Evidence of Macrocyclic Lactone Resistance in Equine Cyathostomin Populations
This open access research article shows clear macrocyclic lactone resistance in a group of Thoroughbred yearlings imported from Ireland to the US in 2019.

Small strongyles or cyathostomes Courtesy Dr. Martin Nielsen

“We have documented for the first time in North America macrocyclic lactone-resistant small strongyles partasites,” said Martin Nielsen, DVM, PhD, DipEVPC, an Associate Professor at the University of Kentucky’s Gluck Equine Research Center, of a recently published study.

The study was published in the International Journal for Parasitology: Drugs and Drug Resistance and was titled, “Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm.” It was authored by Nielsen; Michael Banahan, director of operations for Godolphin’s Darley America operations; and Dr. Ray Kaplan of the University of Georgia.

Nielsen discussed this breaking news in a YouTube video

Click on the image to watch the YouTube video. undefined

In that video he said that the horse industry has three drug classes for treatment of equine small strongyles (cyathostomic parasites). “We have resistance to two out of three drug classes (benzimidazoles and pyrimidines),” said Nielsen. “We have raised concerns about developing or signs of emerging resistance to the third class—the most widely used drug class—macrocyclic lactones. That’s where we have ivermectin and moxidectin products.” 


“Anthelmintic resistance in equine cyathostomins is both widespread and highly prevalent in the benzimidazole and tetrahydropyrimidine classes; however, reports of resistance to macrocyclic lactone (ML) drugs are sparse and sporadic. This study reports a case of clear ML resistance in a group of Thoroughbred yearlings imported from Ireland to the US in 2019. Fecal egg count reduction (FECR) following ivermectin administered in February 2020 demonstrated 100% reduction in the US bred yearlings, but 93.5%, 70.5%, and 74.5% reduction in three groups of the imported yearlings. The two former groups were then retreated with ivermectin, yielding FECRs of 33.8% and 23.5%, respectively. Horses from these two groups were then assigned randomly to two possible treatments; moxidectin or a triple combination of moxidectin, oxibendazole, and pyrantel pamoate. The groups treated with moxidectin had FECRs of 90.2%, 57.3%, and 50.0%, while the triple combination had a 100% FECR in all treated groups. Subsequently, the efficacy of ivermectin was reassessed in June 2020 yielding FECRs of 99.8%, 87.7%, and 62.0% in the three imported groups. The FECRs of the US bred yearlings all remained in the 99–100% range. This is the first study to clearly demonstrate ML resistance in cyathostomins and to confirm the suspicion through reassessment. These data demonstrate that ML-resistant cyathostomins were imported from Ireland and serve to illustrate that the global movement of horses has the potential to quickly spread ML-resistant parasite isolates around the world. The equine industry is strongly encouraged to routinely monitor anthelmintic efficacy, so occurrence of ML resistant cyathostomins can be detected and appropriate interventions implemented as early as possible.”


  • “Reduced macrocyclic lactone efficacy in yearlings imported from Ireland.
  • “Ivermectin efficacy was retested twice and was reduced on all occasions.
  • “100% strongyle fecal egg count reduction in US-bred yearlings on all occasions.
  • “A combination of moxidectin, oxibendazole and pyrantel pamoate was 100% effective.
  • “Global movement of horses likely to spread macrocyclic lactone resistance quickly.”

You can read this open access article through ScienceDirect.com.

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