It is quite possible to spend a lot of time debating the definitions of One Health and strong opinions are sometimes expressed as to which activities or problems are encompassed within a One Health framework and which are not. My sense is that the concept of One Health describes an approach, a way of thinking, that embraces the complexity of interactions and inter-dependencies that shape the health of people and the planet.

My involvement in One Health research came about as a natural consequence of investigating diseases in complex ecosystems, where it rapidly became very clear that most disease problems are multi-facetted, and require understanding and insights from many different disciplines and perspectives. My interest in One Health began with rabies, and initially from the perspective of wildlife conservation, when devastating outbreaks of rabies threatened the survival of endangered African wild dogs (Lycaon pictus) in the Serengeti. But, of course, rabies is a horrifying disease that has much broader relevance – it affects people and domestic animals in communities around the Serengeti, with wide-ranging impacts on human health and wellbeing, public health economics, and animal welfare. For rabies, it is not only that the terrifying nature of the disease and its inevitable fatality are so horrific, but also that there is an enormous economic and psychological toll associated with bite injuries inflicted by rabid animals. These injuries represent a medical emergency, as people have to reach medical facilities to obtain life-saving treatment within 24 hours. It is only when you talk to people that you start to appreciate the scale of the problem, and the heart-breaking challenges and choice that people face, particularly in poor, remote communities. It is probably no surprise that it is people in these communities who suffer most from rabies, but it is clearly unacceptable that because these deaths often pass unnoticed by policy-makers, rabies continues to be a neglected, yet totally preventable, problem.

While rabies provides a striking example of the importance of understanding socio-economic determinants of health, our Zoonoses and Emerging Livestock Systems (ZELS) research in Tanzania (within the Livestock, Livelihoods and Health programme) indicates that similar issues apply to many other endemic zoonoses. As for rabies, it is clear that poor people face many challenges in accessing medical care for treatment of fevers caused by zoonoses, such as brucellosis, Q fever and leptospirosis. Even if patients are able to present at clinics, it can be extremely difficult for clinicians to make a precise diagnosis – many of these diseases present with non-specific clinical signs, and diagnostic tests can often be unreliable. Our ZELS research is making it clear that, as with rabies, we will not be able to tackle these diseases effectively if we rely on medical treatments alone. We need to think more broadly as to how to use One Health to tackle disease problems at source – in the animal reservoir populations.

Animal vaccination programmes offer an important suite of One Health interventions. Mass dog rabies vaccination campaigns have been shown to be highly cost-effective as a means of preventing human rabies deaths, and animal vaccines are also available to prevent many of the zoonoses that we are studying in our research programme. For example, livestock vaccines have been widely deployed in high- and middle-income countries for the control of brucellosis, Q fever, leptospirosis, anthrax and Rift Valley Fever. But they are still only rarely used in low-income countries where these diseases continue to pose substantial health problems.

While social factors will always impact on the ability of people to access health care, One Health interventions can buffer against social and economic inequalities by casting a much wider preventive ‘safety net’. Interventions that control infection from animal source populations (such as livestock vaccination) should convey benefits to everyone in the community regardless of socioeconomic status. Importantly, this benefit can less easily be socially distorted to the detriment of the poor.

One Health Day provides an opportunity to reflect on how we can change the status quo – not only in how we do health research, but also in how we structure health systems, how we design and implement interventions to ensure equitable, cost-effective and appropriate solutions, and how we optimise health – human, animal and ecosystem health – in its broadest sense. Zoonoses comprise an important group of diseases in this context – they affect the health of millions of the most vulnerable people in the world, and they have significant impacts on food security, livelihoods, animal welfare and biodiversity conservation. But perhaps most importantly, good tools already exist that will allow One Health interventions to be implemented. Another programme I am involved with Afrique One-ASPIRE (African Science Partnership for Intervention Research Excellence) is doing exactly this, namely looking into how we can do setting-specific and cost-effective One Health interventions. Now is the time for action.