Worm Egg Counts in Sheep: Welfare-Based Parasite Management

Worm Egg Counting and Sheep Welfare

Worm egg counting (WEC) — measuring the number of nematode eggs per gram (EPG) of faeces — is the cornerstone of targeted selective treatment (TST) for sheep nematode control. This approach replaces blanket anthelmintic treatment, reducing drug resistance while maintaining welfare.

Why TST Matters for Welfare

Gastrointestinal nematodes, particularly Teladorsagia circumcincta and Haemonchus contortus, cause significant welfare harm through anaemia, hypoproteinaemia, diarrhoea (scouring), and wasting. The challenge is treating animals with welfare-threatening burdens while avoiding unnecessary treatment of those with tolerable levels, thereby preserving anthelmintic efficacy for future generations.

FAMACHA System

Developed in South Africa for Haemonchus control, the FAMACHA system matches conjunctival colour to a reference card to identify anaemic animals for treatment. Combined with WEC, it provides practical field guidance for targeted treatment of the most affected individuals.

Worm Egg Count Thresholds

• Lambs: Generally treat above 250-500 EPG depending on system and parasite species; consider clinical signs alongside count.
• Ewes: Periparturient rise requires particular attention; ewes may need treatment in late pregnancy despite lower thresholds.
• Adults: Resistant individuals tolerate higher burdens without welfare compromise — identifying and selecting these animals improves flock resilience.

Anthelmintic Resistance Monitoring

Drench efficacy testing using faecal egg count reduction tests (FECRT) every 2-3 years identifies resistance to specific drug groups. Using an ineffective drug causes welfare harm by failing to control parasites despite treatment.

Integrated Control

• Pasture management: Avoiding high-risk paddocks at high-risk times; rest and rotation strategies.
• Refugia: Leaving some untreated animals on pasture maintains drug-susceptible parasite populations.
• Breeding: Selecting rams with low WEC and high FECR improves genetic resistance in the flock.
• Nutrition: Well-nourished ewes and lambs mount better immune responses and tolerate worm burdens better.

Key Takeaways

Worm egg counting enables evidence-based, welfare-centred anthelmintic use in sheep. By treating animals based on their actual parasite burden rather than calendar-based routines, farmers can maintain effective treatments for animals that genuinely need them while reducing the crisis of anthelmintic resistance.