Impact of farm-level biosecurity on bovine respiratory disease incidence in dairy herds

Thesis Abstract

Bovine respiratory disease (BRD) remains a leading cause of morbidity and economic loss in dairy operations, with farm-level biosecurity practices hypothesized to modulate BRD incidence by limiting pathogen introduction, transmission, and stress-related susceptibility. This study aims to quantify the association between farm-level biosecurity and BRD incidence in dairy herds, while identifying key biosecurity components that most strongly predict reduced BRD risk. The specific objectives are (i) to characterize the spectrum and implementation level of biosecurity measures across commercial dairy farms; (ii) to estimate BRD incidence and antibiotic treatment rates over a 12-month period and assess variation by biosecurity status; (iii) to evaluate associations between defined biosecurity practices and BRD outcomes using multilevel regression, controlling for herd size, management practices, vaccination status, and housing system; (iv) to explore stakeholders’ perceptions of biosecurity barriers and enablers through qualitative interviews, and (v) to develop evidence-based recommendations for prioritizing biosecurity investments. The study adopts a convergent mixed-methods design, integrating quantitative epidemiological data with qualitative insights. The population comprises dairy farms within three major dairy regions, with a target sample of 200 herds selected via stratified random sampling to represent parlor-focused, freestall, and pasture-based systems. Data collection involved (a) a structured farm survey assessing 40 biosecurity indicators across entry controls, animal movement, staff hygiene, visitor protocols, equipment sanitation, quarantine procedures, and disease surveillance; (b) veterinary records abstraction to determine monthly BRD incidence, clinic visits, and antibiotic usage; (c) on-farm observations to validate reported biosecurity practices; and (d) semi-structured interviews with farm managers and veterinary practitioners to capture perceived drivers and barriers. Validated instruments included a composite Biosecurity Implementation Score (BIS) and BRD case definitions aligned with national guidelines. Reliability was evaluated using Cronbach’s alpha for survey scales and inter-rater agreement for observational data. Analytical procedures encompass (i) descriptive statistics to summarize biosecurity practice prevalence and BRD incidence; (ii) multilevel Poisson regression to model BRD incidence rate as a function of BIS, with random effects for herd and region; (iii) logistic regression to examine odds of BRD outbreaks (defined by threshold incidence) relative to biosecurity strata; (iv) mediation analyses to assess whether vaccination status and housing type mediate the BIS–BRD relationship; and (v) thematic analysis of interview transcripts guided by grounded theory to identify contextual factors influencing biosecurity adoption. The theoretical framework integrates the Heterogeneity of BRD Risk and the Protection–Provider Model, drawing on epidemiological theory and health behavior constructs to interpret implementation gaps and adherence patterns. Expected findings anticipate a negative association between higher BIS and BRD incidence, with particular emphasis on strict quarantine and animal movement controls, dedicated staff hygiene protocols, and rigorous vehicle and equipment sanitation as the most impactful predictors. Variation is anticipated across housing systems, with freestall herds benefiting more markedly from cohesive staff adherence, while pasture-based systems may exhibit residual risk due to wildlife exposure and co-mingling during grazing. Qualitative results are expected to reveal that economic constraints, perceived breach consequences, and organizational culture strongly shape biosecurity uptake, mediating the quantitative associations. This study contributes to knowledge by providing robust, farm-level empirical evidence linking specific biosecurity components to BRD outcomes, refining risk stratification for targeted interventions, and informing policy and extension programs. It advances methodological integration of epidemiology and social-behavioral inquiry in dairy health management, offering actionable recommendations such as prioritized investment in quarantine facilities, standardized entry procedures, routine sanitation audits, and workforce training curricula. The anticipated conclusion emphasizes that comprehensive, consistently applied biosecurity programs substantially reduce BRD burden, but effectiveness depends on systemic support, farmer motivation, and feasible implementation tailored to herd structure. Recommendations include development of a scalable BIS toolkit, region-specific biosecurity guidelines, and incentives for routine evaluation of biosecurity practices within dairy operations.