Resistance to moxidectin and abamectin in naturally acquired Ostertagia circumcincta infections in sheep. 
To investigate the occurrence of resistance to macrocyclic lactone (ML) anthelmintics by Ostertagia circumcincta in lambs on a sheep and cattle property in the North Island of New Zealand.  Thirty lambs were randomly allocated to one of five equal-sized groups, consisting of an untreated control and four treatment groups.  The treatments, which were administered at the manufacturer's recommended dose rates, included oral moxidectin, oral abamectin (both at 0.2 mg/kg), an albendazole-levamisole combination, and an albendazole-levamisole-ivermectin combination.  Post mortem worm counts were undertaken 7 days after treatment to determine the efficacy of each anthelmintic.  The albendazole-levamisole and albendazole-levamisole-ivermectin combinations both reduced O. circumcincta burdens to zero whereas for moxidectin and abamectin efficacies of only 72% and 29%, respectively, were recorded.  These results clearly demonstrated the occurrence of resistance to MLs by O. circumcincta.  Although this is not the first occasion where resistance to this anthelmintic family has been detected in this parasite in sheep in New Zealand, it is the first instance that resistance to either moxidectin or abamectin has been reported.