Ivermectin-resistant Ostertagia circumcincta from sheep in the lower NorthIsland and their susceptibility to other macrocyclic lactone anthelmintics

Aim: To confirm the ivermectin resistance status of a strain of Ostertagia circumcincta which was isolated from a sheep farm in the lower North Island of New Zealand and to assess the susceptibility of this strain to other ma crocyclic-lactone anthelmintics. Methods: Twenty-five lambs housed indoors were each infected with 12,000 L3 larvae of the above parasite strain. Approximately 3 weeks after infection the lambs were allocated to 1 of 4 treatment groups (3 groups of 6, and 1 group of 7 lambs), one of which remained untreated while the others were dr enched orally with ivermectin, moxidectin or abamectin at 0.2 mg/kg livewei ght. Faecal egg counts (FECs) before and after treatment, and post-mortem w orm burdens 10 days after treatment were examined to assess efficacies of e ach anthelmintic. Results: Treatment with ivermectin reduced the mean FEC by only 18% and the mean worm burden by only 42%, whereas moxidectin and abamectin reduced FEC s by > 92% and worm burdens by >95%. Conclusion: These results, together with a similar case described recently from the South Island, confirm the emergence of ivermectin resistance in ne matode parasites of sheep in New Zealand. The superior efficacy of moxidect in and abamectin in this case indicates that, following the emergence of re sistance to ivermectin, some short-term practical use may still be made of these other anthelmintics. However, their continued use will undoubtedly re sult in increased levels of resistance and eventual therapeutic failure of these products also.