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.

Hoyt Cheramie, DVM, MS, equine specialist for Merial’s Large Animal Veterinary Services, says to ensure their farms are managed properly for equine parasites horse owners should consider following these steps:

  • Stop treating every horse the same. Research suggests that only 20 to 30% of horses in a herd shed about 80% of the worm eggs. Thus, it doesn’t make sense to treat every horse with the same eight-week frequency.
  • Start working with your veterinarian to establish a more effective parasite control program. Have a fecal egg count test (FECT) run for each horse to identify which parasites are present and whether that horse is a high, medium, or low shedder.
  • Develop a deworming plan with your veterinarian for each horse based on the results of the FECT, the horse’s environment, and the climate.
  • Follow up with fecal egg count reduction tests to determine whether specific products are still effective against the parasites on your farm.
  • Discontinue using any anthelmintic products that are not effective against the parasites on your farm.

Consider incorporating a commercially available broad-spectrum deworming product. One of these broad-spectrum products, Zimectrin Gold (ivermectin/praziquantel), has been proven effective against benzimidazole-resistant small strongyles and is FDA-approved to control tapeworms, which have been recognized as a significant threat to horse health.

Managing parasites, including tapeworms, through a strategic deworming program can help save money in the long run and help protect effective anthelmintic products.<