Is Global Warming Making Us Sicker?

The world is currently plagued by an upsurge in outbreaks of diseases like Ebola, Zika, and MERS. Is global warming responsible?

Global temperature is rising at an alarmingly high rate, and will keep on rising, probably at a faster rate, if left unchecked. For decades, global warming has been implicated in multitude of public health woes. For the last two decades, a sudden surge in outbreaks of new or lesser known diseases like Ebola, SARS, MERS, and Zika have many directly pointing fingers at global warming as the trigger behind these devastating scourges. However, finding a direct link between global warming and infectious diseases is notoriously difficult.

What is global warming?

Over the last century, the temperature of our world has consistently increased as a direct result of many human activities. Fueled by economic growth, we have achieved a dramatically high population size, and rapid urbanization to accommodate more people than ever that are moving into towns and cities. The result of both of these achievements is an unsustainably high consumption rate. To meet the demand of this insatiable consumption, we have, in turn, converted large swaths of primary forests into agricultural land. All these activities either contribute large amount of greenhouse gases (like carbon dioxide and methane that facilitate increase in atmospheric temperature) to the atmosphere or create urban heat islands, which eventually cause global warming. This man-made warming of the world is unlikely to be checked any time soon as developed countries are reluctant to reduce their consumption while developing powerhouses like India and China try to emulate the developed countries by achieving higher economic growth.

How can global warming increase disease risk?

With the increase in global temperature, it was expected that many newer, nastier pathogens will emerge from places such as melting arctic ice that may have ancient pathogens trapped in them. Global warming can also facilitate invasion of vectors, blood-sucking tropical insects, to temperate regions that are becoming warmer. This invasion by vectors, in turn, can spread vector-borne infectious diseases beyond their present distribution [1]. For instance, by 2020, it was predicted that globally 60% more people could be at the risk of getting malaria with an increase in global temperature [2]. The effect of climate change is also likely to be prominent in case of pathogens that are transmitted environmentally. This makes sense because these pathogens, such as fungi, parasitic worms, and water-borne bacteria are dependent on environmental conditions for their development, replication, and transmission [3].

Evidence

A direct link between climate warming and infectious diseases is often difficult to prove. To begin with, we are only starting to understand how a host or a parasitic organism may respond to changes in ambient temperature. Likely, the response will manifest in their physiology, immunology or even behavior. Additionally, global warming brings with it changes in precipitation, humidity, and myriad other environmental factors. How organisms will response to the sum total of these environmental factors are expected to be extremely complex. Furthermore, the effects of climate change in the form of diseases are often visible only in the form of higher-level (population or community) disruptions such as mass mortality events [Box 1]. But, to understand the connection between the two, it is essential to observe the initiation of diseases in individuals, which often goes unnoticed. For instance, two decades ago biologists first observed frog deaths worldwide [4, 5]. Only much later, we came to realize that the reason behind the amphibian extinctions was a group of parasitic fungi called chytrids. These fungi became particularly lethal after changes in environmental temperature that caused depressed immunity and higher susceptibility to diseases in amphibians [6]. However, such seminal and painstakingly done studies connecting evidence between different levels, from individuals to ecosystem, are few and far between.

Finally, interventions by humans often obscure the signature of climate change. This is particularly true for human diseases, since humans regularly intervene with the spread of disease with medical treatments, vector-control, and infrastructural changes. However, it is not often the case for diseases of the wildlife, hence the link between climate warming and infectious diseases is more readily seen in the wild [7]. In spite of these challenges, scientists have recently recorded significant expansion in the geographic range of multiple human viruses including bird flu, coronaviruses (that caused severe acute respiratory syndrome; SARS), Ebola and arboviruses (Dengue, Chikungunya and Zika to name a few) due to factors such as urbanization, globalization, and habitat destruction. Indeed, global data on infectious diseases points to a recent, multifold increase in Emerging Infectious Diseases (EIDs) that are newly spreading, such as Ebola and Zika, or re-surging of diseases that were once considered eradicated or under-control, such as malaria and Lyme disease [8, 9]. There is also evidence that warmer temperature can increase the human exposure to diarrheal diseases such as cholera [10]. With the rise of sea level – a result of global warming, the risk is only likely to increase in near future.

Box 1

 

The Saiga Antelope die-off

In May of 2015, conservation biologists heard about die-offs in Saiga antelopes – an odd looking ungulate (“hoofed animal”) – that once used to roam a vast area spread between Mongolia in the east to Romania in the west. During the nineteenth and early twentieth century, rampant hunting, however, decimated 95% of the historic population size restricting the animals to only a few small populations mainly in Kazakhstan. Die-offs are not uncommon for ungulates, so biologists were not particularly alarmed. However, nothing could prepare them when they visited the Kazakhstan populations. They witnessed the death of whole population – 60,000 animals in total – within four days of their arrival. This scale of decimation – 100% mortality within days – of a wild ungulate population is unprecedented. Later conservationists found out that the die-offs were occurring across the species’ distribution and in total more than 120,000 animals were dead by late May [12, 13]. At that point, due to the lightning speed of the population crash, it was clear that some virulent infectious agents were involved – perhaps Pasteurella or Clostridia – the usual suspects. What was mysterious, however, was the devastating speed at which the infections decimated populations that were far away from each other. What may have triggered such widespread outbreaks? In January, 2018, the bacterium, Pasteurella multocida type B, was finally identified as the pathogen that killed the Saigas. Interestingly, this bacterium is common even in healthy animals but was known to turn virulent with a change in environmental conditions. Indeed, scientists found that the climate in Kazakhstan did gradually turn unusually warmer and humid before precipitating the widespread epidemic of pasteurellosis in Saiga antelopes[13].

 

 

Future

In the last two decades, while risk of some diseases appear to have increased, along with a warmer world, risk of many serious infectious diseases has actually reduced [11]. This rise and fall in infectious diseases risk point to the fact that while climate change can potentially increase risk for some diseases, it can also decrease risk for others. Whatever the case, there is sufficient evidence to support the fact that climate change is altering the way many pathogens interact with their hosts [7]. These changes are often non-linear. To investigate these changes, it is important to combine advanced climate models with ecological models of infectious diseases. Studies may initially focus on how changing temperature may influence different types of diseases such as vector-borne, environmentally or directly transmitted ones, taking into consideration thermal physiology, ecology, and behavior of the organisms involved. Later, other important climatic factors such as humidity can also be incorporated into these models. Furthermore, such investigations are needed to be done at different levels of organizations, from individuals to communities and finally, different ecosystems, such as terrestrial and marine. To trace the mechanisms in the background, the investigations will need to be finally complimented with experiments. It is imperative that we strive to first understand these background mechanisms; before attributing all new maladies to global warming.

References

1. Harvell, C.D., et al., Climate warming and disease risks for terrestrial and marine biota. Science, 2002. 296(5576): p. 2158-2162.
2. Martens, P., et al., Climate change and future populations at risk of malaria. Global Environmental Change, 1999. 9: p. S89-S107.
3. Walther, G.-R., et al., Ecological responses to recent climate change. Nature, 2002. 416(6879): p. 389.
4. Berger, L., et al., Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proceedings of the National Academy of Sciences, 1998. 95(15): p. 9031-9036.
5. Pounds, J.A., et al., Widespread amphibian extinctions from epidemic disease driven by global warming. Nature, 2006. 439(7073): p. 161.
6. Raffel, T.R., et al., Disease and thermal acclimation in a more variable and unpredictable climate. Nature Climate Change, 2013. 3(2): p. 146.
7. Altizer, S., et al., Climate change and infectious diseases: from evidence to a predictive framework. science, 2013. 341(6145): p. 514-519.
8. Daszak, P., A.A. Cunningham, and A.D. Hyatt, Emerging infectious diseases of wildlife–threats to biodiversity and human health. science, 2000. 287(5452): p. 443-449.
9. Jones, K.E., et al., Global trends in emerging infectious diseases. Nature, 2008. 451(7181): p. 990-993.
10. Pascual, M., M.J. Bouma, and A.P. Dobson, Cholera and climate: revisiting the quantitative evidence. Microbes and Infection, 2002. 4(2): p. 237-245.
11. Vos, T., et al., Years lived with disability (YLDs) for 1160 sequelae of 289 diseases and injuries 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010. The lancet, 2012. 380(9859): p. 2163-2196.
12. Nicholls, H., Mysterious die-off sparks race to save saiga antelope. Nature News, 2015.
13. Kock, R.A., et al., Saigas on the brink: Multidisciplinary analysis of the factors influencing mass mortality events. Science Advances, 2018. 4(1): p. eaao2314.

About the author

Debapriyo Chakraborty is a disease ecologist. He is a Senior Research Scientist at EcoHealth Alliance, a global nonprofit based in New York City, USA. Views are personal.

Editors

Arunima Singh, obtained her PhD from the University of Georgia, and is currently a postdoctoral researcher at the New York University. A computational structural biologist by training, she enjoys traveling, reading, and the process of mastering new cuisines in her spare time. Her motivation to move to New York was to be a part of this rich scientific, cultural, and social hub.

 Paurvi Shinde, is a recent PhD, in Biomedical Sciences (Immunology) with expertise in T cell activation pathways. She currently works as a Postdoctoral Fellow at Bloodworks Northwest in Seattle, where she studies the mechanism of how alloantibodies are formed against ‘non-ABO Red Blood Cell antigens’. Apart from science, she loves editing scientific articles to convey the message behind it, in a clear and concise form.

Illustrators

Saurabh Gayali recently completed his Ph.D. in Plant Molecular Biology from National Institute of Plant Genome Research (JNU), New Delhi. He is currently working in the same lab in projects involving the study of abiotic stress response in crop plants apart from actively seeking the post-doctoral position. He has a keen interest in data analysis, visualization and database management. He is skilled 2D and 3D designer with a specific interest in scientific illustration. In leisure, Saurabh plays guitar and compose music, does photography or practice programming. Follow him on Instagram

Disha Chauhan did her Ph.D. in IRBLLEIDA, University of Lleida, Spain in Molecular and Developmental Neurobiology. She has post-doctoral experience in Cell Biology of Neurodegenerative diseases and is actively seeking a challenging research position in academia/industry. Apart from Developmental Neurobiology, she is also interested in Oncology. She is passionate about visual art (Illustration, painting, and photography) and storytelling through it. She enjoys reading, traveling, hiking and is also dedicated to raising scientific awareness about Cancer. Follow her on Instagram

Blog design: Arunima Singh

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