Climate Change: Effects on the Incidence and Distribution of Infectious Diseases

Since the early 1990s, the changing climate has been a hot topic in political circles due to its profound impact on the worldwide economy, ecology, and trade.

But what about the impact of climate change on human health?

The changing rainfall patterns and rising global temperatures are already affecting the incidence and geographic distribution of certain infectious diseases.¹ A recent study pointed to a shift in malaria distribution towards higher altitudes in both Ethiopia and Colombia in warmer years.² Higher rates of illness and death due to malaria have also been observed in highland regions of East Africa.¹

Chikungunya is another vector-borne disease (VBD) with a projected increase in prevalence in the coming years as a consequence of climate change.³ The incidence of water-borne illnesses such as cholera and diarrhea is also affected by episodes of drought and heavy rainfall associated with climate change.^1,4

A recent study points to the potential global spread of a large number of VBDs due to global warming.⁵ Scientists warn that unless there is an adequate response and proactive measures are taken, society will soon have to deal with direct and indirect effects of the changing climate on human health.⁶

In an interview with Infectious Disease Advisor, Emily K. Shuman, MD, assistant professor in the department of internal medicine at the University of Michigan Health System in Ann Arbor, and K. Marie McIntyre, PhD, research associate at the Institute of Infection and Global Health, University of Liverpool, United Kingdom, discussed the impact of climate change on the incidence and spread of infectious diseases.

Infectious Disease Advisor: How is climate change affecting the global incidence and distribution of infectious diseases?

Emily K. Shuman, MD: Many infectious diseases are climate-sensitive, including vector-borne diseases, such as malaria, dengue fever, and Lyme disease, and waterborne diseases such as cholera. Arthropod vectors may expand or alter their ranges as a result of climate change, introducing vector-borne diseases into new areas. Waterborne diseases can occur with heavy rainfall and contamination of water supplies.

Infectious Disease Advisor: What are some of the projections about the continued influence of climate change on the global burden of infectious disease?

Dr Shuman: The World Health Organization predicts that climate change will result in 250,000 additional deaths per year between 2030 and 2050.⁷ Of this increase, 60,000 deaths/year will be related to malaria and 48,000 deaths/year to diarrheal disease. Children and people living in areas with weak health infrastructures are expected to be disproportionately affected.

Infectious Disease Advisor: What are some of the ways the healthcare community can lessen the effects of climate change on disease incidence and spread?

Dr Shuman: The healthcare community can advocate for strategies to reduce greenhouse gas emissions and mitigate the effects of climate change. On a personal level, individuals who work in healthcare can do their part by using alternatives to private vehicles (eg, public transportation, cycling) and engaging in other energy-saving practices (suggestions are available from the Environmental Protection Agency and many other sources). Finally, it is important for scientists to continue to conduct research on the potential impacts of climate change on health.

Infectious Disease Advisor: How is the impact of climate change on infectious disease incidence assessed?

K. Marie McIntyre, PhD: Numerous studies have examined the effects of climate change on specific diseases, mostly using modelling approaches because since measuring the effects of climate change over long periods of time does not lend itself to empirical methods. Approaches include statistical modelling, ecological niche modelling, biological modelling, process-based modelling, climate simulation-based risk modelling, and meta-analytical approaches using electronic data-mining. In addition, the relative impact of climate has been examined in certain diseases and restricted sets of diseases using qualitative and semi-quantitative risk assessment-based methods. Until now, however, no studies have used fully systematic approaches to assess the scale of the effects of climate change on a range of diseases that have a significant impact on human health, and there have been no systematic reviews of their impact on animal health.

Infectious Disease Advisor: What are some of the common characteristics of infectious diseases that respond to climate change?

Dr McIntyre: The effects of climate change on many diseases are emerging. “Emerging” means that the number of diseases is increasing in terms of incidence, expansion into new geographical areas where they hadn’t previously been reported, and emergence in a new host or vector. When diseases appear, they are of high interest to individuals involved in public health, veterinary public health, and research funding; consequently, significant media interest is generated. Recent examples off this process which have been covered extensively in the media include Zika and Chikungunya.

Many other diseases affected by climate change are vector-borne or associated with certain transmission routes, and some are also zoonotic.

Infectious Disease Advisor: Your research group recently published an assessment of the climate sensitivity of important human pathogens occurring in Europe.⁸ What were some of the findings of this study?

Dr McIntyre: Our work suggests that, until now, we have underestimated how many diseases are likely to be sensitive to climate. Climate change could have an impact on a greater range of diseases than we had previously thought, because we found that a higher percentage (63%) of pathogens are climate-sensitive compared with a previous estimate (49%). We also found that many climate-sensitive pathogens are affected by more than just a single or a few climate drivers, such as moisture and rainfall.

Furthermore, certain emerging pathogens were more likely than non-emerging pathogens to be associated with rain or climate change. Good examples of emerging pathogens include Vibrio cholerae (the causative agent of cholera), Fasciola hepatica (which causes liver fluke in livestock), Bacillus anthracis (the cause of anthrax), and Borrelia burgdorferi (the cause of tickborne Lyme disease).

We also examined the extent of the impact on climate-sensitive diseases by examining diseases in which the impact was calculated according to disability-adjusted life years. We found that 37% of disability-adjusted life years result from human infectious diseases that are sensitive to primary climate drivers. Therefore, the impact on more than a third of these diseases could change with climate.

Infectious Disease Advisor: What are some of the impacts of your findings?

Dr McIntyre: Our study is the first to consider the effects climate sensitivity on transmission routes. We found that certain transmission routes — vector-borne, foodborne, soil-borne and waterborne — are more climate-sensitive than others because they were associated with larger numbers of climate drivers. We also found that some pathogen taxa, such as helminth species (parasitic worms) and protozoal pathogens, are particularly climate-sensitive.

Finally, we found evidence that zoonotic pathogens — pathogens naturally transmitted between vertebrate animals and humans — are more climate-sensitive than human- or animal-only pathogens and may therefore be particularly sensitive to climate change. This is worrying because an estimated 75% of emerging diseases are zoonotic, which may make them disproportionally sensitive to climate change. 

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References

1. Shuman EK. Global climate change and infectious diseases. N Engl J Med. 2010;362:1061-1063.
2. Siraj AS, Santos-Vega M, Bouma MJ, et al. Altitudinal changes in malaria incidence in highlands of Ethiopia and Colombia. Science. 2014;343:1154-1158.
3. Fischer D, Thomas Sm, Suk JE, et al. Climate change effects on Chikungunya transmission in Europe: geospatial analysis of vector’s climatic suitability and virus’ temperature requirements. Int. J. Health Geogr. 2013;12:51.
4. Chowdhury FR, Nur Z, Hassan N, et al. Pandemics, pathogenicity and changing molecular epidemiology of cholera in the era of global warming. Ann Clin Microbiol Antimicrob. 2017;16:10.
5. Balogun EO, Nok AJ, Kita K. Global warming and the possible globalization of vector-borne diseases: a call for increased awareness and action. Trop Med Health. 2016;44:38.
6. McMichael AJ, Lindgren E. Climate change: present and future risks to health, and necessary responses. J Intern Med. 2011;270:401-413.
7. World Health Organization. Climate change and health. www.who.int/mediacentre/factsheets/fs266/en/. Accessed November 27, 2017.
8. McIntyre KM, Setzkorn C, Hepworth PJ, et al. Systematic assessment of the climate sensitivity of important human and domestic animals pathogens in Europe. Sci Rep. 2017;7:7134.

This article originally appeared on Infectious Disease Advisor