Internal parasites — particularly gastrointestinal nematodes (GIN) — are the most significant health and welfare challenge in sheep globally. Anthelmintic resistance, now widespread, has transformed parasite management from routine treatment to integrated, targeted approaches.
The primary GIN species affecting UK sheep are: Teladorsagia circumcincta (abomasal worm, most common), Trichostrongylus spp. (abomasal and intestinal), Haemonchus contortus (barber's pole worm, blood-sucking abomasal parasite of greatest welfare significance, predominantly in southern UK and expanding range with climate change), Nematodirus battus (small intestinal, mainly young lambs), and Cooperia spp. Parasite burdens are highest in young lambs and periparturient ewes (periparturient rise from immune suppression).
Subclinical parasitism causes significant welfare costs even without obvious clinical disease: reduced growth rates, impaired feed conversion, anaemia (particularly with Haemonchus), weight loss, reduced wool production, and impaired immune function. Clinical disease causes more obvious welfare problems: protein-losing enteropathy (bottle jaw — oedema under the jaw), severe anaemia causing breathlessness and weakness, diarrhoea, and death. Haemonchus contortus can cause death within days in heavily burdened lambs through acute blood loss anaemia.
Widespread resistance to all major anthelmintic classes (benzimidazoles, levamisole, macrocyclic lactones) means that routine blanket treatments fail to control parasite burdens on many farms. This has profound welfare implications — farmers relying on ineffective treatments develop false confidence in parasite control while animals continue to suffer and die from uncontrolled burdens. Regular flock resistance testing (Drench Check) is essential for knowing whether treatments are working.
TST — treating only animals above threshold parasitism rather than the whole flock — reduces selection pressure for resistance while maintaining effective treatment for those most at welfare risk. Tools for implementing TST include: FAMACHA scoring (conjunctival pallor as an anaemia proxy, particularly relevant for Haemonchus), body condition score monitoring, flock-level faecal egg counts (FEC) with individual treatment above threshold FEC, and identifying 'refugia' — untreated animals that maintain susceptible parasite genotypes in the population.
Integrated parasite management combines: regular FEC monitoring to guide treatment decisions, refugia-based strategic treatment of high-risk animals only, accurate class identification before treatment (preventing treatment with ineffective drugs), pasture management (rotation to reduce larval challenge), genetic selection for parasite resistance, and nutrition management (adequate protein supports immune responses to parasites). This approach maintains effective anthelmintics for longer while providing adequate welfare protection.
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