A: The main protozoan that causes EPM, Sarcocystis neurona, is found in the scat of some opossums. The horse eats feed (hay or grain/concentrates) that is contaminated with S. neurona from opossum feces, and the protozoa gains entrance into the horse’s blood through the intestinal tract. (The protozoan Neospora hughesi also causes EPM, but the definitive host is not known).
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