Anthelmintic Treatment of Sheep and the Role of Parasites Refugia in a Local Context

SIMPLE SUMMARY: The control of gastrointestinal parasites in sheep is often based on the integrated use of anthelmintics and pasture management. As problems with resistance to anthelmintics become more common, it is important to consider the timing of treatment. This is because one way to delay selection for anthelmintic resistance is to take into account that there are susceptible parasites in refugia that are not selected for treatment. In Sweden, stabled ewes are sometimes treated before being let out to pasture with their lambs in the spring. It is thought that this may increase selection for resistance. However, unlike in many other parts of the world, in Sweden, treatment is usually motivated by large numbers of worm eggs detected in faecal samples. In this review, we discuss whether the Swedish model is a risky strategy. In doing so, we consider the experience from several studies conducted in Sweden and abroad. We conclude that the evidence is inconclusive. Moreover, there are gaps in our knowledge, not least regarding the genetic background of resistance formation and how this is influenced by refugia. Further fundamental work is therefore needed on this topic. ABSTRACT: Gastrointestinal nematodes in grazing livestock are ubiquitous and can cause severe damage, leading to substantial losses in agricultural yields. It is undeniable that the integrated use of anthelmintics is often an essential component of successful intensive livestock management. However, anthelmintic resistance has been a major challenge for several decades, especially in pasture-based lamb production. Measures are therefore needed to reduce the risk and prevent further spread. In many countries with more extensive lamb production and pronounced resistance problems than in Sweden, the importance of keeping parasites in refugia is emphasised. To ensure that treatment is necessary, the Swedish model is based on deworming certain groups of ewes based on the parasitological results of a faecal examination and then releasing them with their lambs to safe pastures. This is intended to reduce the risk of infection, which ultimately reduces the number of subsequent treatments. Whether this preventive strategy in turn means an increased risk of resistance is debatable. In this review, we explain the importance of parasites in refugia and how they can help delay the development of resistance to anthelmintics. We also discuss how likely it is that our model contributes to an increase in resistance risk and whether there is reason to question whether it is a sustainable strategy in the long term.