Gastrointestinal nematode (GIN) infections are a common constraint isture-based dairy herds and cause a decrease in animal health, productivity and farm profitability. Current control practices to prevent production losses of GIN infections in livestock depend largely on the use of anthelmintic drugs. However, due to the intensive use of these drugs, the industry is increasingly confronted with anthelmintic drug-resistant nematode populations. This emphasises the need for sustainable control approaches that minimise the selection pressure for anthelmintic resistance (AR). The uptake of diagnostic methods for sustainable worm control could enable more informed treatments and reduce excessive anthelmintic use. However, farmers have been slow in adopting guidelines for sustainable control. Accordingly, in order to successfully implement such control strategies and change the behaviour of farmers, their current perceptions and behaviours need to be comprehended and translated into effective communication strategies. Chapter 1 reviewed the available literature on GIN control in cattle and concomitant threats for the dairy industry. More specifically, it focused on identifying the factors responsible for the limited uptake of current advises and possible future adoption of sustainable methods. This review demonstrates a substantial gap in literature for scientific evidence concerning farmers’ behaviour (intention) in GIN control. Many reports are based on opinions and personal experiences, or are simply based on ‘yes-orno’ questions with immediate relation to farmers’ current or future GIN control, which results in limited insights in farmers’ behaviour and unsubstantiated hypotheses. This stresses the need for more structured and scientific behavioural research, adapted from social veterinary epidemiology, a fairly young discipline with contributions from different fields, such as behavioural psychology and economy. As a response to this emerging need, in Chapter 2 a framework was constructed to identify the socio-psychological factors that influence dairy farmers’ adoption intentions of diagnostic methods before implementing anthelmintic treatments. The framework was based on two grounded models from behavioural and health psychology: the Theory of Planned Behaviour (TPB) and the Health Belief Model (HBM), now commonly used in veterinary social sciences. Data to validate and measure the model were collected through a cross-sectional survey of Flanders’ dairy farmers population (N = 574). In the tested model, adoption intentions (i.e. the proximal determinants of adoption, which captures the motivation to perform this behaviour) were predicted based on attitudes towards anthelminthic drugs, attitudes towards diagnostic methods (i.e. an individual’s positive or negative evaluation of this particular behaviour based on the expected outcomes), subjective norms (i.e. the influence of significant others), behavioural control (i.e. perceived ability to perform this adoption) and perceived risk (i.e. the perceived susceptibility and severity of AR in particular). The factors ‘attitude towards diagnostic methods’ and ‘subjective norms’ had the strongest, positive influence on adoption intention of diagnostic methods. ‘Perceived behavioural control’ had a weak, positive effect on intention. Further, ‘attitude towards the use of anthelmintic drugs’ had a negative effect on adoption intentions, which implicates an effect of current behaviour on future adoption. Moreover, the threat of AR is perceived fairly low, and had no effect on the adoption intentions of diagnostics. This chapter gives a broad, general view of the drivers of sustainable GIN control using diagnostics on dairy farms. However, to be able to provide specific advice, further indepth analyses are necessary to determine farmers’ beliefs and motivations underlying these socio-psychological factors. Moreover, a relatively good intention was measured for the adoption of diagnostics, but low actual usage has been reported, suggesting a gap between intention and behaviour. Therefore, Chapter 3 aimed to dig deeper into the established framework for the beliefs underlying this model, and to identify additional factors impelling this specific behaviour. Data were collected through 22 semistructured interviews with dairy farmers. Results show that the adoption process of diagnostic methods for GIN occurs through three different phases: adoption intention, actual adoption and maintenance. Low infection awareness and low priority (‘top of mind’) of the disease are important barriers for farmers’ positive intentions towards sustainable GIN control. Secondly, different types of motivations influence different sorts of behaviour. Sustainable behaviour such as use of diagnostics will be influenced by moral motives, while management behaviour such as anthelmintic treatment is raised by more economic motives. Thirdly, farmers’ behaviour is guided by two important social norms: the opinion of their veterinarian and their fellow farmers. However, farmers hold an incongruent relationship with both norms throughout the different stages of behaviour: they do not value other farmers’ opinions as a positive reference (intention phase), but they do follow and mimic their behaviour as a group (action phase). The veterinarian is seen as the most important positive reference, but also the responsible actor for GIN control. As such, the farmers do not hold themselves responsible for implementing sustainable control strategies. Finally, not only performing, but also maintaining behaviour is important to fully address the adoption of sustainable worm control. To perform and maintain the adoption on farm, planning could be an important contribution, which could help to surmount other suggested barriers for actual adoption, i.e. habits and responsibility. The insights of Chapter 2 and 3 were used to create and test public service announcements (PSA), which were set up to create awareness of AR and promote sustainable control. In Chapter 4, the aim was to create awareness of anthelmintic resistance and to decrease farmers’ positive attitudes of preventive treatments as a first step towards behaviour change. More specifically, the PSA focused on discouraging this behaviour by using message sidedness in a humorous advertisement. Two-sided argumentation (i.e. both pro and contra argument) and humour as communication strategies are known to generate less negative responses and lead towards improved message acceptance. The effects of the message on sustainable behaviour intentions were measured through a cognitive and affective route of persuasion. The cognitive route is prompted by rational thoughts, while emotions are the drivers of the affective route. Using a 2 (message sidedness: one-sided vs. two-sided) x 2 (humour: humorous vs. non-humorous message framing) between-subjects design (N = 167) the persuasiveness of the advertisement was tested. Results show that a two-sided message without humour evoked more negative cognitive responses (i.e. negative thoughts) than a one-sided message, leading to fewer changes in behavioural intentions. However, a two-sided message resulted in increased sustainable intentions compared to a onesided message when humour was used as a frame in the advertisement. Moreover, the more simplistic PSA with only one argument and no humour presented similar results. Therefore, simple messages towards farmers can be equally effective as more complex strategies (i.e. both two-sided argumentation and humour). Chapter 5 was set up as a ‘call to action’ for sustainable GIN control, the second step towards behaviour change. The chapter investigates how to change farmers’ adoption intentions by using social influence, more specifically, injunctive norms. It focuses on two endorser types (expert vs. peer), considered as distinguished subjective norms, to change farmers’ behavioural intentions. Furthermore, the endorsers’ ability to either discourage (proscriptive message content), or encourage (prescriptive message content) behaviour were established. The effects of endorsers on behavioural intention were measured through three mediators: expertise, trustworthiness and similarity. Using a 2 (endorser type: expert vs. peer) x 2 (content type: proscriptive vs. prescriptive message) between-subjects design (N = 143) the persuasiveness of the advertisement was tested. Results show that an expert endorser (veterinarian) had a direct increased effect on behavioural intention, compared to a peer endorser (farmer). The veterinarian is the most important advisor and the key figure for disease control on farm, while the farmer is perceived as a negative reference, not valued for its opinion. Moreover, the effect was mediated through similarity, which emphasises the power of the in-group for social influence. Finally, the different content types had no effect on behavioural intention, not even when presented by different endorsers. Finally, Chapter 6 assembles the main findings of the socio-epidemiological research performed within this PhD-project, and how these were translated into effective communication strategies. By presenting how the results from qualitative and quantitative studies can be translated into advice and subsequently verified with communication experiments, this chapter contributes to the current knowledge within the field of veterinary parasitology on changing dairy farmers’ behaviours. Firstly, a brief discussion is given on the two main research questions and how these were addressed throughout the PhD project by each separate study. RQ1: What drives the farmer towards the adoption of sustainable GIN control? The first question was addressed with the results from the behavioural research presented in Chapter 2 and 3. This was followed by RQ2: ‘How can we use this newly gained knowledge to encourage farmers to change their current GIN control?’ This second question was tackled with communication experiments presented in Chapter 4 and 5. Furthermore, a general elaboration of the results throughout the whole project is discussed, following the three phases of adoption: intention, action and maintenance. Additionally, the limitations of the project are addressed, along with suggestions for future research on the matter. These limitations are subdivided in three categories, characteristic for this thesis: the object of research, the behavioural methods and theories, and the translation of the results into communication strategies. Lastly, the chapter presents practical and evidence-based guidelines for developing a communication campaign, aimed at raising farmers’ awareness on the importance of a sustainable anthelmintic use.