Resistance to anthelmintic (wormer) drugs in sheep roundworms is widespread and increasing in UK flocks. Ineffective treatment leaves worm burdens that cause weight loss, anaemia, and death in untreated sheep, creating a serious welfare crisis.
Sheep with heavy worm burdens experience progressive anaemia, protein loss, weakness, and death if untreated. Animals that receive wormers that are no longer effective receive no welfare benefit while the treatment expense is wasted. Young stock in their first grazing season are most vulnerable to acute parasitic disease. The welfare consequence of anthelmintic resistance has been emerging for decades and is now at a critical point requiring fundamental management change.