Equine Protozoal Myeloencephalitis (EPM) is a debilitating neurologic disease of horses. It can affect the brain, brainstem, spinal cord or any combination of these three areas of the central nervous system (CNS). The disease may present itself with a variety of different clinical signs, dependent on the location of the damage caused by the organism within the CNS. There is no vaccine currently available.

Life Cycle: The organism causing EPM was given the name Sarcocystis neurona. The horse is a dead-end, aberrant host, as infectious forms of the parasite are not passed from horse to horse or from horse to the definitive or true intermediate hosts. Recent investigation indicates that opossum feces (definitive host) are the source of the infection for horses. Opossums acquire the infection by eating infected birds (intermediate host). Most infections would come from contaminated pasture, hay, grain, and water with opossum feces. Contamination of feed and water with opossum feces may occur indirectly through other mechanisms such as birds and insects. Relocation of opossums away from horses, water, bedding and feed storage environment may be beneficial to reduce exposure. EPM occurs in much of North America and South America. Several surveys revealed that approximately 50% of the horses have been exposed to this parasite. A small survey of Indiana horses indicates that the exposure may be even higher. We know that a positive serum test indicates exposure to the parasite, not necessarily the presence of disease, the incidence of which is much lower. EPM appears to have a sporadic distribution, although outbreaks have been reported on farms in Kentucky, Ohio, Indiana, Michigan, and Florida.

Clinical Signs: EPM is a progressive disease which, if not treated, may eventually lead to death. Any horse that is demonstrating neurologic abnormalities or a subtle lameness that the veterinarian cannot pinpoint after a thorough lameness examination may have EPM. The clinical signs are dependent on the location of the organism within the central nervous system. EPM can affect a horse of any age, breed, or sex. The youngest horse reported affected was two months of age, and the eldest in its thirties. Clinical signs may be triggered or worsened by physiologic stress or the administration of corticosteroids. Clinical signs of the disease include weakness, malposition of a limb, muscle atrophy, spinal ataxia or “wobbling,” head tilt with asymmetry of the face (eyelid, ear, lip). A severely EPM-affected horse may be down and unable to rise. Lameness not traceable to orthopedic disease or any combination of the above signs may occur in early or less severe infections. Other unusual signs may occur. In most cases, affected horses are bright and alert with a normal appetite, although some horses are dysphagic (unable to eat) and may act as if they are choked, with feed material coming from their nose. Hematological and biochemical blood values are usually in the normal range. Sometimes a horse may have more than one disease, e.g., both EPM and cervical stenotic myelopathy (“wobbler”). It is important to realize that all neurologic disease in horses is not EPM, and a complete work-up by your veterinarian is needed in many cases to arrive at a specific diagnosis of the problem.

Diagnosis: Diagnosis of EPM is based on clinical signs and on testing of the horse’s cerebrospinal fluid (CSF). A positive serum test cannot be used to make a diagnosis, but simply indicates exposure to the parasite. Cerebrospinal fluid testing by Western blot is the most useful test to assist in the diagnosis of this disease in the live horse. The presence of these antibodies in the CSF of horses with neurologic signs usually indicates active disease. However, if blood contaminates the CSF sample, a false positive test may result. If the test is negative, and the horse is exhibiting clinical signs, the horse may still have the disease. In some cases, a second test (PCR) may be able to detect very small amounts of parasite DNA (parts of the parasite’s genetic makeup) and may assist in the diagnosis. The PCR is considered most useful as a research tool.

Therapy: Treatment of horses with EPM is expensive. The average range of treatment is 90 to 120 days and may exceed six months in some instances. The appropriate length of treatment and the method to determine adequate treatment duration are unknown. The current approaches to treatment for EPM includes pyrimethamine in combination with a sulfonamide antimicrobial with or without trimethoprim. These medications should be administered one hour prior to feeding hay. Frequent, periodic, veterinary neurologic examinations by your veterinarian are recommended during the treatment period. Discontinuations of therapy may be based on administration of medications 30 days beyond the plateau of clinical improvement. An alternative approach to determining discontinuation of therapy is disappearance of antibody to the protozoa from the CSF. Suboptimal dosing or intermittent therapy has no proven efficacy. Adverse side effects of therapy may include anemia, abortion, diarrhea, and low white blood cell counts. Both medications for treatment of EPM inhibit folic acid metabolism. Supplementation with folic acid (40 mg orally, once a day) may help prevent adverse side effects. Folic acid should not be administered at the same time of day as the antimicrobial drugs. Non-steroidal anti-inflammatory medications and DMSO may be added to the treatment regimen. More recently, diclazuril and toltrazuril are two drugs that have shown some efficacy in the treatment of horses affected with EPM.

Prognosis: Early detection and therapy increase the chance of successful treatment. Response to treatment is highly variable; many treated horses return to their original level of function; however, some may not respond completely. In these horses, too many nerve cells are often destroyed to allow a complete cure. It is also estimated that approximately 10% of the cases relapse after treatment is discontinued. Some horses are currently on medication indefinitely. EPM can be a frustrating disease. If you have questions about the disease, consult with your veterinarian.