What is it?
A neurologic disease caused by a tiny protozoal parasite that migrates into the spinal cord and/or brain. Sarcocystis neurona is the name of the parasite which was first identified in 1991.
These can vary widely depending on where the parasite is living in the spinal cord or brain. Examples of signs include stumbling, incoordination, lameness, inability to stand, muscle atrophy, strange behavior, paralysis, and seizures. These signs can appear suddenly and progress quickly, or they may appear very slowly and progress over time.
How is a horse infected?
EPM is not transferred from one horse to another. Instead, cowbirds carry the protozoa in their muscles but are unaffected by it. Cow birds do not pass the infection directly to horses. Opossums eat the cowbirds, then shed infective protozoa in their feces. Horses get infected by the opossum feces contaminating their hay, grain, pastures or water. Most horses are exposed in areas with a high opossum population. Fortunately most horses that are exposed clear the disease without ever showing symptoms.
How can it be diagnosed?
Diagnosis can be difficult. Blood tests show exposure to the protoza, but not whether they are currently infected by it. Most horses in this area will test positive on a blood test. A CSF (spinal fluid) tap is usually diagnostic. There are occasionally false test results. Diagnosis is sometimes based on treatment response. If the signs are improving as the horse is treated we may recommend continuing through the entire treatment plan.
What is the treatment and rehabilitation?
Treatment is very costly and may not completely cure your horse of clinical signs. The earlier treatment is started in the course of the disease the better the chance of return to normal. Approximately 50% of horses will respond to therapy successfully.
Rehabilitation is important as a horse improves. Gradual but steady exercise helps to rebuild muscles. Riding is not recommended if the horse is unsteady until those signs have completely disappeared. Pasture turn out is better than stall rest unless the horse is very unsteady and could injure itself.
Other therapies that have been tried with mixed success include hydrotherapy, massage and acupuncture.
How to prevent?
This is very difficult as the carrier is the opossum, and they are found everywhere in our area. Trying to prevent opossum fecal contamination of hay and feed is recommended. Utilizing feed tubes that can be cleaned daily is important. Tubs that rest on the ground are well-suited as they can be turned over or removed from the stall readily.