Worm Control in Sheep: Deep Welfare and Resistance Guide
The Worm Burden Problem
Gastrointestinal nematodes — particularly Haemonchus contortus (barber's pole worm) and Teladorsagia circumcincta — are among the most welfare-significant parasites of sheep globally. They cause: blood loss (Haemonchus feeds on blood, causing anaemia and hypoproteinaemia — bottle jaw, pale mucous membranes, progressive weakness, and death in severe cases); reduced feed conversion and growth; reduced wool production; and reproductive failure. The UK climate is highly conducive to roundworm development.
Anthelmintic Resistance
Anthelmintic resistance is the most critical welfare and sustainability challenge in sheep parasite control. Resistance to all three major drug classes (benzimidazoles — BZ; levamisole — LV; macrocyclic lactones — ML) is widespread in UK sheep flocks. Multi-drug resistant Teladorsagia populations are common; Haemonchus with broad resistance is increasingly documented. Over-reliance on blanket preventive treatment has driven resistance, threatening the long-term efficacy of available drugs. Resistance monitoring (DrenchCheck, refugia management) is essential.
FACS/FAMACHA and Targeted Treatment
Targeted selective treatment (TST) reduces drug use and slows resistance development. FAMACHA: conjunctival colour scoring (1-5 scale based on anaemia — pale mucous membranes indicate Haemonchus burden); scores 3-5 trigger treatment. Five-point check (FACS): five indicators of worm burden (FAMACHA score, body condition, dags/soiling, dag score, and bottle jaw/submandibular oedema) identify animals most in need of treatment. Both approaches reduce overall drug use while targeting welfare of most-affected individuals.
Faecal Egg Count (FEC) Monitoring
Faecal egg counts quantify worm eggs per gram of faeces, providing a measure of worm burden at individual or group level. FEC monitoring before treatment (to confirm need), after treatment (to assess efficacy — Drench Efficiency Test), and throughout the grazing season (to monitor uptake) guides rational treatment. FEC reduction test (FECRT) detects anthelmintic resistance. Regular FEC monitoring as part of flock health planning replaces arbitrary treatment calendars.
Pasture Management and Non-Chemical Control
Sustainable worm control integrates pasture management: rotating between pastures (resting pasture allows larval die-off over time); alternating sheep with cattle (cattle are not susceptible to sheep roundworms and act as 'vacuum cleaners' for larvae); maintaining refugia (leaving untreated animals on pasture maintains susceptible worm populations, diluting resistance genes); and selecting breeds with genetic resistance (some breeds, e.g., Texels, show greater natural resistance). Breeding programmes selecting for worm resistance are available in some countries.