Animal melting pot created by climate change could lead to new disease outbreaks

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That could spark new disease outbreaks in many wildlife populations—and in humans as well.

By 2070, assuming the most conservative warming scenario, there will be at least 15,000 new cross-species transmissions involving more than 3000 mammalian species, according to a modeling team led by Colin Carlson, a global change biologist at Georgetown University. “Most of this pattern has probably been set in motion with the 1° of warming we’ve already experienced,” says Carlson, whose study appears online in Nature today.

It’s becoming increasingly clear that climate change will have an effect on the spread of infectious diseases, says Skylar Hopkins, a disease ecologist at North Carolina State University who specializes in parasites, and this “solid study” offers new insights. “They’ve given us a quantitative estimate of how big the risk is for viral sharing in wildlife and where it’s going to happen,” Hopkins says. “But we really don’t know what this means for human health.”

The modelers calculate there are some 21 million possible pairings of mammalian species, and only 7% of those duos currently share a geographic range, which means there’s a tremendous opportunity for new encounters. “Most species on Earth haven’t met each other yet,” Carlson says.

Carlson’s team wanted to know how that might change in the decades ahead. Their models explored four scenarios for climate change, called Representative Concentration Pathways, which range from 2°C to 4°C increases and forecast how each would alter mammalian habitats and mingling. Then, they factored in which first encounters might lead to viral exchanges—only 6% of the possible pairs harbor viruses from the same families, suggesting those could swap pathogens at a future encounter.

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Carlson says several findings surprised him. “We get a completely different geography of risk than we expected,” he says. He anticipated that climate change would drive species north and south toward the poles, but the models showed intense mixing occurs earlier, because species will move to habitats at higher or lower altitudes. Bats, which make up about 20% of all mammals, will have an outsize impact on mixing because their ability to fly allows even nonmigratory species to travel hundreds of kilometers over a lifetime, far more mobility than most small animals. “Bats have the hand on the steering wheel of how bad this is,” Carlson says. And virus swapping will concentrate in high-elevation, species-rich ecosystems of Africa and Asia, not, as other models have suggested, at higher latitudes. “Models let us talk about the nuances of the future,” he says.

For a concrete example, the team did a case study of Zaire ebolavirus, which they predict has 13 possible mammalian hosts based on known susceptibility factors. Adding in dispersal limitations of species, they estimate that the least dramatic climate change scenario could cause the virus to be involved in more than 2000 first encounters between two species, of which nearly 100 might lead to a jump from one species to the other. Those leaps could cause deaths in some of these species but could also lead to Ebola infections in humans in the Horn of Africa, where the disease has never been seen.

Quantifying the impact of climate change on first encounters and new viral transmissions adds an important parameter to the modeling that’s been done earlier, says Erin Mordecai, a Stanford University ecologist who studies climate change and infectious disease dynamics. But Mordecai stresses that the study’s real strength is in the big picture, not the nitty-gritty details, which are often complicated by such factors as predicted dispersal patterns and distinctions within a genus. (Bats alone have tremendous variation.) “The specific numerical and geographic projections in the model should be taken as very coarse hypothetical estimates,” Mordecai says.

Although Carlson says there is no way to undo the species mixing that climate change has already caused, he believes we can better mitigate the risks to humans. Leaders in pandemic prevention, for example, have put great emphasis on the threats posed by wildlife markets and deforestation, which both increase encounters between humans and mammals. But that only accounts for a small percent of the actual risk, Carlson says. Surveillance efforts should also intensify whenever a species is found far from its original habitats, such as the Brazilian free-tailed bats that have greatly expanded their range in the southeastern United States over the past decade. Such long-haul travelers have lots of fresh opportunities to exchange viruses with other species.

“Plan long term around a world where we can’t put this back in the box,” Carlson says. “Wishful thinking is our enemy.”