Equine Protozoal Myeloencephalitis (EPM)

Equine Protozoal Myeloencephalitis (EPM) is a central nervous system disease that is caused by the protozoal organism Soarcocystis neurona.  EPM can affect equines of any age, sex, and in any location in the United States.  The organism reproduces in the opossum and is passed out through the feces.  Horses pick up the organism by eating opossum feces dropped in feed or hay.  Once the horse comes in contact with the sporocysts in the opossum feces, the sporocysts move from the intestinal tract into the bloodstream and move to the blood/brain barrier and attacks the central nervous system.  The horse is noncontagious, but can develop the disease.    

The American Association of Equine Practitioners report that more than 50% of the horses in the US may be exposed to the organism that causes EPM.  Only a small percent which have been exposed will ever develop the disease. EPM causes clinical disease in approximately 1% or less of exposed horses.  Not all horses exposed to EPM will show clinical signs and some may develop an immunity and fight off the disease.

Clinical signs include:

-            Incoordination, stiff movements, abnormal gaits or lameness

-            Incoordination and weakness is worse when going up or down slopes or when the head is elevated

-            Muscle atrophy along topline and large muscles of the hindquarter

-            Facial paralysis

-            Difficulty swallowing

-            Seizures or collapse

-            Abnormal sweating

-            Loss of sensation along the face, neck or body

-            Head tilt with poor balance

There are tests available to diagnose EPM.  There are pros and cons for each of them, so consult with your veterinarian to determine the best test for you and possible treatments.  About 60-70% of affected horses will improve and some will recover completely.  As with most diseases, earlier treatment is more effective.  There is not a vaccine for EPM.  If untreated, EPM can cause devastating and lasting neurological damage

Prevention of EPM includes controlling the contact between opossums and horses in and around the barn.  Keep all feed covered and out of reach (including cat feed).  Use feeders and do not feed on the ground.  Know the signs of the disease and involve your vet early if your horse starts having symptoms.

Here are two articles with more in-depth information:  American Association of Equine Practitioners and Horse eXtension.