Internal parasites — particularly gastrointestinal roundworms (nematodes) and liver fluke (Fasciola hepatica) — cause significant welfare losses in sheep globally. Effective parasite management requires integration of anthelmintic treatment with pasture management and monitoring to control welfare impacts while managing the critical threat of anthelmintic resistance.
Welfare Impact of Parasitism
Heavy roundworm burdens cause protein malnutrition through gut damage and reduced protein digestion, leading to scouring, weight loss, submandibular oedema (bottle jaw), and anaemia in severe cases. Barber's pole worm (Haemonchus contortus) is particularly devastating, causing life-threatening anaemia through blood-sucking feeding. Affected lambs show lethargy, pale mucous membranes, and rapid deterioration without treatment. The welfare cost of clinical parasitism is substantial — severe worm burdens cause considerable suffering before death.
Liver fluke causes acute fasciolosis in lambs following high-level ingestion during wet autumns, causing rapid death from liver destruction. Chronic fasciolosis — slower accumulation of adult fluke — causes progressive liver damage, weight loss, reduced immunity, and impaired welfare over months.
Sustainable Parasite Management
Anthelmintic resistance — the reduced effectiveness of anthelmintic drugs against worm populations that have evolved resistance — is a growing welfare threat. All major anthelmintic drug classes face resistance in some parasite populations, particularly in the UK and Australia. Poorly targeted, frequent anthelmintic use accelerates resistance development, potentially compromising future ability to treat clinical parasitism with the welfare consequences that implies.
Sustainable parasite management integrates targeted selective treatment (treating only animals with sufficient worm burden to require treatment, using Faecal Egg Counts or FAMACHA scoring for anaemia), strategic herd protection treatments only at high-risk times, and pasture management to reduce infective larval challenge. These approaches maintain treatment efficacy for future use while controlling current parasitism welfare impacts.
Refugia-Based Approaches
Maintaining refugia — a proportion of worm populations not exposed to anthelmintics — slows resistance development by diluting resistant alleles with susceptible ones. Leaving the fittest sheep in a flock untreated provides refugia from which susceptible worms can repopulate pasture. This approach requires monitoring of individual animal condition to ensure untreated animals maintain adequate welfare — animals in poor condition should be treated regardless of refugia strategy.