Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm.

The research paper discusses a case of imported macrocyclic lactone-resistant cyathostomins in a thoroughbred farm in the US. It highlights how resistance to anthelmintic drugs can be rapidly dispersed due to the global movement of horses.

Study Background

• The study starts by acknowledging the global issue of anthelmintic resistance in cyathostomins. Cyathostomins are a type of parasitic worm that infect equines such as horses.
• The most commonly observed resistance is against benzimidazole and tetrahydropyrimidine types of anthelmintics. However, macrocyclic lactones (ML), another type of anthelmintic, have shown resilience with sparse and sporadic reports of resistance.

Study Findings

• The researchers discuss their investigation into a case of apparent ML resistance in Thoroughbred yearlings that were imported from Ireland to the US in 2019.
• They conducted fecal egg count reduction (FECR) tests after administering ivermectin (an ML drug) and found that while the US-bred yearlings showed a 100% FECR, the imported yearlings showed lower FECRs of 93.5%, 70.5%, and 74.5% in three different groups.
• The two groups showing the least response were treated with ivermectin again and showed 33.8% and 23.5% FECRs.
• After an organized assignment of the poorly responding horses, they were treated with either moxidectin alone or a triple combination of moxidectin, oxibendazole, and pyrantel pamoate.
• The FECRs improved in the groups treated with moxidectin (90.2%, 57.3%, and 50.0%) and the groups treated with the triple combination showed a complete (100%) FECR.
• However, upon reassessment with ivermectin, the FECRs in the imported groups were again inadequate, validating the ML resistance.

Conclusions and Recommendations

• This study is the first to highlight ML resistance in cyathostomins, underscoring the need for continuous monitoring and reassessment.
• The results drive home the point that ML-resistant cyathostomins were imported from Ireland and these could be a source of rapid global dissemination of ML-resistant parasite isolates.
• The authors call for a regular monitoring of anthelmintic efficacy to detect occurrences of ML-resistant cyathostomins and to implement appropriate interventions as early as possible to control the spread.