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Why Dewormer Rotation Is Over

Dewormer Rotation

Unfortunately, there are plenty of horse owners still following an outdated program (from the 1960s!) of deworming by rotating products and treating their horses every two months or even more often. There’s more to worry about here than simply wasting money on excessive treatments. Here is an insight on why dewormer rotation is over and how parasites are becoming resistant to them.

Over time, overuse of dewormer allowed parasites to develop a tolerance for the drugs. Some parasites were killed, of course, but those that weren’t had better immunity to the deworming drugs, resulting in a parasite population that is becoming increasingly resistant.

We are now seeing widespread resistance to deworming drugs, particularly in the case of small strongyles, one of the most common equine parasites. Roundworms (ascarids), common in young horses, are also showing resistance. Because we only have three classes of deworming drugs available at this time, resistance is a serious issue, as it limits the available options to control parasite populations.

At present, fecal egg count reduction testing (FECRT) is the only method to detect resistance in strongyles, so it should figure into every horse’s deworming program. Your veterinarian can explain how and when to perform this simple test—and discuss the results with you so you know that you’re deworming properly.

Guide to Dewormer Resistance

Resistance among the following common parasites is cause for concern.

Small Strongyles

  • Widespread resistance to fenbendazole, oxibendazole
  • Resistance to pyrantel pamoate
  • Emerging resistance to ivermectin and moxidectin (also referred to as macrocyclic lactones)

Large Roundworms (ascarids)

  • Some resistance reported to fenbendazole, oxibendazole, pyrantel pamoate
  • Widespread resistance to ivermectin and moxidectin (also referred to as macrocyclic lactones)


  • Resistance to ivermectin and moxidectin (also referred to as macrocyclic lactones)


  • No resistance currently reported, but because there are no good resistance tests available for tapeworms, resistance could well be present.

Read more about Regional Parasite Control for Horses >>

This article originally appeared in the April 2019 issue of Horse Illustrated magazine. Click here to subscribe!

Cynthia McFarland

Cynthia McFarland is an Ocala, Fla.-based freelance writer, horse owner and avid trail rider. The author of nine books, her latest is The Horseman’s Guide to Tack and Equipment.

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