Equine protozoal myeloencephalitis (EPM) is a treatable but frequently debilitating central nervous system disease of the horse. It is probably the most commonly diagnosed equine neurologic disease in the Western Hemisphere and results in considerable economic and personal loss. Although nervous disorders of the horse have been known for centuries, the unique pathology associated with EPM was not recognized until the early 1960s. The disease was initially known as segmental myelitis and was found in horses from the northeastern United States. Subsequent cases have been reported among native horses throughout North America, as well as in Central and South America.