Equine protozoal myeloencephalitis

Farm Forum

Equine protozoal myeloencephalitis (EPM) is a neurologic disease in horses caused by the protozoan Sarcocystis neurona. Clinical signs include abnormal gaits, incoordination, loss of sensation to face, muscle weakness and wasting. The neurological exam scale ranges from 0 to 5, where 0 represents a clinically normal horse and 5 represents a horse unable to move.

Neurological Exam Grade Scale

(Adapted from Furr M. and Reed Stephen, 2008.)

· Grade 0: No deficits.

· Grade 1: Deficit just detected at a normal gait, but worsened by backing, turning, loin pressure or neck extension.

· Grade 2: Deficit easily detected at normal gait and exaggerated by backing, turning, swaying, loin pressure and neck extension.

· Grade 3: Deficit very prominent on walking, with a tendency to buckle or fall with backing, turning, loin pressure, or neck extension.

· Grade 4: Stumbling, tripping and falling spontaneously.

· Grade 5: Horse recumbent.

Sarcocystis neurona has a complex life cycle that requires two hosts. The intermediate hosts include the raccoon, armadillo, skunk, domestic cat and sea otter. While in the intermediate hosts, S. neurona goes through asexual reproduction within the digestive system. The sarcocysts harbor within the skeletal muscle of the intermediate hosts. Once the intermediate host dies the opossum eats the infected tissue. Sexual reproduction occurs in the digestive system of the definitive host, the opossum. The opossum’s infected feces are ingested by the horse. The horse plays the role as the aberrant or dead end host in this cycle however, horses are unable to pass EPM to each other. EPM only occurs in North America because the opossum is only native to this continent. The way S. neurona infects the Central Nervous System (CNS) is unknown. However, there is a hypothesis that recommends S. neurona infects leukocytes which then carries the organism to the CNS. Once the organism infects the CNS, rapid production of S. neurona antibodies occur and lesions within the white and gray matter form. Approximately 50% of horses in the United States are seropositive for S. neurona, while only 1% of these horses develop EPM. Reasons for this may be parasite dose, stress and immunosuppression. There are multiple immunodiagnostic tests to use for diagnosis of EPM.

Some of these tests include western blots, ELISA’s and immunofluorescence. There is treatment available for horses with EPM. Treatment is expensive and lengthy. More severe cases of EPM may take longer to treat compared to acute cases. Possible treatment drugs include ponazuril, diclazuril, and sulfadiazine/pyrimethamine combo. However, ponazuril is the most commonly used drug for treatment. Within the future it is imperative to determine the mechanism of how S. neurona enters the CNS, appropriate treatment duration and determine if S. neurona causes immunosuppression in horses. There is a grade scale for veterinarians to use when diagnosing horses with EPM. There are a number of prevention tips horse owners can use. Some of these tips include cleaning waterers regularly, not feeding on the ground and keeping feed container securely closed.