One Health has gained significant traction in recent years as a multidisciplinary research and intervention approach. Its core principle is that the health of humans, animals and the environment are deeply interconnected and cannot be separated.

Key characteristics of One Health:

• Aims to balance and optimize health in all domains.
• Recognizes the interconnectedness of human, animal and environmental health.
• Engages multiple sectors, disciplines and communities.
• Addresses related issues including clean water, energy, safe food and climate action.

Global challenges such as the climate crisis, rapid urbanization, the COVID-19 pandemic and the re-emergence of Ebola virus all focus attention on the complex interrelationship between human health, animal health and environmental health.² These challenges have highlighted the urgency and value of applying a holistic and integrated approach to addressing complex health issues. Figure 1 uses the example of antimicrobial resistance (AMR) to illustrate the multiple determinants impacting disease pathology and their interaction across governance and health systems, people and society, animal health, and environment and climate change.

A One Health approach can be applied at a systems level to improve multisectoral coordination, as well as addressing specific issues such as AMR, zoonoses, vector- and food-borne illnesses and the impacts of climate change.³ The TDR Strategy 2024–2029 advocates for operationalizing a One Health approach to address global health challenges, moving away from a singular focus on a given disease or ill-health itself towards more holistic interventions that engage the most affected stakeholders in their individual contexts.⁴

One Health initiatives emphasize collaboration and learning across disciplines and sectors, creating an enabling environment for change.⁵ One Health and IR share a common focus on interdisciplinary approaches, real world impact, early and sustained community engagement, and systems thinking. The next section will go into more detail on the application of a One Health lens in the context of IR.

A One Health approach is of value when addressing complex health challenges and health events that occur at the animal–human–environment interface.⁶ Researchers should consider applying a One Health lens when investigating health issues such as AMR, vector-borne and other zoonotic diseases, or when considering the impact of environmental changes on an intervention (e.g. climate change, urbanization and deforestation).

By investigating an IR problem multidimensionally using a One Health lens, researchers may discover significant influences beyond the domain of health that they would not have otherwise considered (see Box 2).⁷ Addressing the root causes of health problems in this way – and not just related signs and/ or symptoms – can contribute to a more informed IR design and ultimately strengthen the effectiveness of a health intervention.

Applying a One Health lens can also help researchers understand the long-term impacts and consequences of a health intervention across all three domains. Looking beyond sectoral silos helps optimize risk mitigation and avoid unintended detrimental effects.⁷ Box 2 provides an example of the benefits of applying a One Health approach to a Lassa fever intervention.

The transdisciplinary One Health approach views human–animal–environmental interfaces as complex adaptive systems (CAS), as described in the module Understanding implementation research. Problems occurring at this interface, for example the emergence of new zoonoses, call for more creative thinking and interdisciplinary collaboration.⁸

To be effective, One Health interventions must be complex and adaptive, posing a challenge to conventional research’s focus on finding the ‘right solution’.⁸ Due to its context-specific, multidisciplinary nature, IR provides a practical framework for evaluating and improving the impact of One Health projects. By bridging the gap between research and practice, IR can accelerate the delivery of One Health programmes, as well as improving effectiveness and sustainability.^9,10

For this reason, IR plays a central role in the One Health adaptive management process, as outlined in Figure 2.