Re: Ivermectin-resistant Ostertagia circumcincta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics - Reply

Re: Ivermectin-resistant Ostertagia circumcincta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics - Reply
Peer reviewed

Abstract

Correspondence: Re: Ivermectin-resistant Ostertagia circumcinta from sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics. Allan Murphy (this issue; Murphy, 2001) presents an argument, based on our recent article in this journal (Leathwick et al, 2000), designed to convince the reader that moxidectin is superior to abamectin at controlling ivermectin-resistant parasites and should therefore be the macrocyclic lactone anthelmintic of choice in this country. His letter, however, fails to adequately address several important issues. Our original article compared the efficacy of 3 macrocyclic lactone anthelmintics against a field strain of Ostertagia circumcincta which had previously failed an on-farm faecal egg count reduction test using ivermectin. In our study, the efficacies, based on arithmetic mean worm counts, were 41.7, 99.9 and 95.7% for ivermectin, moxidectin and abamectin, respectively. From these values, Mr Murphy calculates theoretical resistance gene frequencies in the worm population, assuming that ivermectin kills only homozygous susceptible worms (a not unreasonable assumption), and shows that moxidectin must have killed not only all the heterozygous resistant, but also most of the homozygous resistant worms. However, he failed to apply the same calculations to abamectin.

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