Resistance to the main classes of anthelmintic (worm-control) drugs is widespread in sheep gastrointestinal nematodes globally, threatening the ability to control parasites that cause significant welfare harm.
Anthelmintic resistance creates a growing welfare crisis because it reduces the effectiveness of the primary tool for controlling parasites that cause severe suffering. Haemonchus contortus causes haemorrhagic anaemia — affected sheep become weak, breathless, and die from blood loss without effective treatment. Where resistance is high, treatment failures mean animals continue suffering despite intervention. The challenge is that the behaviour driving resistance (frequent whole-flock treatment) emerged from genuine welfare concern — preventing worm burden. Paradoxical as it seems, treating less frequently (targeted selective treatment based on individual need) is now the welfare-preserving approach, because it maintains drug efficacy for the animals that most need it.