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Viruses could acquire thousands of new hosts as climate change causes increasing contact between different mammal species.
Diseases such as Ebola virus are likely to spread further than ever before, while new pathogens could emerge with the potential to affect both animal and human populations.
More than 15,000 instances of diseases crossing into new species could take place in the next 50 years as a result of climate change.
As mammals shift their ranges to adjust to increasing temperatures, thousands of new species will come into contact for the first time.
Even if climate change is limited to less than 2⁰C, more than 300,000 pairs of mammals which have never met before will meet, leaving open the possibility that the viruses they carry will jump from animal to animal or even into humans.
Dr Gregory Albery, who led the paper alongside Dr Colin Carlson, says, 'As the world changes, the face of disease will change too. Because climate change is shaking our ecosystems to their core, the way that we understand viruses and their ecologies will need to evolve.
'We need to start preparing for things not just as they are, but how they will be. By demonstrating that moving mammals will meet each other for the first time and form entirely new communities, we have demonstrated a novel and potentially devastating mechanism for disease emergence that could threaten the health of animal populations in the future.
'This will most likely have ramifications for our health too. This work provides incontrovertible evidence that the coming decade will not only be hotter, but sicker.'
The findings of the study were published in Nature.
Zoonotic diseases are those which can jump between different species. In general, the more closely related two species are, the more likely they are to jump between them.
Some species are more likely to harbour zoonotic diseases than others. Among mammals, bats carry the highest proportion of these diseases, as they have particularly strong immune systems that allow them to tolerate, rather than succumb to, these infections.
Their ability to act as disease carriers, as well as being able to cover large distances in flight, makes them a significant vector for zoonotic events.
In the past, this has been less of a concern for the spread of viruses between different species, as most mammal species only share limited parts of their range with another. A 2020 study estimated that 93% of mammal species pairs shared less than 5% of their ranges, while only 0.5% shared more than half.
However, as Earth's temperature increases, it is likely that this will change. The study estimates that the vast majority of mammal species will overlap with at least one new species somewhere in their future range in any climate scenario, doubling the current level of contact.
Bats in particular will have some of the greatest range changes, with the study estimating that 88% of the new encounters between mammals will be between a bat and another species.
Of these new meetings, around 8,500 will be between bats and primates, resulting in around 110 instances of viruses crossing between these species for the first time.
This could mean that more diseases could spread from animals to humans.
To illustrate how climate change could affect human health, the study simulated a specific group of 13 species, which are potential hosts of Zaire ebolavirus, which causes the majority of cases of Ebola virus disease in humans.
It is believed that the virus is spread through contact between fruit bats and other animals, and is known to infect chimpanzees, gorillas, antelopes and porcupines, among other species.
The scientists found that Ebola virus hosts are likely to encounter around 2,600 new species for the first time, no matter how much the climate changes. This would result in the virus entering around 100 new species, and potentially spreading into West Africa for the first time.
Tropical Africa is likely to be one of the areas with the most new contacts between mammals, along with southeast Asia. These first encounters are most likely to happen in areas of high human population density, such as cities, as well as areas used for growing crops.
The study predicts that cities of southeast Asia, Africa and Europe are among those which could be hotspots of zoonotic crossover in the next 50 years, from which these diseases could spread around the world.
While the study found that many of the crossover events would occur regardless of the amount the world warms by, there are steps that can be taken to help protect animal and human populations from new diseases.
Primarily, this involves observation and monitoring of wild animal populations and their diseases, particularly in disease hotspots, as well as keeping track of human diseases in the same areas. This will allow public health organisations to step in when concerning new diseases emerge.
Gregory says, 'Even now, this process has likely been taking place, mostly unobserved and below the surface, and we need to start looking for it.
'This is happening, it's not preventable even in the best-case climate scenarios, and we need to put measures in place to build health infrastructure to protect human and animal populations.'
While steps to prevent spillover will have some effect, the scientists argue that prevention is no longer enough to prevent new zoonotic diseases.
Colin says, 'We're not going to put the Anthropocene back in the bottle. Our field has been focused on prevention as a mechanism that's more cost-effective and more holistic than preparedness, but our study raises serious questions about whether that is an achievable strategy.
'We have to do both.'