A study published Monday in Nature Climate Change found that climate change is worsening more than half of hundreds of known infectious diseases. These include diseases such as malaria, hantavirus and cholera.
Researchers from the University of Hawaii and the University of Wisconsin-Madison conducted a systematic search for examples of the impact of 10 climate hazards sensitive to greenhouse gas emissions on every known human pathogen.
They found that 58%, or 218 of the 375 infectious diseases that affect humans globally, have at some point been exacerbated by climate hazards — floods, heat waves and drought. Of these, the impact of 16% has decreased. The researchers identified and mapped 1,006 unique pathways where climate hazards, “through different modes of transmission, led to pathogenic diseases.”
“If the climate changes, the risk of these diseases changes,” Jonathan Patz, MD MPH., co-author and director of the Global Health Institute at the University of Wisconsin-Madison, wrote in the study.
He added that doctors should think of these diseases as symptoms of a sick Earth.
Dr. Carlos del Rio, MD, an infectious disease researcher at Emory University who was not involved in the study, told AP News, “The findings of this study are terrifying and well illustrate the enormous effects of climate change on human pathogens . Those of us in infectious disease and microbiology must make climate change one of our priorities, and we must all work together to prevent what will undoubtedly be a climate change disaster.
Effects are not limited to infectious diseases
The research team did not limit their study to infectious diseases. They also look at ailments such as asthma, allergies and animal bites. They identified 286 unique diseases, of which 223 appeared to be exacerbated by climate hazards, nine were reduced by climate hazards, and 54 were both exacerbated and minimized.
Camilo Mora, Ph.D., a climate data analyst at the University of Hawaii, said the study does not predict future cases. “There is no speculation here. These are things that have already happened.”
One example is a case that occurred in Siberia in 2016. A reindeer that had died of anthrax decades ago and had been frozen in permafrost thawed. A child touched it, caught anthrax and caused an outbreak.
The researchers found 3,213 examples of empirical cases in which climate hazards have been implicated in pathogenic diseases. They were associated with 286 unique pathogenic diseases. These include anaphylaxis, black fly bites, conjunctivitis, Diptera, enterobiosis, insect bites, swimmer’s itch, trichinellosis, anthrax, Bacillus cereus food poisoning, chlamydia, cholera, leprosy, Lyme disease, plague, Queensland tick, whooping cough and many others.
Jessica Leibler, DrPH, ScM, an environmental epidemiologist at the Boston University School of Public Health, who was not involved in the study, told NBC News that 58% “seems like a really big number, but it reflects the reality that infectious diseases are driven by what is happening in our environment. Climate is driving habitat change and destruction around the world. It also brings humans into contact with animal species in ways that we have not been in contact with in the past or in the recent past. Our recent pandemic exemplifies the extent to which the leading hypothesis is that bats may have played a role.”
Animal-to-human transmission is increased
Animals that commonly spread infectious diseases to humans include mosquitoes, snakes, birds and rodents. One example, Mora noted, is voles, which use snow cover for winter habitat. But with less snowfall, the small rodents are seeking shelter in homes, where he says they have been documented to transmit hantavirus.
Increased environmental temperatures have also increased the range of many animals and insects, such as ticks, fleas and mosquitoes, which carry West Nile virus, Zika and dengue fever.
A study published in Nature in April noted that there are about 10,000 viruses with the potential to infect humans, and most “circulate silently in wild mammals.” Climate change and changes in land use patterns are increasing the possibility of interspecies transmission of viruses as animals that rarely had contact with humans have increased contact.
The authors write: “While most studies agree that mitigating climate change by reducing greenhouse gas emissions will prevent extinctions and minimize harmful impacts on ecosystems, our results suggest that mitigation alone cannot reduce the likelihood of climate-induced spread of viruses. Instead, the mildest global warming scenarios seem likely to lead to at least as much or even more interspecies transmission of viruses.
They envision the possibility of more than 4,000 viruses that could be shared between different species that have never met. And that sharing could be one virus or all of their viruses, “So when we say 4,000, what we mean is that there will be 4,000 pairs of species sharing viruses for the first time, and that could be 4,000 cross-transmissions of species,” said Colin J. Carlson, PhD, assistant professor of biology at Georgetown University and one of the co-authors of the April Nature study. “It could be 400,000. We just don’t know.”
Dr. Aaron Bernstein, MD, MPH, interim director of the Center for Climate, Health, and the Global Environment at the Harvard TH Chan School of Public Health, added, “It’s not entirely surprising that with habitat loss and climate warming, you’re seeing more -high chances of animals encountering each other – especially animals that have not been in historical contact with each other – and this creates interfaces where pathogens can move from one species to another. For us, that means we have to really think about how we approach emerging infections like COVID-19. And part of that has to be taking seriously the need to prevent proliferation, rather than trying to play catch-up.”
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