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Monepantel Resistance After Two Years

A sheep farm in the Netherlands developed resistance to monepantel after using it for just two years. Monepantel (tradename = Zolvix®) is from a new class of anthelmintics called amino acetonitrile derivatives. It was first introduced in 2009 in New Zealand. It was introduced to the Netherlands in 2011. It is not available in the US.

After a perceived lack of effectiveness in ivermectin and doramectin, the farm started using monepanel in July 2012, and since then, monepantel was used as the sole anthelmintic. Breeding sheep were treated twice each year in 2013 and 2014, and lambs two times in 2012, four times in 2013, and three times in 2014, before monepantel resistance was suspected.

Two groups of twenty lambs were randomly selected from the flock of 80 Swifter lambs to carry out fecal egg count reductions. The calculated efficiency of monepantel was determined to be 0. Pre- and post-treatments were determined to be 100% Haemonchus contortus in the treated group and 99 and 100% in the control group.

It is unknown why resistance of H. contortus to monepantel was established in such a short time. A study in New Zealand showed a similar outcome in a goat herd in which monepantel had been used on 17 different occasions in two years. However, both findings are in contrast with other reports that say resistance to monepantel in H. contortus is not likely to develop within a few generation cycles. Regardless, there is a need to restrict the use of anthelmintics.

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