Living with parasites: exploiting tolerance of infection to reduce the impact of gastrointestinal nematodes in sheep

Gastrointestinal nematodes are a major challenge to the UK livestock industry, and particularly to sheep farming, as a result of their impact on many aspects of animal performance (Mavrot et al, 2015). A recent estimate suggests that helminth infection costs the UK meat sheep industry £41 million annually, £15 million of which is because of the effects on production and £26 million of which is because of treatments with anthelminthic drugs (Charlier et al, 2020). Gastrointestinal nematodes also have an environmental impact, since lambs infected with gastrointestinal nematodes have higher greenhouse gas emissions than uninfected lambs (Kenyon et al, 2013; Fox et al, 2018). These impacts on economic and environmental sustainability have largely been managed with anthelmintic drugs for the past several decades, but reports of resistance to these drugs in worm populations are becoming more widespread and their efficacy is declining (Sargison et al, 2007; Rose Vineer et al, 2020). Anthelmintic residues excreted by sheep also have downstream negative effects on invertebrates, such as pollinators and dung beetles, which are essential to ecosystem health (Vokřál et al, 2023). A general reduction in anthelmintic use is therefore required, to reduce these environmental impacts and maintain refugia where susceptible gastrointestinal nematodes reside (Hodgkinson et al, 2019), increasing the lifespan of anthelmintics and ensuring they can continue to be used as part of integrated parasite management. While there is a need to reduce anthelmintic use, the publication of the Animal Health and Welfare Pathway for England (Department for Enivronment, Food and Rural Affairs, 2024) highlighted internal parasites as a priority for sheep farmers, and it is almost needless to say that their impact must be controlled. It is therefore essential to develop sustainable means of controlling gastrointestinal nematodes that do not rely heavily on anthelmintics (Jackson et al, 2009; Abbott et al, 2012; Forbes, 2023). Developing sustainable parasite control strategies is now, more than ever, an active area of research, although it is not the purpose of this article to discuss them all. Nevertheless, vaccines, grazing management and nutrition are all likely to play a role. Selective breeding to ensure that gastrointestinal nematodes have as little impact on sheep as possible has been an important tool for decades, yet has not been as widely exploited as it could be. Ultimately, breeding acts by altering the biology of the sheep, and as such it is necessary to understand the mechanisms by which sheep naturally fight infection.