Ask the Vet: Equine Protozoal Myeloencephalitis (EPM)

Answered by Sandi Farris, DVM, Harmony Veterinary Services, LLC
Courtesy of AAEP

Question: It has been two years since I have ridden my Thoroughbred consistently. After six weeks of Marquis medication, I treated him herbally and he seemed to be fine. Then he had a relapse and I am treating him with Silver Linings herbs this time. I could not afford the cost of Marquis again and am hoping he will continue to improve. Do horses ever recover completely without future relapses?

Answer: I’m sorry to hear your gelding has not responded to treatment for EPM. Unfortunately, some horses do not fully recover from the damage the protozoa cause within the nervous system. EPM, as you probably know, is caused by protozoal organisms Sarcocystis and Neospora, which are transmitted to horses through the feces of opposum, raccoon, and domestic cats. The organisms cannot complete their life-cycle in the horse but do create significant damage when they reach the brain and spinal cord. Sarcocytis and Neospora are classified as coccidia. Medications against coccidia, including Marquis and Protazuril, are able to slow the progression of the protozoa but do not kill all of the organism in a host. Horses under stress (heavy training, shipment, young or old age, or concurrent disease) are most at risk for contracting EPM. A stressed immune system has a difficult time fighting a coccidia infestation. The organisms populate inside the horses nerve cells of the brain and spinal cord causing permanent nerve damage and symptoms of lameness, unbalance, and odd behavior. Because the medications can only slow the organisms but cannot fully eradicate the organisms from the horses nervous system, a recurrence can occur if the horse experiences a new cause of stress. Multiple rounds or extended lengths of treatment with the coccidiostats may improve relapsing symptoms. Unfortunately, there are occasional patients in whom the nerve damage is too great and neurologic deficits remain the same or worsen with age and time. Safety of the horse and his handlers, as well as quality of life, becomes a consideration in severe or chronic cases. Be sure to contact your local veterinarian for the best diagnostic and treatment plan for your own horse.


Question: How can I help prevent a recurrence of EPM? Should I treat prophylactically every six months?

Answer: Sarcocystis and Neospora, the organisms found to be responsible in EPM, tend to cause new damage when a horse undergoes a stressful period. Nutritional deficiency, illness, severe weather changes, or transport are all potential sources of stress and a compromised immune system in horses. Managing your horse’s daily activities to create a consistent and solid stable routine will go a long way toward decreasing stress. Preventing carrier animals such as opposum, raccoons, and cats from defecating in the feed areas and hay mangers reduce the spread of the EPM organisms. Prophylactic treatment of a previously diagnosed EPM horse is controversial. Some studies have identified that occasional dosing of currently available medications may be useful in decreasing the organism numbers in a horse, while other studies find that a longer duration of initial treatment is more effective. Some clinicians recommend dosing medications during periods of stress. An Ohio State University handout reminds us that “intermittent treatment may increase the risk that parasites infecting a horse develop drug resistance. Therefore, we do not recommend intermittent or periodic treatments.” I suggest discussing options with your local veterinarian to formulate the best treatment and preventative plan for your situation.