Benzimidazole resistance in Trichostrongylus axei in sheep: long-term monitoring of affected sheep and genotypic evaluation of the parasite

This is the first report of benzimidazole (BZ) resistance in the nematode Trichostrongylus axei in sheep. Trichostrongylus axei infects several species of herbivores including sheep, cattle and horses, and the emergence of anthelmintic resistance could lead to significant problems in its control. Benzimidazole resistance in two sheep flocks in central France was detected by post-treatment worm counts. The sequencing of a central region of the isotype 1 beta-tubulin gene from adult T. axei recovered post-mortem revealed only one, non-synonymous single nucleotide polymorphism at position 200 (Phe200Tyr), which had already been reported for other nematodes. Seven years after BZ treatment ceased, T. axei helminths present were still resistant to BZ suggesting these parasites do not revert to susceptibility to this anthelmintic, even when the selection pressure had been removed for many years. The findings also highlight major changes in the make-up of the nematode burden in sheep flocks that accompanies the emergence of BZ resistance.