In 1964, this condition was described as segmental myelitis, noted Kenton Morgan, DVM, Dipl. ACT, a Bayer Animal Health technical service veterinarian at the Horseman's Day seminar during the AAEP Convention. He added that necropsy exams showed sections of spinal cord affected, but at that time scientists didn't know what was causing the problem. In 1976, the problem was given its current name, equine protozoal myeloencephalitis (EPM). "Myelo" refers to spinal cord; "encephalo" refers to brain; "itis" refers to inflammation/infection.

EPM is a protozoal disease that causes inflammation in the brain and spinal cord of the horse. In the late 1970s to early 1980s, Morgan said there was debate whether a toxoplasma or Sarcocystis organism was the cause. They are closely related members of the protozoan family. In the early 1990s, an organism was grown from affected equine spinal cord and given the name Sarcocystis neurona.

S. neurona is what causes EPM in the horse. There is a Neospora organism that can cause EPM, but only in a very small number of cases, and it is considered insignificant as a cause.

This is a disease of the Americas; however, there is one report of a native horse from France suffering from EPM that is under investigation. Previously, every horse which developed EPM in Europe had at some time been in he United States