Equine Protozoal Myeloencephalitis (EPM) is a neurological disorder caused by a parasite. The opossum and a parasite called Sarcocystis neurona have been implicated although current research suggests other hosts (such as raccoons and skunks) and other parasites may be involved in disease transmission. The infective form of the parasite is passed in the feces. If a horse eats contaminated feces then it could develop neurological signs. It should be noted that the majority of opossums are probably not shedding the infective parasite and that of the horses exposed to the parasite, very few will develop EPM.
There are steps you can take to reduce the risk of EPM. Keeping all animals away from horses is advised. Do not kill or trap opossums. This will not work. Unless you remove what is attracting opossums and other animals to your yard then more animals will come. The best thing to do is to encourage animals to leave the area on their own by removing the attractants. This is usually food. Do not leave food outside. Do not leave food for the barn cats at night. Feed cats during the day. Cover garbage cans. Pick up fallen fruit. Cover horse’s feed and grain bins. Do not allow horse to feed off the ground. Place feed in troughs above the ground. Change water frequently. Remove animal hiding places such as dense shrubs and woodpiles.