Equine Protozoal Myeloencephalitis - Nervous System

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Most cases of equine protozoal myeloencephalitis are caused by the apicomplexan protozoon Sarcocystis neurona. Horses are infected by ingestion of S neurona sporocysts in contaminated feed or water. The organism undergoes early asexual multiplication (schizogony) in extraneural tissues before parasitizing the CNS.

Like other Sarcocystis spp, S neurona has an obligate predator-prey life cycle.

The definitive (predator) host for S neurona in the US is the opossum (Didelphis virginiana). Opossums are infected by eating sarcocyst-containing muscle tissue from an infected intermediate (prey) host; after a brief prepatent period (probably 2–4 weeks), infectious sporocysts are passed in the possum's feces. Nine-banded armadillos, striped skunks, raccoons, sea otters, Pacific harbor seals, and domestic cats have all been implicated as intermediate hosts; however, the importance in nature of each of these species is unknown.

Dead-end hosts do not form infectious sarcocysts in their muscles, while intermediate hosts do. Because infectious sarcocysts are only rarely formed in equine muscle tissue, the horse is considered an aberrant, dead-end host for S neurona. When clinical disease occurs in the horse, schizonts and merozoites may be identified within equine nervous system tissues at postmortem, rather than sarcocysts in muscle.

Sporadic cases of EPM are associated with Neospora hughesi, an organism closely related to S neurona. The natural hosts of this organism have not yet been identified. Transplacental protozoal transmission, resulting in infected foals, has been documented for N hughesi but not for S neurona.

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