Reduced efficacy of monepantel against Teladorsagia circumcincta on a New Zealand sheep farm

Aim To evaluate the efficacy of monepantel and a derquantel/abamectin combination against naturally acquired gastrointestinal nematodes in a New Zealand sheep flock.Methods A slaughter study was conducted in a sheep flock located in the North Island of New Zealand, with a history of benzimidazole/levamisole/macrocyclic lactone-resistant Teladorsagia circumcincta, as assessed by faecal egg count reduction test (FECRT), and an indication of monepantel/abamectin-resistant T. circumcincta. Seven lambs per group were treated with either monepantel, a derquantel/abamectin combination, or left untreated. Seven days later lambs were humanely slaughtered, and worm counts were conducted on abomasa and small intestines. FECRT were conducted concurrently using a monepantel/abamectin and an oxfendazole/levamisole/abamectin combination anthelmintic.Results Compared to the untreated controls, monepantel had 85% efficacy against T. circumcincta, which were found in the abomasa of all seven monepantel-treated lambs. Monepantel efficacy against Nematodirus spp. was 95%. Derquantel/abamectin efficacy was >99% against abomasal and small intestinal genera. Based on FECRT, the monepantel/abamectin combination had 96% efficacy against T. circumcincta and 100% against the other nematodes present, while the oxfendazole/levamisole/abamectin combination anthelmintic had efficacies of 82% and 95% against T. circumcincta and Trichostrongylus spp., respectively.Conclusions Reduced efficacy of monepantel against Teladorsagia circumcincta was confirmed using worm counts in a commercial New Zealand sheep flock.Clinical relevance Data from this study, and non-peer-reviewed data from a laboratory network, confirm that monepantel-resistant nematodes are present in New Zealand sheep flocks. This highlights the importance of practices to slow the development of anthelmintic resistance.