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 there is a potential risk of infection – is 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 a scientific journal Nature last week estimates that these animals now have up to 3,695 new contacts with other mammals in different parts of the continent. These encounters will lead to more than 100 virus transmission events. The paper’s authors made the same calculations for thousands of migrating mammals and believe the potential for another SARS-CoV-2-wide pandemic will become even greater in the coming decades.
Science has been able to demonstrate a double (and in some cases triple) response of animals to global warming caused by climate change. Many species migrate to latitudes more in line with their thermal optimum – the temperature to which they are accustomed. Others seek out higher elevations in their surroundings to escape the heat, or in the case of fish species that move further into the depths of the ocean. Geographical reorganization opens up new avenues for the spread and contagion of viruses. On the one hand, because it leads to contacts between species that did not previously share the same habitat, and on the other hand because many of them are genetically related, which facilitates viral jumps between them.
It is this increased risk that a group of scientists is studying, by combining various climate scenarios predicted over the rest of the 21st century, the evolution of thermal amplitudes and the movements of 3,870 mammals classified by genetic proximity. Between 96% and 98% of species will share their habitat with at least one other species with which they had no previous contact within the next 50 years. the Nature the study predicts that this will produce up to 316,000 new contacts between mammals that now live in different environments. As a result of these unprecedented encounters, researchers believe that more than 15,000 new cross-species virus transmission events could occur.
Biologist Colin Carlson, lead author of the article published in Nature, studies the link between climate change and the emergence of infectious diseases at Georgetown University. Carlson presents a comparison that echoes the alleged origin of the coronavirus pandemic. “The closest analogy is with the risks we see in wildlife trafficking,” he says. “We are concerned about these markets [selling wild animals], because the grouping of sick animals in unnatural conditions creates opportunities for the emergence process, as was the case with SARS, which passed from bats to civets and then from civets to humans. But these markets no longer represent a unique case; in a climate change scenario, these processes will become the norm almost everywhere under natural conditions.
Indeed, the increase in virus transmission events between species will occur in virtually any part of the planet. The areas most at risk will be concentrated in Southeast Asia, sub-Saharan Africa and South America, although such events are also expected in Western Europe and North America. This concentration in tropical regions seems to contradict the direction of animal migrations highlighted in other studies.
Carlson admits that this discovery surprised him “completely”, but he sees a certain logic in it: “Species move towards the poles, but they do it together, so that they do not meet any other species.” That is, migrations to higher latitudes do not imply a greater risk of viral infection, since the number of new encounters with other species is relatively low because animals that already shared an environment move in parallel. But the other great migratory movement is altitudinal; localized movement to higher altitudes. This is where species from different habitats can coincide. Thus, the results of these surveys indicate that the urgency of new contagions is occurring in the highest regions of the tropics. “When they can move horizontally and reach the mountains from different sides, that’s when they can meet [different species] for the first time,” says Carlson.
Phylogeny, or genetic proximity, is a key factor in a virus’ ability to jump between species. The more genetically similar two animals are, the more likely a pathogen will find refuge in new hosts. Greg Albery, researcher at EcoHealth Alliance and co-author of the Nature study, emphasizes that the greatest element of risk remains the movement of animals from one area to another. “In reality, phylogeny is a bigger determinant than geography in terms of influence, but that doesn’t change. Species never change their level of [genetic] proximity on the ecological time scale that interests us, from 2020 to 2070, but they change in their geographical distribution,” he told EL PAÍS by email.
The authors of the Nature point out that climate change has already overtaken other anthropogenic alterations that traditionally affect the risk of contagion between species and, ultimately, between humans. Deforestation, agriculture and the extension of urbanization, as well as the reduction of 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 focused on changes in the environment in which they are produced.
The problem is that even if greenhouse gas emissions are reduced and the rate of global warming is reduced, it may still be too late to stop these shifts in geographic distribution. As Albery points out: “The reason is [these species] are already migrating and have been doing so for some time. They will continue to do so for some time to come, because even our best efforts will only delay global warming rather than reverse it, so the motivation to move will always be there.