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.

在苯并咪唑类和四氢嘧啶类中，马细胞抗虫药的抗蠕虫药性既普遍又非常普遍；然而，对大环内酯 (ML) 药物耐药的报道很少且零星。本研究报告了 2019 年从爱尔兰进口到美国的一组纯种一岁鸽出现明显 ML 抗性的案例。 2020 年 2 月施用伊维菌素后粪便卵数减少 (FECR) 表明美国繁殖的一岁鸽减少了 100%，但减少了 93.5% 、70.5% 和 74.5% 的三组进口一岁鸽减少。然后用伊维菌素重新治疗前两组，分别产生 33.8% 和 23.5% 的 FECR。然后将这两组的马随机分配到两种可能的治疗中；莫昔克丁或莫昔克丁、奥苯达唑和吡喃苯甲酸酯的三联组合。莫昔克丁治疗组的 FECR 分别为 90.2%、57.3% 和 50.0%，而三联疗法在所有治疗组中的 FECR 为 100%。随后，伊维菌素的疗效于 2020 年 6 月重新评估，三个进口组的 FECR 分别为 99.8%、87.7% 和 62.0%。美国繁殖的一岁鸽的 FECR 都保持在 99-100% 的范围内。这是第一项明确证明细胞分裂素对 ML 耐药并通过重新评估确认怀疑的研究。这些数据表明抗 ML 的 cyathostomins 是从爱尔兰进口的，并有助于说明马的全球运动有可能将抗 ML 的寄生虫分离株迅速传播到世界各地。强烈鼓励马业定期监测驱虫效果，