Sheep Internal Parasites and Worm Control

Gastrointestinal nematodes are the greatest welfare and production threat to sheep worldwide, causing suffering and death when poorly managed.

Key Facts

Haemonchus contortus (barber's pole worm) can kill sheep within days through blood loss
Anthelmintic resistance is widespread globally, with triple resistance common on many farms
FAMACHA scoring assesses anaemia in individual sheep to target treatment only where needed
Strategic refugia-based management slows resistance development by leaving some worms untreated
Pasture management, nutrition, and genetics all reduce worm burden without relying solely on drugs
Lambs and ewes around lambing are most vulnerable to worm challenge

Welfare Considerations

Worm burdens cause significant welfare harms including anaemia, bottle jaw (fluid accumulation), diarrhoea, weight loss, and death. Sheep in pain from worm damage show reduced feeding, social withdrawal, and lethargy. The rise of anthelmintic resistance means many farms are losing the ability to treat effectively, threatening future welfare at scale. Integrated parasite management - combining targeted selective treatment, pasture rotation, nutrition, and breeding for resistance - offers the only sustainable path forward for both welfare and production.

What You Can Do

Learn FAMACHA scoring to target worm treatment to individual animals in need
Work with a veterinarian to develop an integrated parasite management plan
Use refugia-based dosing strategies to slow anthelmintic resistance
Select breeding animals with genetic resistance to worms (e.g. Barbados Blackbelly crosses)
Rotate pastures and avoid overgrazing to reduce infective larvae on pasture
Monitor flock condition scores and faecal egg counts regularly