Equine Protozoal Myeloencephalitis

EPM: A Frustrating Disease for Horse Owners

by R.M. Nervig, DVM, MS

Equine protozoal myeloencephalitis (EPM) is a disease of horses that affects the spinal cord and brain. It is caused by a protozoa, Sarcocystis neurona. The effect of the parasite on the horse’s central nervous system (CNS) produces symptoms of neurological disease. It is an important equine disease in the U.S. due to the tremendous economic losses. Based upon studies conducted in 1998 by the U.S. Department of Agriculture, the estimated cost of EPM was $27 million. The losses were related to lost use of horses, veterinary services, drugs, and death loss.

Equine protozoal myeloencephalitis is not a new disease. The first report of EPM appeared well over 20 years ago. It is found throughout the Western Hemisphere and can involve horses of any age, breed, or sex. The incidence of EPM is unknown. Many horses mildly affected may never be diagnosed with the disease. It has a sporadic distribution. However, outbreaks have been observed on horse farms. Serologic surveys have shown an exposure rate of greater than 50% in some areas. Young horses appear to be more susceptible to EPM with the highest risk in horses one to five years old. More Thoroughbreds have been diagnosed with EPM than any other breed, followed by Standardbreds then Quarter Horses. However, the disease has been confirmed in Appaloosa, Arabian, Morgan, Paint, Belgian, crossbreeds, and other breeds.

Sarcocystis neurona has been isolated from the CNS of horses with EPM. This pathogenic protozoa has a complicated life cycle. The opossum is the definitive host for the disease and becomes infected by eating the secondary host, birds, such as cowbirds, starlings, sparrows, and grackle. Horses become infected by consuming grass, hay, grain, or water that has been contaminated with droppings of the opossum. The ingested parasites migrate into the horse’s brain and spinal cord. The horse is a dead-end host so there is no transmission to other horses or to other animals. Many horses are exposed to this organism but not all become infected.

The clinical signs of EPM vary widely among horses. This is because the protozoa can cause damage at various locations in the CNS. Many times the signs begin subtlely and progress gradually. However, they may appear suddenly and rapidly worsen. The most common signs are: incoordination, weakness, muscle atrophy, and cranial nerve damage. The incoordination and weakness, characteristic of spinal cord disease, are common findings in horses with this disease. The signs involve only one side of the body. Many times what is seen is a weak spastic, swaying, and possibly stumbling gait. Horses can exhibit what appears to be a low-grade lameness. In severe cases, the horses may fall and are unable to get up. The muscle atrophy associated with EPM is not symmetrical and is most apparent over the hindquarters. Cranial nerve damage is manifested by facial paralysis, drooping ear, lazy eyelid, head tilt, and difficulty eating and drinking. Horses with EPM appear bright and alert with no fever. They have a normal appetite, even though they may have problems eating. In a few horses seizures were the only clinical signs observed. Locking up of the stifle and back pain have been associated with EPM. There is evidence that stress can trigger clinical signs of EPM in a horse which has the organism in its system. This is a progressive disease which, if not treated, may eventually lead to death.

Diagnosis of EPM by clinical signs alone is very difficult, because there are several other neurologic diseases than can cause a similar clinical picture. Laboratory diagnosis is based on testing cerebrospinal fluid (CSF) for either antibodies to Sarcocystis neurona or the organism’s DNA. Blood contamination during CSF collection can cause false positive test results. Testing blood, as compared to CSF, for the presence of anti-Sarcocystis neurona antibodies is of questionable value, because positive results only mean the horse has been exposed to the parasite in the past. It does not mean that the organism caused, or will cause disease, or even still exists in the body. Therefore, requesting a blood test for EPM during a prepurchase examination is debatable, especially when there is no evidence of neurologic disease.

Treatment of EPM most commonly involves: (1) antibiotics that inhibit replication of the protozoa (pyrimethamine and sulfadiazine), (2) anti-inflammatory drugs that reduce inflammation, and (3) supportive therapy for any secondary problems. The average duration of treatment is 90 to 120 days, and may exceed six months in some cases. Discontinuation of therapy should be based on negative test results on CSF. Premature withdrawal of treatment often leads to relapse, which is many times associated with a poorer prognosis for improvement or recovery. Treatment of horses for EPM is expensive, so it is important that the horse owner discuss the cost of therapy with his/her veterinarian before making a decision to treat the animal.

Early detection increases the chance of successful treatment. Practitioners report that treatment appears to result in a successful outcome in 60% of the horses which are suspected to have EPM. Many treated horses return to their original level of function, however, some may not respond completely. It is also estimated that approximately 10% of horses relapse after treatment is stopped. New drugs and treatment therapies are being studied and show great promise. Recently, the U.S. Department of Agriculture has issued a conditional license for an EPM vaccine. However, studies have not been done at this time to demonstrate efficacy.

Good management is the best defense in protecting your horse. Limiting exposure to wildlife, particularly, opossum and wild birds is paramount. Birds are difficult to control, but they should be kept out of barns. Since opossum feces represent the most significant source of contamination by Sarcocystis neurona, the most effective recommendation for prevention of EPM is to minimize the risk of your horse eating feedstuffs contaminated with opossum feces. Every effort should be made to reduce the opportunities for opossums to interact with horses. Opossums should never be allowed inside barns, where horse feed is stored or where horses are fed. Keeping a dog in or near the barn has been suggested as a useful method in keeping opossums away from the horse barn.

The opossums are omnivores and are attracted to grains, cat and dog food, fruit and garbage. Keeping horse and other animal feeds, such as food for barn cats, securely contained helps keep the opossums away from the barn. Horsemen should not set morning feed out overnight. All uneaten or discarded grain should be cleaned up; spilled grain should not be allowed to accumulate; carcasses of dead animals should be disposed of properly, and fallen fruit should be picked up quickly. Horses should not be allowed to drink from slow running streams or stagnant ponds, especially in wooded areas. Fresh water should be provided in clean containers or from automatic waterers. Good housekeeping in and around the barn goes a long way toward reducing the risk of environmental contamination by opossums.

Even though a conditionally licensed vaccine is available, its effectiveness has not been proven. Therefore, limiting the exposure of horses to opossum is the best preventative approach. The disease can be managed if detected early and treated appropriately for a sufficient period of time. Hopefully, increased awareness of EPM can lead to an earlier diagnosis and therapy with a greater chance of successful treatment.