I think you want to know which animal can pass the infection on to the horse. the main one is the opossum
Opossoms
mice
As odd as it sounds horses get the illness from opposoms who get it from birds.
Oppossums
mice
Opossums
equine refers to horses
Equine Protozoal Myelitis
yes, Equine Protozal Myelitis is a "bug" that can be passed.
Opossums can spread equine protozoal myeloencephalitis (EPM) to horses primarily through their feces, which may contain the protozoan parasite Sarcosystis neurona. When horses ingest food or water contaminated with opossum feces, they may become infected with the parasite. The parasite then travels to the horse's central nervous system, leading to neurological issues associated with EPM. Effective management of horse feeding areas and reduction of opossum populations can help mitigate this risk.
Horses Equine- Is the study of horses -Maurenna
Raccoons are the animal most commonly implicated.