New Research on Dewormer Resistance in Horses, Explained

For most of the last two decades, the working assumption in a hunter/jumper barn was that fenbendazole and pyrantel had gone soft, but ivermectin and moxidectin could still be trusted to finish the job. A new three-year French survey removes that fallback. For the first time, researchers have documented resistant parasite populations against all four of the anthelmintic drug classes available in the country, in the same national dataset, at the same time.

What Was Actually Tested

The study, led by Gabin Brucelle of Anses, the French Agency for Food, Environmental and Occupational Health Safety, ran fecal egg count reduction tests across 749 equids kept at 70 facilities over three years, completing 104 full test cycles. It was published in July 2026 in the International Journal for Parasitology: Drugs and Drug Resistance. The design is a straightforward, unglamorous one: dose a group of horses with a given drug, count strongyle eggs in the manure before and after, and see how far the egg count actually drops. A drug doing its job should flatten the count close to zero. One that no longer works leaves the parasites, and the eggs, largely untouched.

Fenbendazole and pyrantel failing at that scale is not news. What is new is the other half of the table. Ivermectin and moxidectin are the two drug classes the industry has leaned on precisely because they were assumed to still work everywhere, and more than one in five French facilities tested now has a strongyle population that survives them. There is no longer a drug class on the shelf that functions as a guaranteed backstop.

The Rotation Was Never the Safeguard It Felt Like

The instinct behind rotating dewormers through the year was reasonable on its face: vary the drug, and no single parasite population gets enough repeated exposure to adapt. In practice, rotating blindly on a fixed calendar does close to the opposite. Every horse in the barn gets treated whether or not it is carrying a meaningful parasite burden, which means every available drug class gets pushed against the entire population on a schedule, rather than against the horses that actually need it. The American Association of Equine Practitioners rewrote its parasite control guidelines in May 2024 specifically to say so, recommending that barns stop deworming every horse on a fixed interval and stop rotating drug classes blindly, in favor of annual fecal egg counts that sort horses into low, medium, and high shedders, with treatment weighted toward the horses actually carrying the load.

A hunter/jumper training or boarding barn is close to the ideal setting for resistance to spread this way. Horses share turnout, share arenas, and often share a single farm-wide deworming calendar set by whoever manages the barn rather than by each horse's own fecal egg count. The low shedders in that population, dewormed on the same schedule as everyone else, contribute nothing to the parasite population's vulnerability and a great deal to the drug exposure driving resistance in it.

What a Resistant Population Actually Costs

None of this means a dewormer dose stops being useful once resistance shows up in a region. Resistance in these studies is measured at the population level, not as a light switch, and a drug that fails on 27 percent of farms still works cleanly on most of them. The risk is treating the drug label as proof of performance in your own barn, when the only way to know whether a given anthelmintic is still doing its job on your own horses is to test for it directly, with a fecal egg count reduction test of your own rather than an assumption carried over from the last decade.

That is the real shift this research asks for. The question a barn manager should be answering is no longer which drug class to buy this month. It is whether the drug class already in the feed room has been checked against the parasites actually living on that property, this year, rather than trusted on reputation.

The Practical Read

An annual fecal egg count reduction test, run by a veterinarian on a representative group of horses, is a modest cost against what a barn already spends on tack, footing, and show fees. It is also the only tool that tells a barn manager something a label cannot: whether the dewormer in current rotation is still killing parasites on that specific farm, or quietly maintaining a population that has already adapted around it. A rotation schedule built on faith in last decade's drug hierarchy has become a guess rather than a plan, and the French data says the odds on that guess are getting worse every year it goes untested.