Anthelmintic resistance in gastrointestinal nematodes of sheep is one of the most significant emerging animal welfare threats in the UK livestock sector.
Sheep with untreatable worm burdens deteriorate progressively — losing condition, developing pale membranes and eventually dying. When all anthelmintic classes fail, the only welfare option is euthanasia of heavily affected animals. Resistance is partly a product of overuse — treating all animals routinely rather than strategically. Veterinary guidance on targeted treatment and refugia is essential to preserve drug efficacy and protect future sheep welfare.