Dewormer Drug-Resistance Panic

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Concerns about dewormer resistance in parasites have risen in recent years. And, of course, we’ve seen conflicting advice regarding what to do about it.

Rotational deworming has been widely promoted as a way to delay the emergence of resistant parasites. Equine supply stores and catalogs promote rotational packages with a different dewormer for each use, even offering year-long supplies and a deworming calendar, so they basically do the work for you. Manufacturer websites offer rotational schedules.

The truth of the matter is that there is no solid data to support that these rapid (switching with each treatment) rotational strategies decrease resistance. Many experts believe that rotation may even increase the risk of resistance by exposing parasites to a wide variety of dewormers. Even more importantly, the question is moot. The fact is that resistance is here, it’s widespread, and you need to understand how to protect your horse.

The major parasites of horses in North America are cestodes (tapeworms) and nematodes (all the rest). There are no known resistance problems with the cestodes, but nematodes are a different story. Researchers usually follow the major nematode parasite of young horses (roundworms) and adult horses (small strongyles) to monitor for parasite resistance. There are documented resistance problems worldwide for ivermectin with roundworms and all drugs except ivermectin and moxidectin for small strongyles.

Bottom Line. We’ll discuss deworming strategies in detail in an upcoming article, but the bottom line here is clear. There’s no point in deworming your horse with a product that doesn’t work.

If you want to continue to use specific drugs that are known to have resistance problems, at least check with a fecal-egg count two weeks after using the product (your vet can run the sample). You could be throwing away your money and risking your horse’s health by using an ineffective drug.

Article by our Veterinary Editor, Dr. Eleanor Kellon.