Professor Camilo Mora feels the impacts of climate change in his knees.
During a 2014 visit to his native Colombia, heavy rains caused the worst flooding his hometown had seen in decades and boosted the mosquito population. A mosquito bit Mora, transferring the chikungunya virus and making him a patient during an unprecedented outbreak in the region.
His joints ache still today. He blames a warming world.
In a study published Monday, Mora and his colleagues at the University of Hawaii canvassed tens of thousands of studies to analyze the global impacts of climate change on infectious diseases that affect humans. They determined that nearly 220 infectious diseases — 58% of the total studied — had become bigger threats because of climate hazards.
“Systems have been evolving for millions of years and now humans have come along and changed things,” Mora said. “We are punching nature, but nature is punching us back.”
The study, which ultimately analyzed more than 3,200 scientific works, is one of the most thorough examinations of climate change’s overall impact on diseases worldwide.
“It’s only in the recent past of infectious disease research that we really focus in on climate change as a driver of infectious disease,” said Jessica Leibler, an environmental epidemiologist at the Boston University School of Public Health who wasn't involved in the research.
Fifty-eight percent “seems like a really high number,” she said, “but it reflects the reality that infectious diseases are driven by what’s going on in our environment.”
The research is not without limitations. Scientists often have a hard time quantifying how much climate change is contributing to disease outbreaks, since it’s an indirect process.
Climate hazards also diminished some impacts of infectious disease. For 16% of diseases, these hazards reduced the ailments’ impact or produced mixed results.
Climate hazards bring people and animals closer together
When Mora and his team examined the effects of 10 climate hazards on 375 infectious diseases, they found more than 1,000 ways that climate change spurred disease transmission. Rising temperatures were the biggest driver of pathogenic diseases, followed by precipitation, floods and drought.
Most often, infectious diseases were spread to humans from animals such as mosquitos, snakes, birds or rodents.
Voles, for instance, depend on snow cover for winter habitat, Mora said. But diminishing snowpacks have sent the creatures seeking shelter inside people’s homes, where they have been documented transmitting hantavirus.
“Climate drives habitat change and disruption around the world. That also brings humans into contact with animal species in ways that we were not in contact with them historically, or haven’t been in the recent past,” Leibler said. “Our recent pandemic is an example to the extent that the leading hypothesis is that bats might have played a role.”
Rising temperatures have also increased the habitat ranges for creatures like ticks, fleas and mosquitoes, growing the footprint of infections like West Nile virus, Zika and dengue fever.
“Mosquitoes are obviously the big one that cause a tremendous amount of mortality internationally,” Leibler said.
Other climate-linked diseases spread directly to humans through food, water or air. Fecal pathogens like E. coli or salmonella, for instance, can enter drinking water after a flood or hurricane, and rising temperatures may increase their chance of survival.
“There’s a good deal of evidence that as temperatures rise, it’s more likely that different sorts of pathogens will be present in drinking water globally,” Leibler said.
Climate hazards even put direct stress on the human body and make people more vulnerable to infection.
“What happens with warming countries in particular is that drought, because it undermines nutrition and increases malnutrition, compromises our body’s ability to fight infection,” said Amir Sapkota, an epidemiology and biostatistics professor at the Maryland Institute for Applied Environmental Health. Sapkota was not involved in the research.
Mora said heat waves could be pushing some viruses, through natural selection, to tolerate higher temperatures. That’s bad news, he said, because one of the human body’s key weapons against a viral invader is heat from a developing fever.
Scientists worry about a ‘Pandora’s box’ of new pathogens
Mora’s study also raises concerns about the potential for new diseases to spread.
In the Arctic Circle, for instance, ancient pathogens in the bodies of animals frozen beneath permafrost have begun to re-emerge with some nasty effects. Through genetic analysis, scientists traced a 2016 anthrax outbreak in Siberia to buried prehistoric animals exposed during a heat wave.
“Melting permafrost may expose pathogens that are frozen in time,” Sapkota said. “We don’t even have any idea of what they are and what they would be like if they were to infect us today.”
Mora said it’s possible that rising temperatures in the Arctic could open a “Pandora’s box” of new pathogens for which human immune systems haven’t had exposure.
Scientists are also worried about the potential for new viruses to spill over from animals to humans.
“With drought, what happens is animals may start to move over larger areas in search of food, so that brings this opportunity for what we call this viral spillover event,” Sapkota said. “The same thing goes with regards to humans encroaching on their area.”
Mora’s study found that some disease-carrying creatures are becoming more prevalent or developing new advantages in a warmer world. So scientists said it’s important to increase surveillance in areas where humans and animals closely interact.
“One question that comes to mind is: What if that new viral spillover event is something very unique?” Sapkota said. “It’s as efficient as the coronavirus in terms of spreading from one person to another, but as efficient as Ebola virus in terms of killing people.”