Joining the Resistance Against Preventable Antimicrobial Use: Epidemiological Insights into the Determinants of Antimicrobial Consumption in Broiler, Pig and Veal Calf Farms in the Netherlands

Abstract

OUTLINE OF THE THESIS This PhD thesis includes eight chapters: Chapter 1 (“General Introduction”), the present chapter, introduces the reader to the topic of AMU and AMR, the objectives and the rationale behind this thesis. Chapter 2 (“Farm-level risk factors for antimicrobial use in pigs, broilers and veal calves: a systematic review”) presents, by means of a literature review, the state-of-the art studies on risk factors for AMU in broilers, weaners, and veal calves. Chapter 3 (“Factors associated with antimicrobial use in pig and veal calf farms in the Netherlands: A multi-method longitudinal data analysis”) presents the results of an epidemiological analysis examining the factors associated with AMU in pig and veal calf farms in the Netherlands. The analysis utilizes data from previous research based on a longitudinal study design and compares multiple modelling approaches. Chapter 4 (“Biosecurity and antimicrobial use in broiler farms across nine European countries: toward identifying farm-specific options for reducing antimicrobial usage”) presents the results of an epidemiological (cross-sectional) study of the factors associated with AMU across nine European countries using data from the EFFORT project. Additionally, it details the development of an algorithm that generates farm-specific AMU reduction plans, incorporating both biological and economic considerations. Chapter 5 (“Diseases associated with antimicrobial use in pig farms and risk factors thereof: a cross-sectional study in the Netherlands”) and Chapter 6 (“Risk factors for antimicrobial use in pig farms: a cross-sectional study in the Netherlands”) present the analyses of newly collected (cross-sectional) data from Dutch pig farms for the year 2019. The study investigates not only farm characteristics associated with AMU, but also farm characteristics associated with the occurrence of specific pig diseases. Furthermore, it identifies which diseases are primary drivers of AMU in pig farms. Chapter 7 (“Risk factors for antimicrobial usage and diseases in Dutch veal calf farms: a cross-sectional study”) seeks to answer the same questions as in Chapters 4 and 5, but with another methodology suited to the needs of newly collected data from rosé veal calf starter farms in the Netherlands in 2021. Lastly, Chapter 8 (“General Discussion”) discusses the main findings from all the studies and synthesizes the observed associations into a network diagram using the newly collected data on pigs (Chapter 5 and 6) and calves (Chapter 7), which also include disease etiology for AMU treatments. Furthermore, a critical appraisal of the entire body of work is presented, highlighting commonalties in results and methodological challenges across chapters. Finally, concluding remarks outline potential future developments for the AMUlet tool and propose directions for advancing epidemiological analyses of this kind.