Haemonchus contortus resistance to monepantel in sheep.

In a sheep farm in the Netherlands with a suspected Haemonchus contortus resistance to monepantel (Zolvix®, Novartis Animal Health), a fecal egg count reduction test was carried out in two groups of lambs, according to the method of the World Association for the Advancement of Veterinary Parasitology. Group 1 was the untreated control group, and group 2 was treated with monepantel at the manufacturer's recommended dose rate. Efficacy of treatment with monepantel was 0%. Larval identification of pre- and post-treatment coprocultures revealed 100% H. contortus larvae. On this farm, after a perceived reduction in efficacy of ivermectin and doramectin, the sheep farmer started using monepantel 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, and confirmed three weeks later. Although the frequency of monepantel treatments on this farm was relatively high with treatments on thirteen separate occasions in two years time, possibly establishing favorable conditions for a competitive advantage for resistant H. contortus, it is remarkable that resistance to monepantel was established in such a very short period. This study confirms, to the best of our knowledge, the first case of H. contortus resistance to monepantel occurring in the field.