One Health epidemiology: A scoping systematic review of observational studies and their epidemiologic units

Abstract Background : The One Health approach, which integrates human, animal, and environmental health, is crucial for addressing complex global health threats. However, the epidemiological methods underpinning this approach have not been systematically evaluated. Methods : We conducted a scoping systematic review to characterise the design of self-labelled observational One Health epidemiological studies, define their epidemiologic units, and assess their adherence to the Checklist for One Health Epidemiological Reporting of Evidence (COHERE). We searched Ovid Medline, Scopus, and CABI for observational studies published since 2004 with the term “One Health” in the title. Results : From 1874 unique records, 79 studies were included. Our analysis revealed a marked increase in publications since 2020, and predominance of cross-sectional designs (79.7%). We classified epidemiologic units as either “singular” (e.g., households, farms), where components are naturally co-located, or “intended” (e.g., villages, communities), where domains are sampled independently within a geographic boundary. Intended units were more common, reflecting practical challenges in integrated sampling. The assessment of 23 studies that included all three One Health domains against the COHERE checklist revealed strong adherence to conceptual items, such as justifying the One Health approach (100%). However, we found significant reporting gaps in various methodological areas, including the description of the interdisciplinary study team (13% compliance), results for animal participants (34.8%), and environmental findings (60.9%). Conclusions : One Health epidemiology is a rapidly expanding field, but it is characterised by methodological heterogeneity and inconsistent reporting. Future progress depends on developing integrated analytical frameworks that can manage multi-domain complexity and promote better adherence to reporting standards like COHERE. This will enhance methodological rigour, improve study replicability, and strengthen the evidence base for policies that protect human, animal, and ecosystem health. One Health impact statement This article presents a first scoping systematic review of One Health observational epidemiological studies, showing that the One Health field is moving towards a more holistic, integrated era. We conceptually define the One Health epidemiologic unit and provide a critical baseline for researchers and policy makers to advance study design and analytical frameworks. The added value of this work lies in the comprehensive assessment of studies that highlights the urgent need for more integrated One Health reporting and methods, which is essential to strengthening the evidence base for practical solutions. This research was a collaborative effort by a transdisciplinary team of epidemiologists, veterinarians, public health experts, mathematicians, and immunologists. This unique integration of expertise was essential to advancing our understanding of a complex field and identifying the necessary steps for improving future studies and interventions.