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Unfortunately, there are plenty of horse owners still following an outdated program (from the 1960s!)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.

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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)

Pinworms

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

Tapeworms

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

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

6 years ago

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.