Equine Protozoal Myeloencephalitis

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Equine Protozoal Myeloencephalitis, or EPM, is a disease cause by a protozoal infection of the central nervous system of horses.

The neurologic signs that it causes are most commonly asymmetric incoordination (ataxia), weakness and spasticity, although they may mimic almost any neurologic disorder. Clinical signs among horses with EPM include a wide array of symptoms that may result from primary or secondary problems. Some of the signs cannot be distinguished from other problems, such as lameness, which can be attributed to many different causes. Airway abnormalities, such as laryngeal hemiplegia (paralyzed flaps), dorsal displacement of the soft palate (snoring), or airway noise of undetermined origin may result from protozoa infecting the nerves which innervate the throat. Apparent lameness, particularly atypical lameness or slight gait asymmetry of the rear limbs are commonly caused by EPM. Focal muscle atrophy, or even generalized muscle atrophy or loss of condition may result. Secondary signs also occur with neurologic disease. Upward fixation of the patella (locking up of the stifle) is among the most common findings among horses with neurologic disease. Another common side effect of EPM is back soreness, which can be severe. The actual method by which the Sarcocystis neurona infects a horse is still unknown, however it is thought to preferentially infect leukocytes in order to cross the blood brain barrier.

This disease is caused by the parasite Sarcocystis neurona. In order to complete its life cycle this parasite needs two hosts, a definitive and an intermediate. In the laboratory raccoons, cats, armadillos, skunks, and sea otters have been shown to be intermediate hosts. The oppossum is the definitive host of the disease. Horses most commonly contract EPM from grazing in areas where an opossum has recently defecated. However, horses cannot pass the disease among themselves. That is, one horse cannot contract the disease from another infected horse. The horse is the dead-end, or aberrant, host of the disease.

Control of this disease includes a recently released vaccine against the parasite, and control of opposums in an area. The vaccine, however, has only been conditionally approved by the USDA until efficacy tests are available.

EPM was discovered in the 1960's by Dr. Jim Rooney. The disease is considered rare, though recently, an increasing number of cases have been reported. Research at the University of Kentucky has labeled the opossum as the definitive host of the disease.