Cause and Effect | As the earth warms, the rising threat of infectious diseases - Hindustan Times
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Cause and Effect | As the earth warms, the rising threat of infectious diseases

ByTannu Jain
Mar 29, 2023 09:00 PM IST

Jarring numbers on this threat were revealed in a review of decades of scientific papers on all known pathogens to map risks aggravated by climate-related hazards

218. This is the number of infectious diseases that scientists believe will be aggravated as the planet warms.

Scientists are still to determine how the coronavirus came to first infect humans. (AFP) PREMIUM
Scientists are still to determine how the coronavirus came to first infect humans. (AFP)

While this number alone might cause alarm, what really puts it in perspective is the number of diseases known to affect humans worldwide: 375.

These jarring numbers were revealed in a review of decades of scientific papers on all known pathogens to map risks aggravated by climate-related hazards.

The origin of diseases and how pathogens come to affect humans has been one of the most controversial topics since the Covid-19 outbreak — scientists are still to determine how the coronavirus came to first infect humans.

The future threats are against the backdrop of a number of diseases that threaten to trigger wider outbreaks. Avian influenza (H5N1 and H3N2), the Marburg virus disease (from the filovirus family, which includes the Ebola virus), and monkeypox have recently prompted these fears with the world still reeling from the once-in-a-century pandemic that brought variants, pathogens, pandemic, vaccines, flu shots, daily infection trackers and — for most Indians — Dolo to popular discourse.

Vector-borne diseases (103) have shown the highest increase among pathogenic diseases. (AFP)
Vector-borne diseases (103) have shown the highest increase among pathogenic diseases. (AFP)

Scientists from the University of Hawaiʻi at Mānoa, United States, in the study, Over half of known human pathogenic diseases can be aggravated by climate change, published in August 2022, looked at what was increasing incidence of diseases caused by all varieties of pathogens.

They discovered that extreme climatic events, made more common and more severe by global warming, affected diseases triggered by viruses, bacteria, animals, fungi and plants.

Vector-borne diseases showed the highest increase among pathogenic diseases at 103, followed by waterborne (78), airborne (60), direct contact (56) and foodborne (50 diseases).

The study focused on 10 climate-related hazards linked to rising greenhouse gas emissions: Atmospheric warming, heatwaves, drought, wildfires, heavy precipitation, flooding, storms, sea-level rise, ocean warming and land cover change.

Warming (160 unique diseases), precipitation (122), floods (121), drought (81), storms (71), land cover change (61), ocean climate change (43), fires (21), heatwaves (20) and sea level (10) were all found to influence diseases triggered by viruses (76), bacteria (69), animals (45), fungi (24), protozoans (23), plants (12) and chromists (9).

The figure shows the pathways by which climatic hazards aggravate specific pathogenic diseases. The lines’ thickness is proportional to the number of unique pathogenic diseases. The colour gradient indicates the proportional quantity of diseases, with darker colours representing larger quantities and lighter colours representing fewer. The numbers indicate the number of unique pathogenic diseases. An interactive display is available at https://camilo-mora.github.io/Diseases/. Credits: word clouds, WordArt.com; bacteria, Wikimedia Commons (www.scientificanimations.com); other images, istockphoto.” Source: Mora et al. 2022 via Nature
The figure shows the pathways by which climatic hazards aggravate specific pathogenic diseases. The lines’ thickness is proportional to the number of unique pathogenic diseases. The colour gradient indicates the proportional quantity of diseases, with darker colours representing larger quantities and lighter colours representing fewer. The numbers indicate the number of unique pathogenic diseases. An interactive display is available at https://camilo-mora.github.io/Diseases/. Credits: word clouds, WordArt.com; bacteria, Wikimedia Commons (www.scientificanimations.com); other images, istockphoto.” Source: Mora et al. 2022 via Nature

The study cited shifts in the geographical range of species as one of the most common ecological indications of the climate crisis.

Warming and precipitation changes, for instance, were associated with a range expansion of vectors such as mosquitoes, ticks, fleas, birds and several mammals implicated in outbreaks by viruses, bacteria, animals and protozoans, including dengue, chikungunya, plague, Lyme disease, West Nile virus, Zika, trypanosomiasis, echinococcosis and malaria.

Habitat disruptions caused by warming, drought, heatwaves, wildfires, storms, floods and land cover change were also associated with bringing pathogens closer to people.

The pathogens in these cases jump from one species to another, eventually evolving to infect humans — a phenomenon known as zoonosis. The World Health Organization defines zoonosis as an infectious disease that has jumped from a non-human animal to humans.

Spillovers from viruses (for example, Nipah virus and Ebola), were associated with wildlife (for example, bats, rodents and primates) moving over larger areas foraging for limited food resources caused by drought or finding new habitats following wildfires.

Apart from facilitating contact between people and pathogens, climatic hazards also enhanced aspects of pathogens, including improved climate suitability for reproduction, acceleration of the life cycle, increasing length of likely exposure, enhancing pathogen-vector interactions and increased virulence.

Another study, Climate change increases cross-species viral transmission risk, published in April last year warned that as global temperatures rise, many animal species will migrate to cooler, higher altitudes, taking their parasites and pathogens with them and facilitating viral sharing between species that previously had no interactions.

The study predicted that at least 15,000 new cross-species viral transmissions were likely to happen by 2070, in the worst-case scenario of a 2-degree Celsius rise in temperature.

The rising risk of cross-species viral transmission. (Credit: Hindustan Times/E-paper May 4,2022)
The rising risk of cross-species viral transmission. (Credit: Hindustan Times/E-paper May 4,2022)

The study, published in Nature, suggested that the new spillovers will be concentrated in the mountainous and species-rich parts of tropical Africa and southeast Asia. The researchers highlighted the Sahel region, Ethiopian highlands and the Rift Valley in Africa; and eastern China, India, Indonesia and the Philippines. Some European population centres may be in the transmission hotspots as well.

While the study highlighted the Covid-19 pandemic (an unknown coronavirus passed from a wild bat to a human), and a simian immunodeficiency virus making a host jump from monkeys to chimpanzees and gorillas facilitating the origins of HIV, it came just weeks before cases of monkeypox were confirmed outside Africa in May.

“This work provides us with more incontrovertible evidence that the coming decades will not only be hotter, but sicker,” Gregory Albery, a disease ecologist at Georgetown University in Washington DC and a co-author of the study, had said.

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