The opossum is the definitive host for Sarcocystis neurona, meaning that the protozoa can mature and reproduce within its body.
1. The Opossum ExcretES The Parasite eggs, called oocysts, in its feces.
2. The oocysts release a secondary stage, called sporocysts, which may contaminate feed or water and be consumed by other animals.
3. The horse may ingest sporocysts. Horses are considered aberrant hosts because, so far, no evidence has been found that the protozoa complete their life cycles in horses.
4. In some cases, the protozoa may cross into the horse’s central nervous
/ or brain, causing equine protozoal myelo-encephalitis (EPM).
5. Other animals—including raccoons, skunks, cats and armadillos— may ingest the sporocysts and become intermediate hosts.
6. Once inside the intestine of an intermediate host, the sporocysts hatch and go through other life stages. Eventually, they invade the muscle tissue and form sarcocysts, which contain parasite spores.
7. When the intermediate host dies, its carcass may be scavenged by an opossum, which ingests the sarcocysts. The parasites mature in the opossum’s intestine, and the cycle begins again.
Note: The life cycle of Neospora hughesi is less understood, but it appears that horses do not have to eat infected food or water to contract it: Mares who carry the organism can pass it to their offspring during gestation. This means that EPM may be a possibility even in areas where opossums are not found.