Hoyt Cheramie, DVM, MS, Dipl. ACVS, is a member of the Merial Veterinary Professional Services team. He has expertise in performance horse medicine, is a board-certified surgeon and has teaching experience at the Virginia-Maryland Regional College of Veterinary Medicine. He has practiced in Kentucky, Louisiana, Georgia, and Illinois. Cheramie earned his doctor of veterinary medicine from Louisiana State University School of Veterinary Medicine. Here, he answers a question about deworming.

Question: Can too much deworming actually be a bad thing?

Answer: While it is unlikely a horse will become ill or suffer harmful effects from being dewormed too often, in the long term, all horses’ health can be compromised by the development of parasite resistance to dewormers.

When deworming strategies were developed in the 1960s, the protocol was simple: treat every horse on an eight-week schedule with the newly available benzimidazole anthelmintics (an anthelmintic is a medication causing parasitic intestinal worms to be expelled or killed). A dramatic reduction in mortality from parasitic disease resulted. During the next two decades, as new anthelmintics became available, veterinarians recommended rotating between classes of products, but still treating every horse the same