Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis (EPM) is a disease that affects the nervous system of horses. Although many horses are exposed to the causative agent, Sarcocystis falcatula, clinical signs result from the migration of this agent into the central nervous system. These clinical signs vary from mild gait abnormalities or behavioral changes to severe seizures and paralysis. Success in treating this disease depends on early diagnosis. Once a diagnosis has been made, it is important to monitor your horse for adverse side effects of the drugs used and to continue the treatment until full remission. This disease is the most frequently diagnosed equine neurological disease at Mississippi State University's College of Veterinary Medicine.

Life Cycle

Equine Protozoal Myeloencephalitis occurs when a one-celled protozoal organism called Sarcocystis infects the nerve tissue in a horse. The organism was originally thought to be Sarcocystis neurona, but now it is believed to be Sarcocystis falcatula. The organism normally causes a nonclinical intestinal infection in the opossum and is shed in opossum manure. In order to reinfect other opossums, the organism in opossum manure must be eaten by a bird, where it forms cysts in the bird's muscle (sarcocysts). Other opossums become infected after eating bird meat that has been infected with sarcocysts.

When horses eat or drink food or water that has been contaminated by infected opossum manure, two possibilities can occur. The horse's immune system may clear the organism from the intestine and the horse may not have a problem, or the organism is not cleared by the immune system and migrates to the nervous system, where it reproduces and causes neurological signs. The horse is not contagious to other animals, and thus it is termed a dead-end host. (See graphic.)

Clinical Signs

The clinical signs displayed as a result of this disease can be extremely variable, ranging from an unnoticeable lameness to spasticity, loss of coordination, and paresis. You can make these signs more noticeable by walking the horse up or down a slope, or by leading the horse with its head up. Often an injury is the main problem noticed because these horses are more prone to accidents resulting from lack of coordination. Approximately 10 percent of affected horses will display signs of brain involvement, which in turn are manifested by facial paralysis, inability to chew or swallow, difficulty breathing, or seizures.

Diagnosis

The most reliable test is a laboratory test, the Western Blot Immunoassay, performed on cerebrospinal fluid (CSF). This test can be performed on blood serum also, but a positive response to this means only that the horse has been exposed. Clinical signs occur only when the parasite has migrated to the brain or to the spinal cord of the horse. Therefore, exposure is not always significant. It is also important to rule out other diseases such as herpes virus, rabies, moldy-corn poisoning, and other neurological diseases.

Treatment

Antiprotozoal drugs such as pyrimethamine in combination with a sulfonamide are currently recommended. Give these drugs for at least 4 months. Monitor your horse closely during this period. About 60 percent of horses treated will respond. The others will show no, or very little, improvement, or they will improve, then relapse after the treatment is discontinued. Anti-inflammatory drugs are often indicated in acute cases of EPM. These drugs include flunixin, phenylbutazone, DMSO, ketoprofen, and corticosteroids. All of these products are prescription medications, so follow your veterinarian's instructions for their use.

As many as 10 percent of horses treated will experience a treatment reaction or crisis. This reaction is an inflammation of the nervous system resulting from the body's response to the dying organisms. This reponse may actually cause the neurological signs to worsen. In these cases, the use of anti-inflammatory drugs is defintely needed.

A new, experimental drug treatment seems to be more effective than the pyrimethamine/sulfonamide combination and to have fewer side effects. This drug is the antiprotozoal product diclazuril.

Prevention

There is no vaccine or definitive prevention/control program at this time, and development of a vaccine for EPM is unlikely.

Follow these recommendations toward wilflife involved in the life cycle of the infective agent:

• Provide fencing or screening to keep birds and opossums away from feed and bedding.

   

• Properly discard leftover pet feed to avoid attracting opossums, birds, and rodents.

By Richard M. Hopper, D.V.M., Leader, Extension Veterinary Medicine

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Information Sheet 1586
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

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