The opossum does not become sick, but may shed the parasites for months. Horses represent an aberrant host of S. neurona . Sporocysts are ingested, but never encyst in the tissues of the horse. Instead, they migrate to the central nervous system, where they continue to undergo asexual reproduction intracellularly in neurons, without forming tissue cysts. Horses cannot transmit the organism to other horses, or even to opossums .
Horses probably eat the opossum sporocysts inadvertently while eating grass, hay or grain.
The horses had detectable serum antibodies at about 3 weeks after infection, and all of the horses that ultimately developed EPM had spinal fluid antibodies about a week later. Those that did not develop EPM never had antibodies in the spinal fluid, even as long as 4 months later. This disease may be preventable by some simple measures. Anything that may attract opossums into barns should be tightly covered, or put away, especially at night. This includes cat food, garbage and grain.