxia, asymmetry, and atrophy are the hallmark signs of an active EPM infection.

-We know that most horses are exposed to EPM but not all of them get sick. Do we know why some horses develop symptoms while others don’t?

Dr. Craig: Most horses’ immune systems are able to fight off an EPM infection, but several factors may play a role in a clinical case being presented. We know there are individual variations in a horse’s ability to fight infections and those with a lower threshold for this organism will always be at a higher risk.

Rate of exposure can be another factor. Some areas of the country (particularly the southern states and eastern seaboard) have much higher populations of opossums who transmit the disease. Anytime you have more organisms in the environment, the chance of disease is going to be elevated.

Stress also plays a significant role. Any horse that is placed in a situation that may cause anxiety to the animal, suppresses their immune system and therefore increases the probability of disease.

– How is EPM treated?

Dr. Craig: There are three FDA approved treatments in the marketplace. All of them are administered orally and the treatment period ranges from 28 days to up to 270 days. Protazil® (1.56% diclazuril) Antiprotozoal Pellets is an alfalfa-based pellet that is simply top dressed on the feed once a day for the treatment period.

Marquis® (15% ponazuril) is a paste that is administered orally daily for the duration of treatment.

ReBalance® (sulfadiazine and pyrimethamine) is an oral solution that is administered for a period of between 90 and 270 days.

-At one point, an EPM diagnosis was the end of a horse’s riding career; what’s the rate of success like for EPM treatment these days?

Dr. Craig: It depends on how you measure success. If a horse has advanced symptoms or long-term deficits, while we can many times stop the progression of disease, they may never return to original performance levels. However, if we are able to catch the signs early and begin treatment, our chances of returning to normal go much higher.


PROTAZIL® is contraindicated in horses with known hypersensitivity to diclazuril. The safety of Protazil in horses used for breeding purposes, during pregnancy, or in lactating mares, and use with concomitant therapies in horses has not been evaluated. Do not use in horses intended for human consumption. Not for human use. For complete safety information, refer to the product label.

Please refer to Marquis® and ReBalance® product labels for important safety information.

Leave a Comment

Your email address will not be published. Required fields are marked *