Study links climate threats to 58 percent of infectious diseases

Climate hazards such as floods, heat waves and droughts have exacerbated more than half of the hundreds of known human infectious diseases, including malaria, hantavirus, cholera and even anthrax.

Researchers have reviewed the medical literature on established cases of the disease and found that 218 of the known 375 human infectious diseases, 58 percent, appear to be made worse by one of 10 types of climate change-related extreme weather, according to a study in Monday’s journal Nature Climate Change.

Doctors dating back to Hippocrates have long linked disease to weather, but this study shows just how far-reaching climate’s impact on human health is.

In addition to studying infectious diseases, the researchers broadened their search to include all types of human diseases, including non-communicable diseases like asthma, allergies, and even animal bites, to see how many diseases they could link in some way to climate hazards, including infectious diseases. They found a total of 286 unique diseases, and of those, 223 appeared to be made worse by climate hazards, nine were reduced by climate hazards, and 54 cases had both been aggravated and minimized, the study said.

The new study does not perform calculations to attribute specific disease changes, probabilities or magnitude to climate change, but finds instances among many cases where extreme weather was a likely factor. The study has shown the 1,006 connections between climate change and disease.

“As the climate changes, so does the risk of these diseases,” said study co-author Dr. Jonathan Patz, director of the Global Health Institute at the University of Wisconsin-Madison.

Doctors like Patz said they had to look at the diseases as symptoms of a sick earth.

“The results of this study are startling and provide a good illustration of the enormous impact of climate change on human pathogens,” said Dr. Carlos del Rio, an infectious disease specialist at Emory University who did not participate in the study. “Those of us involved in infectious diseases and microbiology need to make climate change one of our priorities, and we all need to work together to prevent what will undoubtedly be a climate change catastrophe.”

The study’s lead author, Camilo Mora, a climate data analyst at the University of Hawaii, said it’s important to note that the study isn’t about predicting future cases.

“There is no speculation here,” Mora said. “These are things that have already happened.”

An example that Mora knows firsthand. About five years ago, Mora’s home in rural Colombia was flooded – for the first time in his memory there was water in his living room – and Mora contracted Chikungunya, a nasty virus transmitted through mosquito bites. And although he survived, he still has joint pain years later.

Sometimes climate change works in strange ways. Mora includes the 2016 case in Siberia, when a decades-old reindeer carcass that had died of anthrax was unearthed as permafrost thawed from warming. A child touched it, got anthrax and triggered an outbreak.

Mora originally wanted to browse medical cases to see how COVID-19 intersects with climate hazards, if any. He found cases where extreme weather both worsened and decreased the likelihood of COVID. In some cases, extreme heat in poor areas led people to gather to cool off and exposure to the disease, but in other situations, heavy downpours reduced the spread of COVID because people stayed home and indoors, away from others .

Longtime climate and public health expert Kristie Ebi of the University of Washington warned that she had concerns about the way the conclusions were reached and some of the methods used in the study. It’s a proven fact that the burning of coal, oil and natural gas has led to more frequent and intense weather extremes, and research has shown that weather patterns are linked to many health problems, she said.

“But correlation is not causation,” Ebi said in an email. “The authors did not discuss to what extent the studied climate hazards have changed over the period of the study and to what extent changes have been attributed to climate change.”

But dr Aaron Bernstein, interim director of the Center for Climate, Health, and the Global Environment at Harvard School of Public Health, Emory’s del Rio and three other outside experts said the study was a good warning for now about climate and health’s future. Especially as global warming and habitat loss bring animals and their diseases closer to humans, Bernstein said.

“This study underscores how climate change may cause the dice to favor unwanted contagious surprises,” Bernstein said in an email. “But of course it only reports what we already know, and what’s still unknown can be even more compelling about how preventing further climate change can prevent future disasters like COVID-19.”

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