In the world of human medicine, you’ve likely heard about concerns of bacteria becoming increasingly resistant to antibiotics. It works like this: Each time a person takes an antibiotic, sensitive bacteria are killed. Resistant germs, however, are left to grow and multiply, ultimately creating a population of "supergerms" that don’t respond to traditional antibiotics.

What’s that got to do with horses and parasites? Well, a similar theory applies. Over the years, parasites have developed resistance to certain commonly used anthelmintic classes. What does that mean for horse owners? Their horses could have parasites that are resistant to some of the deworming products owners are currently using.

So not only could owners be wasting money on products that are no longer effective in controlling some parasites, but they could also be putting their horse’s health at risk and helping create bigger populations of resistant parasites.

Horse owners might be asking how that can happen if they’ve carefully adhered to the decades-old recommended practice of rotational deworming. The answer is simple: Parasites have responded to the anthelmintic challenge by developing resistance. In the case of small strongyles, identified as the most prevalent parasite in adult horses today, there is evidence of their widespread resistance to two of the three major dewormer classes: benzimidazoles and pyrantels