Deworm all your horses, do it on a frequent and regular schedule, and rotate your product choice through different classes of dewormers. Right?

Wrong. Equine parasitologists now advise you to rethink the traditional rotational deworming protocol. Conventional wisdom has changed, and it’s forcing a need to relearn what we’ve assumed to be “fact” about parasite control. The following is a quick fact check for you to use before you deworm again.

FACT. Rotating dewormers is not only unnecessary, it actually may be harmful.
“Most horse owners think they have been doing the right thing to fight resistance by rotating dewormers. But using several products, some of which may not be effective for parasites infecting horses on your farm, doesn’t work,”1 says Frank Hurtig, DVM, MBA, director, Merial Large Animal Veterinary Services. “And indications are that this practice may actually contribute to resistance.”2,3,4

Rotational deworming, according to Dr. Hurtig, is an outdated practice that originated primarily because early dewormers weren’t entirely effective against all worms. 4,5 They also believed rotating product classes would help keep resistant worm populations from arising on a farm or in an area, but it does not appear to be slowing the development of resistance.2,3

Resistant worms occur by chance but they cannot be diagnosed on a farm until they make up at least 25 percent of the worm population.6 By then, it is too late to correct.7 This fact highlights the importance of deworming only the horses that need it and utilizing Fecal Egg Count Reduction Tests (FECRTs) as part of an overall strategic deworming protocol.7,8 By using this strategy, you can regularly monitor the efficacy of your program and the products you are using, as well as help determine if resistance exists on your farm.7 It is important to remember that resistance develops to classes of drugs and not just individual products within classes.1,2,4,5

FACT. As it turns out, leaving a residual worm population on a farm not only does no harm, it’s actually necessary.
True. It’s called “refugia.” As illogical as it may sound, you should actually welcome a population of parasites on the farm that are susceptible to dewormers. By leaving that refuge population of non-resistant (susceptible) worms to interbreed with resistant parasites, you help reduce the development of resistance.1,4,9

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