One Health recognises that human health links to the health of animals, environment and ecosystems. It is an appealing concept, almost Zen-like in its emphasis of the whole. But beyond philosophy, One Health uses data and science to understand some human diseases as aberrations in a larger system of connections. First proposed in 2004, the One Health framework has become a practical and necessary approach to public health, especially in the face of emerging, and reemerging, infectious diseases.
One Health covers a wide range of diseases. But its role may be most relevant to prevent and manage two groups of infectious diseases. One is zoonoses: diseases that move from animals to humans. The second is diseases of livestock, which have a direct bearing on the livelihoods of many. Livestock also act as conduits for infections from wildlife to move to humans, bringing us back to zoonoses.
And One Health to combat zoonoses rests on two premises. First: wildlife and livestock present shared reservoirs for pathogen exchange and spread. Second: many infectious diseases result from human-animal interactions. Pathogens often leap across species with human activities that increase contact with wildlife. Disrupting natural ecosystems, bushmeat hunting, wildlife trade and large-scale animal husbandry operations all offer fertile ground for zoonotic outbreaks. Where pathogens intersect with human behaviour is the focus of One Health interventions.
Indeed, a key consideration for One Health is to regulate human contact with wildlife, either directly (e.g. eating bushmeat) or though their byproducts (e.g. guano). Take for instance the Nipah virus outbreak in Malaysia in 1998-1999. The outbreak among humans happened through infected farm pigs that contracted the virus from eating fruits dropped by bats that carry the virus as reservoirs. Instead of a knee-jerk response of culling bats, One Health-style policies focused on keeping pigs away from fruit trees where bats ate.
Going from reactive to proactive One Health mode also makes economic sense. According to a 2008 estimate by the World Bank, the cost of implementing a One Health workforce will be around $852 million in the 49 countries with highest risk of emerging diseases. Compare this with the $40-50 billion spent to control a single SARS pandemic.
Let’s be clear though. One Health in no way undermines the need for vaccines, antimicrobials or a mechanistic insight into disease pathology. Instead, One Health seeks to complement traditional approach to understand and treat disease, and offers a robust scaffolding to prevent and control disease outbreaks. Research agendas for diseases tended to focus on singularly on microbiology, pathology or pharmacology. One Health, by contrast, considers all components that lead to disease emergence and spread. These include ecological and wildlife components as well as domestic animals and human factors.
To recognise that these links exist is one thing; to establish them through rigorous science is quite another. Studying the relationships between so many factors can be complicated. There is much uncertainty in the causes and consequences of different drivers and likely solutions. It can be tricky to pull so much information together in a cohesive public health policy.
That’s why One Health relies on team efforts, bringing together professionals from a variety of disciplines. This includes human medicine, veterinary science, epidemiology, ecology and agriculture. The team also needs social scientists to assess human behaviours that promote disease emergence and spread, and suggest behavioural change that can help mitigate risk. There will also be ethical and legal concerns in implementing One Health, such as choices between individual and common good. These hurdles have to be tackled by the right experts to improve the scope and relevance of the approach.
Getting together so many different experts for interdisciplinary work can be difficult. For example, scientists from lab-based or more experimental domains tend to be critical of the less mechanistic sciences that seek to understand environmental, economic and social drivers of disease. Differences in training and philosophies among disciplines can make it hard to arrive at a common research agenda. This can delay or prevent consensus on which research goals to prioritise and the methods by which to approach the study.
Implementing One Health can also be mired by the rigidity of administrative set-ups. Bureaucratic red-tape often hinders cross-sectoral coordination across departments to implement a unified policy. Maybe that is why, despite its appeal, the One Health approach has been applied only in a handful of diseases (rabies, lyme disease and, recently, Kyasanur forest disease). However, these few examples offer much promise and good lessons in the real-world workings of the concept.
So what does a One Health agenda look like? The first step is to know which diseases most need a One Health approach. These exercises are already underway. For each disease, experts from different disciplines have to assess the relevant links between humans, animals and ecosystems. The goal is to identify risk factors, map known and unknown pathways of disease emergence and spread, and establish research gaps. This information will help locate at-risk clusters and develop monitoring programmes. The expert teams would regularly share knowledge to adapt research and monitoring efforts.
Regular surveillance and robust monitoring are key to One Health practice. The aim is to detect rare and isolated health events and identify when they might become a threat. Avian influenza presents a good example of field ecologists working with human and animal health workers. These teams act in concert to track pathogens in animal and human reservoirs. They will also aim to identify novel and emerging pathogens (e.g. USAID’s PREDICT project). The surveillance networks need support from a well-organised public health workforce, integrated across human and animal health sectors.
Effective programs for One Health research, policy, and monitoring needs a workforce with the right training. Novel academic and skill development programs can train practitioners with the systems thinking and subject basics needed to understand the many parts of One Health. Regular workshops and refresher courses will ensure that all stay updated on the latest knowledge and practice.
In sum, the One Health framework may disrupt existing intellectual, disciplinary and administrative silos. Research questions, problems, and solutions will have to transcend their subject-specific dimensions. Human health will have to be seen as an emergent property of a complex system of interactions – the whole being much more than the sum of its parts.
We have much to know about the links between ecosystem change, disease regulation and human well-being. COVID-19 served us a reminder that human and nonhuman health can’t be separated. To think of systems as a whole will not be easy. But it’s a challenge that deserves our best shot.
Meghna Krishnadas is a project scientist at the CSIR-Centre for Cellular and Molecular Biology, Hyderabad.