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What Is Rotational Worming In Horses?

As a horse owner you have probably heard of “Rotational Worming” or “Wormer Rotation” so here is a little bit of information to help explain what it is and why the practice is important.

Wormer rotation refers to periodically changing the horse wormer you are using to worm your horse. But it is not about changing the wormer brand but more specifically you must change the active ingredient that you are worming with.

The reason you should rotate wormers is because some intestinal worms can build up a resistance to the active ingredient in wormers and therefore their effectiveness can be significantly diminished or become completely ineffective.

Every wormer has either one or a combination of different active ingredients which is the chemical/drug that kills the worms. This is what you need to change when you are rotational worming. The most common active ingredients in horse wormers are “mectin” based like ivermectin or abamectin and other ingredients that end in “ectin” or the “BZ” or “Benzimidazole” group which contain active ingredients ending in “azole” like oxfendazole. “Anthelmintics” like Praziquantel is another ingredient commonly seen in combination with other active ingredients and these are only effective against specific worm species.

Most manufacturers recommend that a good rotational worming program is to worm one year with one active ingredient and then the next year change to another active ingredient. Therefore if year one you worm with “mectin” based wormers the next year you should change to “Bendimidazole” based wormers. You should check the label to see how often you need to use the specific wormer and always follow the instructions.


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