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mice
mice
Opossoms
Opossums
The carrier in the US is the raccoon.
Oppossums
If by "possums disease" you mean Equine Protozoal Myeloencephalitis, the answer is a few horses will, but these were the minimally affected horses. Severely affected horses (ie, those that are having difficulty walking or standing) generally do not recover fully and many are euthanized for humane reasons when they become infected.
Raccoons are the animal most commonly implicated.
As odd as it sounds horses get the illness from opposoms who get it from birds.
I think you want to know which animal can pass the infection on to the horse. the main one is the opossum
Opossums are the definitive host for Sarcocystis neurona, which is the parasite that causes EPM (equine protozoal myeloencephalitis. The parasite is excreted from opossums in their manure which can then be ingested by the horse while grazing or drinking water.
The feces of opossums may contain sporocysts - cysts that contain spores that can reproduce asexually. Horses can ingest these sporocysts with feed, grass or water contaminated with opossum feces. The protozoa can leave lesions on the spinal cord and brain stem. It is this neurological damage that can cause the various symptoms of EPM.