Possums ingest the sporocyst and it is transferred to horses when they come in contact with possum feces.
http://www.epmhorse.org/The_Disease/Opossum.htm
https://www.merckvetmanual.com/nervous-system/equine-protozoal-myeloencephalitis/overview-of-equine-protozoal-myeloencephalitis
http://www.epmhorse.org/The_Disease/Opossum.htm
https://www.merckvetmanual.com/nervous-system/equine-protozoal-myeloencephalitis/overview-of-equine-protozoal-myeloencephalitis