COVID-19, many scientists believe, originated in bats, while monkeypox, which was found in a fourth Massachusetts resident last week , is thought to have spread to humans from African rodents.
A new study found that many more diseases could spread from animals to humans as climate change forces species to migrate into new landscapes and habitats, “shaking the planet like a snow globe,” said Colin J. Carlson, an assistant professor of biology at Georgetown University who led the new research.
As warming temperatures and changing ecological conditions alter global habitats, many wild animals are fleeing their homes. That means species are intermingling in new, unprecedented ways. Some of those creatures end up in regions with large human populations. And many are bringing diseases with them, greatly increasing the risk of pandemics.
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Scientists don’t expect monkeypox to ravage the world the way that COVID-19 did. But in the future, other diseases from animals could. In fact, nearly every pandemic of the past 100 years — from the Spanish flu to COVID-19 — is thought to have been the result of pathogens spreading from animals to people, said Aaron Bernstein, interim director of the Center for Climate, Health, and the Global Environment at Harvard’s T.H. Chan School of Public Health.
“These newly emerging infections, the new ones, the things we’ve never heard before, are essentially all from animals,” said Bernstein.
A major reason viruses from animals are so dangerous: Our immune systems are poorly equipped to deal with unfamiliar threats.
“In the past, we haven’t had much contact with these species and so we haven’t been exposed to these diseases,” said Richard Primack, a biology professor at Boston University who focuses on climate change. “Now, people are being much more exposed.”
Tens of thousands of viruses could be lurking in the bodies of wild mammals, according to an April study published in the journal Nature, which Carlson led. Using computer simulations, the authors estimated that as many as 10,000 of those viruses have the capacity to jump to humans, a phenomenon scientists call “zoonotic spillover.”
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Over the next 50 years, as many as 4,000 of them will make that jump, the report projects.
Globally, some regions will be more affected than others. Southeast Asia is most probably vulnerable to cross-species disease transmission, Carlson said, largely because it’s home to a variety of bats, whose wings allow them to carry their illnesses farther than other mammals. The mountains of the African tropics could also be a hot spot for spread, because animals may flock toward their peaks from all sides in search of cooler temperatures.
But that doesn’t mean the United States is immune.
“There are risks everywhere,” Carlson said.
Not all viruses spread straight from other mammals to humans, said Primack. Many move through ticks and mosquitoes, which can also be more abundant due to climate change.
“When I was growing up, we just really hardly ever saw ticks,” he said. “Now, they’re everywhere.”
For instance, research shows that global warming has contributed to the expanded range of ticks, which heightens the risk of contracting Lyme disease and other tick-borne illnesses. Often, they’ll pick up those illnesses from rodents, then pass them to us, Primack said.
The April study focused on mammals, but other creatures can also spread disease. And climate change makes it more likely that they will.
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This week, for instance, Massachusetts officials warned residents to stay away from the hundreds of sea birds recently found on Martha’s Vineyard. They suspect highly pathogenic avian influenza — which can spread to humans, though it rarely does — killed them. And there’s evidence that climate change increases avian flu’s transmission.
Ducks and other waterfowl can carry other flus and other viruses, too, which can get passed on through mosquito bites, said Primack.
And there’s evidence that due to warming sea-surface temperatures, Vibrio species — a kind of bacteria that can spread to shellfish and then to humans — are creeping northward into New England, said Bernstein.
“They’re only able to live up here because the water’s warmer now,” he said.
Mitigating climate change by phasing out fossil fuels and curbing planet-warming pollution won’t stop the cross-species spread of illnesses, Carlson said. In fact, his study found that could exacerbate it.
The reason why, he said, is “really, really depressing”: In a rapidly warming world, more species will go extinct before they can share illnesses, but if we slow climate change, species are more likely to be able to adapt to their changing conditions and continue to spread diseases.
That doesn’t mean we should keep using fossil fuels and spewing out planet-warming pollution, Carlson said. But we’ll need to focus on preparedness and managing risk.
To do so, he said, leaders should work to monitor diseases in wildlife and track early outbreaks of animal-borne diseases, especially in places where people have more contact with animals, like livestock operations and zoos. This could keep disease from spreading and could even prevent the next pandemic before it starts.
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Curbing deforestation could also minimize risk, Bernstein said, because it would cut back on another factor that can destroy habitats, which drives animals into new territories.
Such practices have long been dismissed, Bernstein said, because there’s no direct financial incentive for preventing disease spillover. But this preventive approach, he said, is key.
“We are being completely foolish if we do not spend a lot more money on preventing spillover in the first place,” he said.
Even if we implement these practices, some illnesses are still likely to spread to humans. That makes universal health care all the more important, said Carlson, not only to protect infected individuals but also to limit spread.
“We could live in a world where, if we fight for health justice, spillover rates are going up, but we never have another COVID-19,” he said. “But we have to completely rethink the way that we handle outbreaks if we want to achieve that.”
Dharna Noor can be reached at dharna.noor@globe.com. Follow her on Twitter @dharnanoor.