Equine protozoal myeloencephalitis (EPM) is a progressive and potentially fatal neurologic disease in horses caused by protozoal (single cell) microorganism, most commonly Sarcocystis neurona, which causes inflammation in the brain and/or spinal cord.

Opossums, the primary intermediate host in the life cycle of S. neurona, excrete parasite oocysts (akin to an egg) in their feces, which subsequently develop into sporocysts (infective spores). Horses become inadvertently infected when they ingest the sporocysts while grazing.

Clinical signs of EPM are usually asymmetrical (worse on one side of the body). Signs can be as mild as a slight decrease in performance or as severe as narcolepsy, seizuring, and collapse.