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How Climate Change Is Worsening Infectious Diseases Across the Globe

Health logo Health 8/16/2022 Cathy Cassata

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The global COVID-19 pandemic, followed in short-order by a worldwide monkeypox outbreak, have opened the world's eyes anew to the harm infectious diseases can cause. And it seems these types of global disease-related challenges may only intensify amid climate change.

A new study published by researchers at the University of Hawai'i is bringing awareness to the fact that climate change—including increasingly frequent occurrences of heat waves, droughts, and wildfires— has a major impact on infectious diseases. In particular, the study notes that 58% percent of infectious diseases confronted by humanity have at some point been aggravated by various climatic hazards. The same study revealed 1,006 unique pathways in which climatic hazards, via different transmission types, led to pathogenic diseases.

These findings, according to the study authors, point to the urgent need to control greenhouse gas (GHG) emissions.

"Climate change is a human health issue, not just an environmental issue," Erik Franklin, associate research professor at the Hawaii Institute of Marine Biology at the University of Hawai'i at Manoa, told Health.

"Based on prior research, we expected to find some relationships between climate hazards and diseases but the sheer number of pathways (over 1000) was a wake-up call that climate change will have a dramatic impact on human health," Franklin said.

While the study's results focus on impacts that have already occurred and don't predict future outcomes, Franklin said the results are "sobering." Here's a closer look at the study, its findings and its ramifications.

The Link Between Climate Change and Infectious Disease

The University of Hawai'i study, which was published in the journal Nature Climate Change, reviewed medical literature that linked climatic hazards to specific pathogenic diseases. The review found that 218 out of the known 375 human infectious diseases appear to be made worse by one of 10 types of extreme weather connected to climate change.

Some of the extreme weather types considered by study included drought, heatwaves, wildfires, flooding, sea level rise, and changes in land cover. Based on the study, researchers uncovered four basic relationships between climate and human disease:

  • Climate hazards, such as warming, floods, and droughts, affected insects and animals carrying diseases by bringing them in closer contact with humans.
  • Humans, in turn, reacted to climate hazards by moving closer to the sources of the diseases.
  • Some diseases were also strengthened by climate hazards.
  • In some cases, humans' immune systems were weakened by climate hazards, which made them more susceptible to diseases.

The researchers did not limit their investigations to the microorganisms that cause infectious diseases. They expanded their scope to include all types of human illnesses, meaning the study also encompassed non-infectious sicknesses such as asthma and allergies. The goal was to determine if these health issues were also impacted by climate change.

"Reducing the [study] scope to just microbes would have excluded plant and fungal allergens, which are aggravated by warming, floods and storms," said the study, noting that these things and indeed becoming a serious health problem for non-communicable outbreaks of asthma, skin, and respiratory illness.

How Climate Change Exacerbates Disease

To help better understand the direct connection between climate change and diseases, William Schaffner, MD, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Nashville, Tennessee, explained that "as temperatures rise, they can influence the range of insects, practically mosquitoes, but also ticks and other insects that can carry infectious disease."

For example, the deadly disease malaria is transmitted by certain mosquitos, which typically don't survive above 5,000 feet high, so mountainous areas tend to be mosquito-free.

"But with global warming their range has increased because it's a little warmer at higher altitudes than it used to be and it's been discovered that the mosquitos will follow the increasing temperature so the risk of malaria increases geographically and so new populations are involved," explained Schaffner.

Similarly, ticks that transmit Lyme disease have typically thrived in the northeast and upper Midwest regions of the United States. "Yet those have been increasing and it's thought that at the very least, the warming climate has provided an opportunity for those tick populations to expand somewhat," said Schaffner.

The study results show that society needs to take immediate action to reduce global greenhouse gas emissions and in turn reduce the future risk of negative human health outcomes from climate change, Franklin said. "From a health standpoint, humanity will need to adapt to a changing climate that brings a growing risk of pathogenic diseases," he said.

Schaffner agreed, noting that the infectious disease community supports a range of activities that are recommended to help diminish the impact of climate change and protect human health.

"This research is certainly noteworthy. It reinforces the notion that we are a very small and getting smaller global community in terms of distance because people travel so much. So, whatever is over there in other countries can soon be over here," Schaffner said.

However, Schaffner also stressed that other factors not discussed in the research, such as an expanding population and more people traveling over greater distances, also contribute to the present expansion of infectious diseases. The recent COVID-19 and monkeypox outbreaks are an example of this point, said Schaffner.

"Within a month of COVID having been first described in China, this virus had been detected on every continent in the world except Antarctica and that's because the COVID virus didn't move on its own. It was people; people got on airplanes," Schaffner explained.

The same holds true for monkeypox, which began in west African and now is detected in Europe, Canada, and the United States.

"That's not global warming but international travel," said Schaffner.

Schaffner added that today people spend more time in primitive areas of the world like jungles for work and recreation, which puts them in contact with more insects and wild animals, providing even more opportunities for viruses to jump from animals to humans.

Society Should Take Note—Not Panic

An August report titled Climate Endgame: Exploring Catastrophic Climate Change Scenarios, published by the Proceedings of the National Academy of Sciences (PNAS), warned that society needs to seriously look at worst-case scenarios of accelerating climate change. The scientists who developed the report highlighted the importance of what they call the "four horsemen" of the climate endgame: increases in infectious diseases; risks of famine; extreme weather disasters; and conflict over resources.

The point of the study is to motivate world leaders and create an understanding of the worst-case scenarios of climate change—which include the global population decreasing by 10% and even total human extinction.

"Prudent risk management requires consideration of bad-to-worst-case scenarios," states the report. "Yet, for climate change, such potential futures are poorly understood. Could anthropogenic climate change result in worldwide societal collapse or even eventual human extinction? At present, this is a dangerously underexplored topic. Yet there are ample reasons to suspect that climate change could result in a global catastrophe."

When it comes to the increase in infectious diseases now being highlighted by scientists, continued and diligent monitoring of such developments will be an essential task in the face of climate change.

"From time to time, national political leadership wants to cut back on the international public health perspective that the CDC has, and all of us in public health think that's a bad idea," said Schaffner. "We need to station people in other countries and help other countries do surveillance for infectious diseases for their own benefit and for ours because the more we know what's happening around the world, the better prepared we will be here."

On an individual basis, Schaffner said, we all need to begin considering a common phrase that is used regularly by environmentalists: "Think globally, act locally."

"Everybody can do what they can do in order to contribute to conservations and saving the environment and that includes being attentive to climate change and letting your elective representatives know you think climate change is real and important and want them to vote in favor of doing those things that reduce our impact on climate change," Schaffner said.

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