Internal Parasite Management in Sheep: Welfare and Resistance

Internal parasites are the most significant health and welfare challenge in sheep farming globally. Sustainable management balances parasite control with preventing anthelmintic resistance.

Key Facts

Haemonchus contortus (barber's pole worm) is the most pathogenic sheep parasite in warm climates
Heavy worm burdens cause anemia, weight loss, bottle jaw (submandibular edema), and death
Anthelmintic resistance is widespread globally, threatening the effectiveness of chemical control
FAMACHA scoring (conjunctival color assessment) allows targeted selective treatment of anemic animals
Refugia-based management — leaving some untreated animals to dilute resistant worms — slows resistance
Genetic selection for parasite resistance is a growing tool in sustainable parasite management

Welfare Considerations

Internal parasite burden causes chronic suffering in sheep — progressive anemia, weight loss, and protein loss cause weakness, poor growth, and eventually death. The welfare impact disproportionately affects young lambs and pregnant ewes. Treatment with anthelmintics rapidly relieves suffering when parasites are susceptible, but resistance is making this increasingly difficult. Refugia-based approaches reduce resistance development but require leaving some animals with parasite burdens. Flock monitoring through targeted selective treatment identifies most affected individuals for priority treatment. Pasture management, including spelling paddocks, reduces larval contamination and welfare burden.

What You Can Do

Learn FAMACHA scoring to identify anemic sheep requiring treatment
Implement targeted selective treatment rather than blanket whole-flock dosing
Rotate anthelmintic classes to delay resistance development
Use refugia-based management strategies to maintain drug efficacy
Consult a veterinarian for flock-specific parasite management planning