Bluebonnet Equine Humane Society

EPM - Equine Protozoal Myeloencephalitis

By Sharon Martin-Holm

EPM is a term that strikes fear in the hearts of horse owners. Much of it is fear of the unknown ... How did he get it? How can it be diagnosed? Can I afford the treatments? Will he ever get well? Most of these questions are very difficult to answer.

A protozoa is an organism that is a little bigger than a bacterium, but still only consists of one cell. It can only reproduce within a host organism. The protozoan in the case of EPM is named Sarcocystis neurona. S. neurona did not set out to destroy our beloved horses. It is meant to spend its life cycle in opossums; the horse is a “dead end” host for S. neurona. A horse who accidentally grazes near a place where a ‘possum has defecated can pick up the S. neurona sporocysts. Most horses, when exposed this way, never get sick because their immune systems recognize the intruders and get rid of them. Only a few horses go on to get symptoms of infection. An affected horse can not “give” the disease to another horse; it is not transmitted from one horse to another.

The S. neurona travels through the blood stream of the horse until it gets to the central nervous system -the brain and spinal cord- which is where the real problems start. Parts of the spinal cord are damaged by the parasites, and depending on where the cord is damaged will determine what symptoms the horse has. Some look lame. Some get very off balance. Some can’t seem to keep their front and back feet coordinated. Muscle wasting (atrophy) can happen, so can seizures or collapse.

There are other diseases that can have these presentations as well - West Nile Virus can cause a similar reaction. So can the other encephalitis diseases: Eastern/Western/Venezuelan Equine Encephalitis (the EEE, WEE, VEE viruses). It is very important that you seek immediate veterinary attention as soon as you notice any unusual symptoms in your horse.

EPM can be very difficult to diagnose. Just doing a blood test won’t give you an answer. All horses who have ever been exposed to the protozoa, regardless of whether it’s still present in their body or not, will test positive on blood samples. The gold standard so far has been spinal fluid analysis.

It is also very difficult to treat. There are just a few treatments available, all are expensive, and none can guarantee a cure. Marquis (ponazuril) was the first to be FDA-approved for treatment. It still costs several hundred dollars per month and more than one month may be needed. A sulfa combination called pyrimethamine/sulfadiazine is now marketed under the trade name ReBalance. It is recommended for use from 120 to 150 days. Neither drug has better than a 50-60% cure rate. Some people recommend folic acid and vitamin E supplements to improve and support nerve tissue function, but these have not been “approved” as treatments. At one point there was a vaccine in production, but it was found to be ineffective.

You can try to prevent infection by making your barn and property inhospitable to opossums. One very important precaution is to not leave out cat or dog food, as this definitely attracts the homely nocturnal mammals.