Horses that are diagnosed with equine protozoal myeloencephalitis (EPM) have a few options for treatment of the disease, including Marquis (ponazuril) and Protazil (diclazuril). While resistance to anthelmintics and antimicrobials are real concerns, resistance to antiprotozoals is not, Dr. Nicola Pusterla of the University of California, Davis, School of Veterinary Medicine tells The Horse.
Pusterla says that this concern is unfounded for two reasons: EPM medications would need to be overused, allowing for the hardiest organisms to survive, and this is not happening, he said. Second, Sarcocystis neurona, the primary EPM-causing organism, dies with the host, even if it becomes resistant (except if opossums scavenge the dead horse to complete the protozoa's lifecycle, which is highly unlikely).
S. neuroa requires two hosts: a definitive host, which is an opossum, and intermediate hosts, which can include raccoons, cats, skunks and armadillos. The protozoa lives in these animals for a period of time, but does not affect the animals. To complete the protozoa's life cycle, the opossum must ingest tissue from the intermediary host that contains the protozoa.
Opossums are often infected with S. neurona and are the main source of infection in horses. The horse then contracts S. neurona by ingesting food or water that has been contaminated with the opossum's feces. A horse cannot spread disease to other horses, nor can intermediate hosts spread the disease.
Because of this, a horse treated with an antiprotozoal is unlikely to harbor resistance forms of S. neurona that can be ingested by opossums, completing the life cycle of the parasite. Additionally, all FDA-approved EPM treatments have concentrations in excess of what inhibits S. neurona growth.
Read more at The Horse.
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