Anthelmintic therapy of equine cyathostomin nematodes – larvicidal efficacy, egg reappearance period, and drug resistance

[Display omitted] •Encysted cyathostomin larvae were apparently resistant to fenbendazole (10 mg/kg for 5 days)•The larvicidal efficacy of moxidectin (0.4 mg/kg) was within historic ranges.•A reduced egg reappearance period was associated with immature luminal L4 cyathostomins.•Four cyathostomin species were associated with early egg reappearance following treatment with moxidectin.•Adults of 12 cyathostomin species were found resistant to larvicidal regimens of fenbendazole. Cyathostomins are ubiquitous in grazing horses across the world, and anthelmintic resistance has been reported with increasing levels over past decades. The aims of the present study were (i) to investigate the efficacy against encysted larval stages of moxidectin (0.4 mg/kg) and fenbendazole (10 mg/kg daily for five consecutive days) and compare these regimens at 2 and 5 weeks post-treatment, (ii) to investigate individual cyathostomin species associated with shortened egg reappearance periods, and (iii) to document species exhibiting decreased susceptibility to the evaluated compounds. Thirty-six ponies were allocated to treatment groups with half euthanatized 2 weeks post-treatment, and the remainder necropsied after 5 weeks. Luminal and mucosal worm counts were conducted and strongyle egg counts were determined at weekly intervals. At 2 weeks, mean reductions of early L3s were 50.4% and 73.8% for fenbendazole and moxidectin, respectively. At 5 weeks, the respective efficacies were 51.3% and 71.8%. Two week efficacies against late L3s and L4s (LL3s/L4s) were 70.8% and 74.6% for fenbendazole and moxidectin, respectively, whereas very low numbers were found in all three groups at 5 weeks. None of the mucosal counts were significantly different between treatment groups. Fenbendazole and moxidectin reduced luminal worm counts by 93.2% and 98.3% at 2 weeks following administration, with moxidectin group adult counts being significantly lower than the other two groups (P