To learn more about updated deworming recommendations, please read “Deworming: No Longer One Size Fits All,” originally published in the May 2025 issue of the Chronicle.

Currently, there are four classes of deworming medication labeled and sold for use in horses in the U.S., and these are the only options experts expect will be available for the foreseeable future.

Unfortunately, parasite species are already showing either early indications or widespread evidence of resistance to each of the three principal classes of equine dewormer—making it more critical than ever for managers to be strategic in terms of which product they are using, when, and on which horses.

“The classes are different based on how they work, how they actually kill the parasite we are trying to target, and what stage of the parasite they target or kill,” explained Julia Wilkinson, DVM, IVCA, M.Sc, clinical assistant professor at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, Massachusetts.

The following represent the four classes of equine dewormer, some of the specific drugs that fall into that category, and where resistance has been seen.

When purchasing a deworming product, managers need to look past the brand name to identify the actual pharmaceutical compound contained within it.

• Class: Macrocyclic lactones
Drug Names: ivermectin, moxidectin
Incidence of Resistance: Widespread in pinworms and parascaris species; early indications in cyathostomins

• Class: Benzimidazoles
Drug Names: fenbendazole, oxibendazole
Incidence of Resistance: Widespread in cyathostomins, early indications in parascaris species

• Class: Pyrimidines
Drug Names: pyrantel
Incidence of Resistance: Widespread in cyathostomins, early indications in parascaris species and tapeworms

• Class: Praziquantel
Drug Name: praziquantel
Incidence of Resistance: Early indications in tapeworms (its only use)

Note that in the U.S., praziquantel for horses is always sold in combination with a macrocyclic lactone.