At least 13 animal species, from bats to gorillas, are hosts to the Ebola virus. The geographic distribution of these species – and, as such, the areas where potential risk of infection exists – are limited to parts of western central Africa. However, driven by climate change, they are moving to new habitats and carrying their pathogens with them. A study published in scientific journal Nature last week estimates that these animals now have up to 3,695 new contacts with other mammals in different regions of the continent. These encounters will lead to over 100 virus transmission events. The authors of the paper made the same calculations for thousands of migrating mammals and believe that the potential for another pandemic on the scale of SARS-CoV-2 will become even greater over the next few decades.
Science has been able to demonstrate a double (and in some cases triple) response by animals to global warming provoked by climate change. Many species are migrating to latitudes more in accordance with their thermal optimum – the temperature to which they are accustomed. Others are seeking higher elevations in their environment to escape the heat, or in the case of fish species moving further towards the ocean depths. The geographical reorganization is opening new paths for the spread and contagion of viruses. On the one hand, because it is leading to contact between species that previously did not share the same habitat, and on the other because many of them are genetically related, which facilitates viral jumping between them.
This risk increase is what a group of scientists have been investigating, by combining various predicted climate scenarios over the course of the rest of the 21st century, the evolution of thermic ranges and the movements of 3,870 mammals classified by genetic proximity. Between 96% and 98% of species will share habitat with at least one other species they previously had no contact with over the next 50 years. The Nature study forecasts that this will produce up to 316,000 new contacts between mammals that live in different environments today. As a result of these unprecedented encounters, the researchers believe over 15,000 new virus transmission events between species could occur.
Biologist Colin Carlson, lead author of the paper published in Nature, investigates the link between climate change and the emergence of infectious diseases at Georgetown University. Carlson lays out a comparison that echoes the suspected origin of the coronavirus pandemic. “The closest analogy is with the risks that we see in the trafficking of wild animals,” he says. “We are concerned about these markets [selling wild animals], because bringing together sick animals in unnatural conditions creates opportunities for processes of emergence, as was the case with SARS, which jumped from bats to civets and then from civets to humans. But these markets no longer represent a unique case; in a scenario of climate change, these processes will become the norm almost everywhere under natural conditions.”
In effect, the increase in virus transmission events between species will occur in practically any part of the planet. The areas of greatest risk will be concentrated in South East Asia, sub-Saharan Africa and South America, although such events are also predicted in Western Europe and North America. This concentration in tropical regions appears to contradict the direction of animal migrations highlighted in other studies.
Carlson admits that this finding “completely” surprised him, but he sees a certain logic in it: “Species move towards the poles, but they do so together, so they don’t run into any other species.” That is to say that migrations toward higher latitudes do not imply a greater risk of virus infection, because the number of new encounters with other species is relatively low as animals that already shared an environment move in parallel. But the other great migratory movement is altitudinal; localized movement to higher elevations. That is where species from different habitats can coincide. As such, the results of these investigations indicate that the emergency of new contagions is taking place in the higher regions of the tropics. “When they can move horizontally and reach the mountains from different sides, that is when they can meet [different species] for the first time,” says Carlson.
Phylogeny, or genetic proximity, is a key factor in the ability of a virus to jump between species. The more genetically similar two animals are, the more likely it is that a pathogen will find a home in new hosts. Greg Albery, a researcher at EcoHealth Alliance and a co-author of the Nature study, emphasizes that the greatest element of risk remains the movement of animals from one zone to another. “In reality, phylogeny is a more important determinant factor than geography in terms of influence, but it does not change. Species never alter their level of [genetic] proximity on the ecological timescale we are interested in, from 2020 to 2070, but they do change in their geographical distribution,” he tells EL PAÍS via email.
The authors of the Nature paper highlight that climate change has already exceeded other anthropogenic alterations that traditionally affect the risk of contagion between species and, ultimately, among humans. Deforestation, agriculture and the spread of urbanization, as well as reducing distances between species, also lead to imbalances that facilitate the emergence of viral events. To reduce the risk, the researchers propose aligning disease surveillance systems in nature with real-time studies focusing on changes in the environment in which they are being produced.
The problem is that, even if greenhouse gas emissions are cut and the pace of global warming is reduced, it may still be too late to halt these changes in geographical range. As Albery points out: “The reason is [these species] are already migrating and they have been doing so for some time. They will continue to do so for a while yet, because even our best efforts will only delay global warming rather than reversing it, so the motivation to move will still be present.”