The aim of this project was to improve control of gastro-intestinal nematodes (GIN) in sheep by understanding barriers to sustainable use of medicines used to treat GIN infections. Through engagement with farmers, we identified potential interventions and developed a mathematical model that tests the likely effect of proposed control strategies on parasite transmission and lamb live weight gain.
GIN are common and highly pathogenic in sheep, especially first season grazing lambs. They are traditionally controlled using routine veterinary medicines or anthelmintics. In recent years, populations of GIN, resistant to all three major anthelmintic classes and multi-drug resistant parasites have been reported on many farms in the UK.
To reduce the rate of development of resistance, advice to farmers has changed and is now focussed on using anthelmintics only when indicated e.g. by high faecal egg counts or failure to meet weight gain targets and using alternative strategies such as pasture management to reduce exposure. Many farmers do not engage with these messages. The aim of this study was to identify the barriers behind this lack of engagement and, by developing a mathematical model, to demonstrate the impact of sustainable parasite control measures, on a typical farm.
An online questionnaire was used to provide a snapshot of management and worming practices on a cross section of sheep farms across the UK. Focus groups on sustainable parasite control were held across the UK, either in person or online between December and June 2023. Nine meetings were held with farmers, including some ‘seldom seen’ farmers, i.e. people who had no history of engaging with their Veterinary Practice’s knowledge exchange programmes. In addition, one focus group was held with farm animal veterinarians and two with registered animal medicines advisors. Discussions were recorded and transcribed for thematic analysis.
A mathematical model of a ‘typical’ sheep farm was developed, informed by data collected through the online questionnaire and evaluated using data from previous studies (Kenyon et al 2013). By adopting an individual-based modelling framework that links environmentally driven pasture contamination with parasite burdens within individual hosts we were able to consider impacts of management decisions and interventions at the level of the farm whilst retaining detail on parasite burden at the level of individual sheep. Four proposed treatment scenarios were evaluated: (i) Neo-suppressive treatment (NST): 4-weekly treatments from weaning to sale; (ii) Strategic prophylactic treatment (SPT): treatment at weaning and 6 weeks later; (iii) Treatment minus 10 (T-10): 4-weekly treatments that exclude 10% randomly selected animals from the heaviest half of the flock; (iv) Targeted treatment (TT): treatment of the whole group based on monthly composite faecal egg counts from ten lambs.
The farmer focus groups identified four main categories of barriers to change; not seeing any need due to the perceived success of current strategies, a lack of information and support on how to change, a lack of faith in proposed new methods, and practical and logistical issues. There were also positive findings, notably an appetite for information and knowledge, an awareness of anthelmintic resistance and its consequences, growing uptake of faecal egg counts which were proving to be beneficial to their businesses. Our mathematical model can be used to help address the first three of the issues described above by exploring the potential efficacy of a range of GIN control strategies. The model predicts that approaches that reduce the number of treatments can provide similar body weights at a flock level while reducing the number of treatments administered. The NST approach, which administers the most anthelmintic treatments predicts the greatest body weights across the flock in the absence of resistance; however, the efficacy of this treatment regime decreases notably under increasing anthelmintic resistance. Under the assumption that the alternative treatment scenarios would at least slow the development of resistance, we see that over time we would expect better performance in terms of body weight under the alternative treatment scenarios e.g. comparing NST under 40% resistance to T-10 under 20% resistance.
Conclusions: Amongst farmers there was a deep sense of anxiety around leaving lambs untreated and changing their normal worming practices for fear it might compromise the health and productivity of their animals, which could impact their business. To address these barriers, we present a mathematical model developed to explore the potential impacts of proposed treatment regimes, showing that any short-term losses due to adopting some of these novel approaches may be offset by slowing the development of anthelmintic resistance. The model has been built with flexibility in mind following consultation with stakeholders and so can be readily adapted and employed to explore a wide range of novel control scenarios such as weight-based targeting of treatment or rotational/mob grazing. Such model outputs could be used to identify promising control strategies for trial on farm and, together with bespoke educational programmes, to convince farmers to focus their efforts onto more sustainable control of GINs.
Contact details:
Dr Diana Williams
Professor Infection Biology & Microbiomes
Liverpool University