Design, implementation and evaluation of a herd-level antimicrobial stewardship program for dairy cattle

Thesis Abstract

Antimicrobial resistance (AMR) presents a critical threat to dairy production systems through the emergence of resistant pathogens and limited treatment options, underscoring the need for evidence-based stewardship at the herd level. This study addresses the gap between antimicrobial use policies and on-farm practices by designing, implementing, and evaluating a comprehensive herd-level antimicrobial stewardship (HAS) program targeted at large commercial dairy operations. The aims are to reduce non-therapeutic and inappropriate antimicrobial use, improve veterinary oversight, enhance record-keeping and decision support, and assess consequent impacts on animal health, productivity, and resistance indicators. Specific objectives include (1) developing a HAS framework grounded in responsible-use guidelines and behavioral change theory, (2) implementing standardized protocols for antimicrobial prescribing, withdrawal times, and infection prevention across three dairy herds (n=2,500 cows total), (3) integrating decision-support tools and farmer/veterinarian training to promote evidence-based decisions, (4) measuring changes in antimicrobial consumption metrics (defined daily doses per 1,000 cow-days), treatment outcomes, and udder/ruminal infection rates, (5) evaluating microbiological and molecular indicators of resistance in sentinel samples, and (6) analyzing economic and production consequences to determine cost-effectiveness. A mixed-methods design combines quantitative and qualitative approaches within a before-and-after framework over 18 months. The population comprises commercial dairy herds within a temperate-climate region, with three adjacent farms selected for intervention and one matched control farm. A sample size of approximately 3,000 milkings and 600 cow-years will provide sufficient power (>80%) to detect a 15% reduction in antimicrobial usage with a 5% significance level, accounting for clustering at the herd level. Data collection instruments include (i) a standardized antimicrobial stewardship protocol manual; (ii) electronic farm records capturing antimicrobial prescriptions, indications, durations, and withdrawal intervals; (iii) quarterly herd health dashboards recording somatic cell count, clinical mastitis incidence, reproductive metrics, milk yield, and culling rates; (iv) environmental and milk microbiology sampling for resistance surveillance (mecA, blaCTX-M, mcr-1 genes) from a rotating subset of isolates; and (v) semi-structured interviews and focus groups with farm managers, veterinarians, and herd workers to explore adherence, perceived barriers, and facilitators to change. Validity and reliability will be ensured through pilot testing of the data collection instruments, triangulation across data sources, and inter-rater reliability checks for qualitative coding. Analytical methods include interrupted time-series analysis and mixed-effects regression models to quantify changes in antimicrobial use and health outcomes, adjusting for confounders such as seasonality, herd size, and baseline disease prevalence. Economic evaluation will employ a cost-effectiveness analysis (CEA) and a cost-benefit analysis (CBA) using net present value and return on investment metrics. Resistance data will be analyzed using logistic regression models to identify associations between stewardship adherence levels and the incidence of resistant isolates, complemented by Bayesian hierarchical modeling to incorporate uncertainty. The theoretical underpinnings integrate the Theory of Planned Behavior to explain anticipated changes in prescribing practices, and the Diffusion of Innovations framework to interpret adoption rates among farms and personnel. The study is expected to demonstrate that a structured HAS program reduces overall antimicrobial consumption by 15–20% while maintaining or improving animal health outcomes and milk production, with favorable economic implications and a measurable shift in resistance indicators. The contribution to knowledge includes empirical evidence on the feasibility and effectiveness of herd-level stewardship interventions in a real-world dairy setting, operationalization of a replicable HAS framework, and insights into the behavioral and economic drivers of sustainable antimicrobial use. The main conclusion anticipated is that integrated HAS programs, combining policy, education, decision-support tools, and rigorous monitoring, can achieve meaningful reductions in antimicrobial use without compromising productivity, while also contributing to the global effort to curb AMR. Recommendations include scaling the program to diverse dairy systems, refining resistance surveillance protocols, and integrating HAS metrics into routine farm audits and extension services.