Importation of Resistant Small Stronglyes into the US

Whenever possible I like to point out that our deworming products are becoming less and less effective. And that a truly effective parasite control program depends on controlling environmental factors and properly timed dewormings based on a aggressive monitoring system to assess efficacy. This is the basis of our articles on deworming and the importance is demonstrated in this summary. Just to help you read this cyathostomins are small strongyles and the macrocytic lactones are a class of dewormers that include ivermectin and moxidectin.
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Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm
Int J Parasitol Drugs Drug Resist. 2020 Sep 30;14:99-104. doi: 10.1016/j.ijpddr.2020.09.004. Online ahead of print.
Authors
M K Nielsen 1 , M Banahan 2 , R M Kaplan 3
Affiliations

1 M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
2 Godolphin, Jonabell Farm, 3333 Bowman Mill Road, Lexington, KY, USA.
3 Department of Infectious Diseases, University of Georgia, GA, USA.
Abstract
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.