An investigation of factors influencing antimicrobial resistance in dairy herds

McLaughlin, Daniel (2023) An investigation of factors influencing antimicrobial resistance in dairy herds. PhD thesis, University of Nottingham.

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The emergence of antimicrobial resistance (AMR) as a result of the selective pressures placed on both commensal and pathogenic bacterial populations as a result of overuse and misuse of antimicrobials is one of the greatest issues facing human healthcare. Antimicrobials are widely used in agriculture for the maintenance of health and welfare, but their use also contributes to the issue of AMR and poses a risk to human health via the food chain. In order to tackle the challenge, governments and organisations across the world have committed to reducing antimicrobial use (AMU) in agriculture and to implement surveillance programmes to monitor AMR. Although AMU in agriculture in the United Kingdom (UK) is reducing, there remains a knowledge gap regarding the dynamics which exist in terms of AMU/AMR associations and the influences of the wider farm environment.

This context provided the rationale behind the research carried out and presented in this thesis. Chapter 1 provides an overview of available literature to explore the context and an outline of research aims.

In Chapter 2, a study group of sixteen dairy farms were recruited to investigate the associations between historical trends of AMU and AMR as part of a longitudinal study. AMU was determined over the course of six years and AMR was measured according to the minimum inhibitory concentration of sentinel bacterial species isolates from bulk tank milk samples. The findings of this Chapter demonstrated that higher levels of AMU did not necessarily represent higher levels of resistance and led to an interest in other influencing factors.

Chapter 3 outlines a cross sectional study investigating the influences of farm management practices on levels of resistance on dairy farms. Data was sourced from two study groups, one of which represented the herds recruited in Chapter 2, and utilised questionnaire responses collected during farm visits. Data was analysed using a robust modelling procedure and highlighted a range of management procedures existing across the dairy farm which may be associated with levels of resistance in sentinel bacteria.

Chapters 4 and 5 sought to outline a new laboratory based methodology which could be employed in the monitoring of on farm AMR via sampling of bulk tank milk. Initial investigations took the form of a pilot study, in which raw bulk tank milk samples were enriched using selective nutrient broths. The results of these initial investigations helped to inform a potential antimicrobial susceptibility testing (AST) methodology. This was further investigated with validation to compare experimental methodology with already established testing standards. Comparisons of the final methodology with validation steps demonstrated viability of the AST method.

Investigations of AMU/AMR interactions were once again considered in Chapter 6. AMU data, collated from farm medicine use records, were obtained from farms where bulk tank milk samples were sourced as part of investigations in Chapter 5. Analysis indicated that where statistically significant relationships between AMU and AMR existed, these relationships were negatively correlated.

Together, the findings of each of the Chapters presented in this thesis help to further our knowledge of the dynamics which exist with regards to AMR in the dairy farm setting, and provide an opportunity to further develop AMR surveillance across the industry.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Green, Martin
Bradley, Andrew
Keywords: Antimicrobial resistance; Antimicrobial use; Surveillance programmes; Dairy farms; Farm management; Milk sampling
Subjects: S Agriculture > SF Animal culture
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Veterinary Medicine and Science
Item ID: 74347
Depositing User: McLaughlin, Daniel
Date Deposited: 13 Dec 2023 04:40
Last Modified: 13 Dec 2023 04:40

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