Background: The animal welfare and production consequences of gastrointestinal nematode infections, namely Nematodirus battus, on sheep are compelling. The control of gastrointestinal infections has been heavily reliant on the administration of anthelmintics since their introduction into mainstream use in 1960. However, their frequent and often excessive use has resulted in anthelmintic resistance being reported extensively in several gastrointestinal nematode species. The prevalence of anthelmintic resistance as well as the increase in the number of cases of multiple-drug resistance in nematode populations now poses substantial hindrance on the viability of sheep production systems. However, despite the threat N. battus poses to the sustainability of sheep farming, there are very few publications on this nematode.

Aim: The aim of this study is to investigate the prevalence and epidemiology of N. battus in sheep across Ireland and the United Kingdom between the years 2011 and 2021. Relevant studies were reviewed from the literature to identify N. battus resistance to the benzimidazole and macrocyclic lactone anthelmintic groups and to assess anthelmintic efficacy using in vivo and in vitro methods. Preliminary data was also gathered from the distribution of a survey to identify the perceived efficacy of anthelmintic treatments and the management factors that may be contributing to anthelmintic resistance. The dissemination of the survey was primarily shared via social media platforms.

Results: The results obtained from the literature highlighted variation in the epidemiology of N. battus. Although the historically proposed lifecycle which occurs during the spring period is prevalent across Ireland and the UK, clinical nematodirosis in autumn is becoming more frequent. Resistance to a number of the broad-spectrum anthelmintics was identified in 60% (n=10) of the publications from the UK, with the benzimidazoles being reported as the least effective. The assessment of anthelmintic efficacy highlighted that there is a lack of effectiveness and sensitivity in the in vivo and in vitro methods that are readily applied in field conditions. From the 134 responses obtained from the survey, a number of concerns were raised with management practices. It was concluded that many flock owners have become complacent with incorrect dosing and a lack of calibrating of dosing equipment, as seen in 69% and 63% respectively. In addition, the prevalence of N. battus is becoming more evident on farms across Ireland and the UK, with 62% of farms having an outbreak of N. battus infection on the farm within the last five years. Although many of these respondents identified clinical signs in lambs during the predicted spring period, a number of respondents (6.7%) proposed that infection occurred during the autumn period, which concurred with previous findings.

Conclusion: The extensive use of anthelmintics on sheep production systems has led to the rapid progression of anthelmintic resistance across Ireland and the UK, particularly to the broad-spectrum anthelmintics. Without the use of effective and reliable testing methods and the implementation of stringent monitoring strategies, the fight against anthelmintic resistance will be a losing battle. It is critical that there is more emphasis on the need for improving worming strategies on sheep farms. Without effective worm control and a resistance management flock plan, animal health and anthelmintic efficacy will continue to deteriorate.

Keywords: Gastrointestinal nematodes, anthelmintic resistance, Nematodirus battus, multiple-drug resistance, sheep, sustainability, prevalence, epidemiology, Ireland, United Kingdom, in vivo, in vitro.