Internal parasites — particularly Haemonchus contortus — are a leading welfare and economic problem in sheep farming, with growing anthelmintic resistance threatening control options.
Internal parasite burdens cause significant chronic suffering in sheep: anaemia reduces oxygen delivery, causing weakness and fatigue; protein loss leads to fluid accumulation and muscle wasting; heavily parasitised lambs fail to grow and may die. The emergence of anthelmintic resistance means that the drugs previously relied upon are losing efficacy, leaving farmers with fewer tools to control infections. This creates a genuine welfare crisis: without effective anthelmintics, parasite burdens will increase and welfare will deteriorate unless alternative strategies are adopted. Integrated parasite management — combining targeted selective treatment, refugia preservation, pasture management, and selection for resistant breeds — is essential to protect both animal welfare and long-term drug efficacy.