Issues and principles for legitimate and meaningful One Health monitoring

Although the One Health (OH) approach has gained prominence in health governance, no widely accepted method exists to assess the overall state of health across these interconnected domains in geographically defined settings. Available data remain fragmented, and existing evaluation frameworks largely focus on OH implementation rather than its outcomes. Two composite indices-the One Health Index and the Global One Health Index-have attempted to operationalize OH measurement, but their high level of aggregation, limited transparency, and top-down design constrain interpretability and legitimacy. We outline conceptual, logistical, and political challenges associated with measuring and monitoring OH outcomes, examine existing indices to illustrate these limitations, and argue that progress in OH measurement relies less on developing a single universal index than on establishing shared methodological principles. We propose core principles grounded in transparency and contextual relevance and highlight the need for collaborative efforts to develop clearer methodological guidance for more legitimate and meaningful OH monitoring systems.