The dewormer rotation most barns run was built on an idea that made sense at the time: vary the drug class, and no parasite population gets enough exposure to adapt. It turns out that treating every horse on a fixed calendar does roughly the reverse, because the low shedders take the drug exposure and contribute nothing in return, according to new three-year French data.
Source: Nottinghill Equine, LinkedIn, October 3, 2026. Link. 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.
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 . . . meaning there is no longer a drug class on the shelf that functions as a guaranteed backstop.”
Image: Link.
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