Before the 20th century, we lived in an era when only a few well-known "big" diseases, such as malaria, tuberculosis, and childhood diseases, seemed to exist; global pandemics were rare, occurring on average every 100 years. This is no longer the case: we are witnessing the emergence of a myriad of little-known or even totally new diseases. Even worse, their frequency is accelerating: in the last 30 years, it is easy to see that COVID-19 is just the most recent example in a long and daunting list of emerging or re-emerging diseases (e.g., Zika, Ebola, H5N1, H1N1, MERS-CoV [Middle East Respiratory Syndrome Coronavirus]). We are clearly in an era of emergence.
What is the origin of these novel infectious diseases that emerge in human populations? It has now been scientifically demonstrated that 75% of novel diseases are zoonoses—that is to say, animal diseases that jump to human populations. Now that we are aware of this, the next obvious action is to understand the processes responsible for these transfers. Again, it has been scientifically established that biodiversity loss is strongly implicated. But why is that? Ecosystems rely on fragile equilibria, and microbes are a key part of wildlife biodiversity—they are even crucial for well-functioning ecosystems. However, when humans alter the balance within ecosystems (e.g., deforestation), they also alter microbe communities. To make a long story short, this has two main consequences. First, it increases the circulation of infectious diseases among wildlife species; second, it multiplies the transfers of these pathogens to human populations, either directly or indirectly through domestic animals. These novel pathogens can mutate and acquire the capacity to spread among humans. When this point is reached—given current globalisation—they have the potential to cause a pandemic.
What can be done? The global strategy is organised around the triptych of control–preparedness–prevention, but currently almost all efforts have been concentrated on control. That is to say, we wait for epidemics or pandemics to occur then we try to control them, often in crisis situations and with varying degrees of success. It is increasingly recognised that it would be more effective to focus on preparation or, even better, on prevention. As a result, a growing number of scientists around the world are working to develop long-term, science-based prevention strategies based on the One Health approach. It is now clear that accelerated emergences and re-emergences of diseases are just one aspect of a global syndrome of human effects on the planet. Thus—more than ever—the concept of One Health is relevant.
What crucial anthropic factors need to be considered? According to recent syntheses, land-use change, agricultural and livestock booms, and urbanisation are responsible for more than 30% of emerging disease events. Wildlife trade—both legal and illegal—is likely to increase disease emergence because it brings species together that usually do not interact, at least so closely, and this favours the transfer of pathogens among these species and humans. Climate change also has major consequences because it forces many species to flee their natural habitats and geographic distributions. Consequently, many species that had never interacted will rapidly be doing so. According to a recent publication in Nature (May 2022), it was estimated that 15,000 novel transmissions could occur among species before 2070. Another concern is the thawing of Arctic permafrost. Not only does this cause a climate disaster due to the massive amounts of methane released into the atmosphere, it also releases "zombie" viruses and bacteria. An example of this occurred in Russia in 2016, when an anthrax outbreak took place after a heat wave thawed a 75-year-old reindeer carcass that was full of dormant spores of this dangerous bacteria. Is this an isolated case? According to a 2021 synthesis published in Nature Climate Change, the answer is no. This is worrying because we do not know what impact these microbes—which evolved with species that are now extinct, like mammoths or giant sloths—will have on organisms living in today's ecosystems, including humans.
Despite the bleak outlook, we can still escape the era of emergence. The solution is in our hands: we need to change our practices and our relationships with biodiversity and ecosystem management. We need a paradigm change; we need to accept that prevention is better than control. Prevention is not only early detection and surveillance, it is also the protection of biodiversity if we want to tackle the problem at its root. Biodiversity loss is the spark causing the forest fire, since biodiversity provides what is called a “dilution effect”. Recent quantifications have also demonstrated that, from a financial point of view, pandemic preventions would be a hundred times less costly than control, in addition, of course, to saving human lives. However, biodiversity protection is complex because the immediate drivers of biodiversity loss often have an indirect causation, being the consequence of changing human demographics, economic activities, cultural habits, poverty, conflicts between humans, and so on. Thus, we need to create synergies between public health, economic concerns, and environmental protection strategies.
Concretely, the first recommendation for a preventive One Health approach is to identify and better understand the risks. It is for instance estimated that there are still 1.7 million unknown viruses in mammals and birds, including 800,000 potentially able to infect humans. We need to encourage academic research at the interface between animals, humans and the environment. Indeed, the problem is not limited to animals and their pathogens, nor to humans or the environment, it is at the interface between the three, and therefore interdisciplinary research is needed for accurate modeling. When we better understand the risks, we can change our behaviours and routines to reduce these risks. To succeed, operational programmes inspired by translational research must follow the path of sustainability, food security, respect for cultures, religions, and so on, which implies cooperation, co-construction processes, among the concerned partners. Education is also crucial, starting with children. We need to prepare young people to be critical thinkers who will inherently understand connections among human, animal, and ecosystem health, and to be future leaders and responsible citizens in our diverse, complex, and ever-changing world. Early detection and rapid response are the next steps in the event of an outbreak: we need to reduce the time it takes to detect dangerous microbes before they acquire pandemic potential. To do this, we need to develop early detection methods and health infrastructure at the local level and strive for the most equitable access possible among the world's different populations. Finally, we need global surveillance at multiple scales, as an outbreak anywhere in the world now has the potential to become a pandemic due to global connectivity.
To summarise, it is no longer enough to control epidemics and pandemics; rather, it is crucial on many fronts to prevent them from occurring in the first place. If we do not adopt a preventive approach, epidemics and pandemics will recur, perhaps as frequently as every decade or even more often, and it is possible that viruses much more virulent than SARS-CoV 2 will appear. For this task, we need scientific studies that improve our understanding of the drivers and mechanisms of disease emergence at different scales. We also need to encourage more connections between fundamental academic sciences and translational research to turn this knowledge into concrete strategies that are easily adaptable by local stakeholders around the world. Finally, given that pathogens do not stop at borders, international coordination and collaborations with interdisciplinary scopes are key to the success of preventing and controlling pandemics. The COVID 19 pandemic has revealed the current unsatisfactory integration of these issues.
While we still have a lot of work to do, we can still escape the era of emergence if we break down the silos and adopt a "One Health" perspective.
