Resistance to Equine Dewormers

Despite aggressive research efforts* over the past decade, some horse owners continue to stick to tried-and-true ways of deworming their charges with chemical anthelmintics (dewormers). Be it a rotational program or a “when-I-remember” approach using a favorite product, such strategies are no longer appropriate or economical, according to equine parasitologist Martin Nielsen, D.V.M, Ph.D., assistant professor at the University of Kentucky Gluck Equine Research Center.

“We only have a limited number of deworming products to use in horses, and widespread parasite resistance to these has been demonstrated in several scientific studies,” explains Nielsen.

The only deworming products used in horses include the following three drug classes: benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel), and macrocyclic lactones (ivermectin, moxidectin). Of those, the roundworm Parascaris spp., is resistant to ivermectin and small strongyles, cyathostomins, are resistant to both fenbendazole and pyrantel and beginning to show resistance to ivermectin and moxidectin.

Dewormers introduced in the 1960s contributed greatly to improving the health and welfare of horses. For example, large burdens of cyathostomins that reside in the large intestinal wall can cause fatal colitis, characterized by diarrhea, if encysted parasites emerge from the wall at the same time. Further, large burdens of roundworms, most frequently seen in foals, can cause impactions and colic, and migrating Strongylus vulgaris can also cause severe colic.

Such parasite-related diseases are now only seen occasionally, but with no new dewormers available or in development, if a resurgence of these diseases occurs, there will be no way to deworm and treat affected horses.

“The continued indiscriminate use of anthelmintics is like shooting ourselves in the foot,” warns Nielsen.

And if you haven’t had the misfortune of having any parasite-related health problems in your horses and remain unconvinced that resistance is brewing then take Nielsen’s “worm challenge.”

“Deworm a young horse with fenbendazole and then perform a fecal egg count 10-14 days later. The egg count should be zero. If there are still eggs in the sample then resistance is developing, leaving the horses on that farm at risk,” Nielsen advises.

Once convinced of the perils of anthelmintic resistance, consider some of these approaches to deworming your horses:

• Use chemical anthelmintics intelligently. Discuss with your veterinarian how to test the need of particular dewormers by fecal egg counts, for example, and how to choose the most appropriate product and timing of administration;

• Once a dewormer has been selected, follow the manufacturer’s instructions;

• Accurately estimate your horse’s weight to ensure a full dose is administered; and

• Administer the full dose. Dewormer that falls from the horse’s mouth and lands on the stall floor is not effective.

*Matthews, J.B. 2014. Anthelmintic resistance in equine nematodes. International Journal of Parasitology: Drugs and Drug Resistance 4(3):310-315.