Gastrointestinal parasitism is the most economically and welfare significant health challenge in sheep farming worldwide. Science-based worm control integrates sustainable anthelmintic use with genetic, nutritional, and management strategies to maintain sheep welfare while preserving the long-term efficacy of available treatments.
Multiple nematode species parasitise sheep, with significant variation by region and season. Haemonchus contortus (barber's pole worm) is dominant in warm, humid climates—a blood-sucking parasite capable of killing sheep rapidly. Teladorsagia circumcincta and Trichostrongylus species are more significant in temperate regions, causing chronic production loss rather than acute mortality. Nematodirus battus causes an acute disease in lambs that can cause sudden high mortality in spring.
Resistance to benzimidazoles (white drenches), levamisole (yellow drenches), and macrocyclic lactones (clear drenches) is widespread globally. Multi-drug resistance (resistance to multiple classes simultaneously) is increasingly common. Several countries have reported resistance to newer compounds (monepantel, derquantel). The development of anthelmintic resistance is the most significant threat to sustainable sheep parasite control and, consequently, to long-term sheep welfare.
The fundamental principle of sustainable worm control is maintaining a population of susceptible worms 'in refugia'—worms not exposed to anthelmintic selection pressure. Treating all animals simultaneously eliminates refugia and accelerates resistance selection. Targeted selective treatment (TST)—treating only animals that need treatment based on welfare or production criteria—leaves refugia intact. This approach requires robust assessment tools (FAMACHA, body condition, faecal egg count) to identify individuals requiring treatment.
Significant variation in genetic resistance to parasites exists within and between breeds. Estimated Breeding Values (EBVs) for Faecal Egg Count (FEC) enable selection of rams with better parasite resistance, gradually improving flock resilience. Breeds with better natural resistance include Merino variants, some Texel-crosses, and native breeds adapted to high-challenge environments. Genetic selection for resistance is a long-term, sustainable welfare improvement that reduces dependence on anthelmintics.
Adequate protein nutrition is critical for maintaining immune responses against GI parasites. Protein-deficient sheep are significantly more susceptible to worm burdens and less able to mount protective immunity. Periparturient ewes—in negative protein balance—show a characteristic 'periparturient rise' in FEC as immunity is suppressed. Targeted protein supplementation of ewes around lambing maintains immune function and reduces the worm challenge to lambs.
Faecal egg counts (FECs) quantify worm burden and guide treatment decisions, assess treatment efficacy (Faecal Egg Count Reduction Test—FECRT), and monitor refugia (untreated groups). Regular FECRT testing is essential for detecting resistance early. Decision support tools (e.g., SCOPS guidelines in the UK, SCOOPS, SMALL RUMINANT ADVISOR) integrate FEC data with clinical assessment, nutritional status, and epidemiological risk to generate evidence-based treatment recommendations.