Eventually the organism was found in opossum feces, and now it is assumed that most infections come from contamination of horse feed and water with opossum feces. Dr. John Madigan, one of the seminar participants, believes that there must be other animals which also spread the offending sarcocytis. It is known that this sarcocystis encysts in the breast muscles of scavenging birds. When these birds die, they are eaten by wild animals, the opossum included, and then these animals shed the organism into horse feed, water, or pasture. It is not known why the disease seems to have a sporadic distribution.
Some areas have a higher incidence than other areas.
It is difficult to estimate how long EPM has been affecting horses. The symptoms of the disease are much like other neurologic diseases in the horse. In diagnosis, it is important to carefully distinguish between half a dozen neurologic diseases. The Sarcocystis antibodies may be carried in the blood without an actual invasion of the parasite into the spinal cord. So, the Western blot test of serum does not necessarily mean that the organism has actually invaded the nervous system.