Factors affecting farm-level antimicrobial resistance among bacteria in livestock: a scoping review

Antimicrobial resistance (AMR) is a global threat, and responsible antimicrobial use (AMU) in animals is crucial. However, AMR development is influenced by numerous factors. This scoping review aims to describe the association between AMR, AMU, and other risk factors in European livestock farms. Understanding the impact of different risk factors is essential for mitigating AMR. The results confirm that the AMR prevalence varies between livestock farms and that AMU affects this prevalence. At herd level, factors related to infection pressure and AMU have been predominantly studied and are often associated with AMR. Only a few other factors such as temperature and metals or disinfectants in the farm environment have been investigated. Overall, the associations between AMR and the studied farm level. Risk factors, including AMU, were variable and complex. The studies varied in their methodological approaches. Different AMR indicators were used, with phenotypic resistance in indicator Escherichia coli being most common, and resistome characterisation gaining interest in recent years. Some studies did not include AMU data or had insufficiently detailed data, while some calculated treatment incidences in different ways. Standardised measures for risk factors and outcome variables would facilitate comparisons between studies and meta-analyses. For deeper insights into resistance dynamics, resistome characterisation is preferable, whereas phenotypic characterisation of pathogenic bacteria is more appropriate for developing clinical guidelines, and resistance profiles in indicator bacteria are useful for monitoring. In conclusion, farm-level risk factors are complex and risk factor studies including AMU, using validated methods, sufficient sample sizes, and sound statistical analyses are still needed.