Internal parasites, particularly gastrointestinal nematodes, cause enormous welfare harm in UK sheep populations and require integrated management combining treatment, genetics and pasture rotation.
Internal parasite welfare in sheep is an endemic crisis driven by decades of excessive anthelmintic use that has created resistant parasite populations across UK farms. Heavily parasitized sheep suffer chronic anemia, protein loss, diarrhea and wasting that causes prolonged welfare harm. The development of drug resistance means that some farms have lost effectiveness of all three main drug classes, leaving them with limited treatment options. Sustainable parasite management that preserves remaining drug efficacy through targeted selective treatment, refugia maintenance and pasture management is both a welfare imperative and a practical necessity. Genetic selection for worm resistance is a long-term welfare solution.