EQUUS ‘Farm Calls’ Podcast Episode 5: The Science of Equine Deworming

In this episode of the EQUUS "Farm Calls" podcast, we talk to Martin Nielsen, DVM, one of the foremost equine parasitologists in the world about equine parasite resistance and solutions for it.

Equine parasite resistance is a problem, but there are solutions.

Dr. Martin Nielsen and friends, courtesy the University of Kentucky

The EQUUS “Farm Calls” podcast is brought to you in 2022 by Farnam—Your Partner in Horse Care.

The bad news in equine deworming is that all three of the drug classes used to combat internal parasites in horses now have some degree of resistance—meaning that some percentages of the parasites don’t die when treated. The other bad news is that there hasn’t been any new anthelmintic (dewormer) created in the last 40 years.

The good news is that all three of the drug classes are still efficacious against some parasites. The other good news is that there have been scientific advances in determining when and how to deworm our horses in order to keep them healthy, and more research looks promising.

The three drug classes of equine dewormers are benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel) and macrocyclic lactones (ivermectin, moxidectin). Each class works best against specific parasites, but not all parasites.

In this episode of the EQUUS “Farm Calls” podcast, we talk to Martin Nielsen, DVM, PhD, Dipl. ACVM (parasitology), Dipl. EVPC (European Veterinary Parasitology College), one of the foremost equine parasitologists in the world. Nielsen is a professor of Equine Infectious Disease at the University of Kentucky’s Gluck Equine Research Center and Associate Chair of the Department of Veterinary Science. He’s also the director of Graduate Studies. Nielsen is the lead author of the AAEP Parasite Control Guidelines, which is available to veterinarians and horse owners. Originally from Denmark, Nielsen joined the Gluck Center in 2011.

First, Nielsen wanted horse owners to understand that it is “normal” for a horse to have some internal parasites, and that those are not a problem unless they cause health issues in the individual animal. He reminded horse owners that it is beneficial for a horse to have some worms.

The best way to tell whether your horse has parasites that are resistant to dewormers is to do a fecal egg count (FEC) reduction test. That is best done in the entire group of horses that live together because the parasite population is shared by all horses in a herd—but not all horses have the same type of parasite burden.

“There is an assumption that all horses have the same worms or worm burden,” said Nielsen. “For example, one horse may have tapeworms, and another not.”

He reminded horse owners that it is their responsibility to combat drug resistance. “Once resistance is there, it doesn’t go away,” Nielsen said. “And if we do get a new drug, let’s make sure we don’t make the same mistakes.”

Topics that are covered in this podcast include:

  • University of Kentucky equine parasite research (Neilsen has been part of more than 100 research papers since he started at UK)
  • Global collaborative efforts in equine parasite research that are helping
  • How horse owners can use fecal egg count reduction tests to determine resistance
  • Different use of products, depending on that test
  • The future of deworming
  • Resources for horse owners


Nielsen has several ways that horse owners can learn more about equine parasitology and his research, including some award-winning videos!

EQUUS “Farm Calls” is a production of the Equine Network LLC.




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