The Proximity of People & Pathogens: Understanding the Impacts of Climate Change on Infectious Disease

BY LILIA POTTER-SCHWARTZ

Introduction: Widespread impacts of climate change on infectious disease

A 2009 Lancet Commissions report declared climate change as the biggest global health threat of the 21st century, estimated to affect the wellbeing of billions of people worldwide.¹ Since the Industrial Revolution, climate change has caused significant shifts in the social and environmental determinants of health. Humans have released unprecedented levels of greenhouse gasses into the earth’s atmosphere through the burning of fossil fuels. The abundance of carbon dioxide, methane, and nitrous oxide in the earth’s atmosphere is at its highest in over 800,000 years, increasing the earth’s surface temperature.² Far-reaching direct and indirect impacts of human-caused climate change include extreme temperatures, flooding and acute weather events, poor air quality, melting of permafrost, changing patterns of vector-borne diseases, human displacement, and food insecurity. While many health conditions will be impacted by these factors, the spread of infectious disease is of particular concern.

Groundbreaking research at the University of Hawai’i at Mānoa found that 58% of human infectious diseases have been historically aggravated by climatic hazards.³ Such climatic hazards (e.g. droughts, heatwaves, wildfires) cause changes in microbial evolution and human behaviors that can exacerbate the spread of infectious diseases. The increase in climatic hazards has been caused by an excess of greenhouse gasses leading to atmospheric warming and accelerated soil water evaporation.³ Strengthened precipitation and sea level rise have also caused flooding and storms. According to the University of Hawai’i study, of the 1,066 unique pathways by which climatic hazards have exacerbated disease spread, 160 pathways were due to warming, 122 due to changes in precipitation patterns, 121 due to floods, and 81 attributed to droughts.³

In regards to the mechanisms driving this increased infectious disease spread, the University of Hawai’i study emphasized the ability of climatic hazards to bring people and pathogens closer together.³ The geographic distribution of vectors (e.g. mosquitos, ticks, fleas, birds, several mammals) will continue to expand due to warming climates and changes in precipitation patterns.³ More specifically, historically colder areas that experience warming temperatures will become more hospitable for tropical vectors and pathogens. Animals searching for new habitats will also result in greater human-vector contact.³ Many spillover events from major viruses in history – like Ebola – are associated with wildlife searching for new habitats.³ Droughts can cause a congregation of mosquitos around water sources, while an increase in recreational water-related activities in response to heat waves may lead to greater transmission of waterborne diseases through increased contact between humans and waterborne pathogens.³

As the scientific community examines the impact of climate change on infectious disease spread, it is essential to understand 1) changes in the geographical distribution of vectors, 2) increases in human displacement and migration, and 3) changes in antibiotic resistance attributed to a warming climate.

Changes in the Distribution of Vector-Borne Diseases

Vector-borne diseases can be spread by blood-feeding arthropods including mosquitoes, ticks, and fleas. As the deadliest animal worldwide, the mosquito is responsible for killing more humans than any other creature.⁴  Mosquitoes survive at higher rates in warmer climates and are responsible for the spread of major global vector-borne diseases including Malaria, Dengue, Chikungunya, Yellow Fever, and Zika. Malaria causes the most deaths among vector-borne diseases, accounting for 620,000 deaths in 2017 – almost 15 times more than the second most deadly disease, Dengue Fever (with 40,500 deaths per year).⁵ Given the lethal spread of vector-borne diseases, significant research and resources have focused on endemic areas.  

An increase in mean global temperatures (estimated  4-5°C by the end of the century) could pose a grave threat to control measures against vector-borne diseases.⁵ With increasing temperatures, researchers predict that vector abundance, vector survival, and pathogen development rates within the vectors will increase.⁵ There is also a complex relationship between precipitation and vector abundance. Increased precipitation may result in more vector breeding sites, while droughts could provide more breeding sites due to increased use of rainwater collection and storage.⁶

Existing data on climate change effects on vector-borne diseases confirms the predictions outlined previously. A time-series analysis of monthly Malaria cases in Colombia and Ethiopia highlands showed a shift in the distribution of Malaria towards higher altitudes in warmer years.⁵ Furthermore, a study of Dengue Fever prevalence in Sri Lanka found the risk of Dengue Fever to be highest 6-10 weeks after precipitation of over 300mm per week.⁵ Vector-borne diseases place morbidity, mortality, and economic burdens on individuals and health systems – particularly in communities that do not have prior experience managing the diseases.⁶ 

Human Displacement Caused by Climate Change will Exacerbate Disease Spread

Human displacement due to extreme weather events will bring people closer together, thereby exacerbating infectious disease spread. Natural disasters including storms, droughts, and unpredicted rainfall can destroy agriculture and livestock, resulting in increased food insecurity and forced displacement.⁷ These same factors of food and clean water insecurity also aggravate conflict and conflict-related displacement. Former vice-president of the World Bank Ismail Serageldin predicted over 25 years ago that twenty-first century conflicts would be over water, instead of oil.⁶ Water insecurity can heighten existing conflict and forced displacement on a local level between communities as well as a national level, such as disputes over dam-building and water management projects.⁷

On average over the past decade, climate change has caused 23 million people to be displaced from their homes each year.⁸ The United Nations High Commissioner for Refugees (UNHCR) estimates that without significant changes, by 2050, 200 million people will require humanitarian assistance annually due to climate change.⁸ In 2020, 30.7 million people were internally displaced by disasters including droughts in Afghanistan, severe winter conditions in Mongolia, floods in Yemen, bushfires in Australia, and coastal risks in Samoa.⁹ In fact, climate disasters lead to three times as much displacement (30 million people) as displacement caused by conflict and violence (9.8 million people), despite the two being often closely interconnected.⁹

As individuals are displaced due to climate disasters, people are forced to live and seek refuge together, thereby exacerbating opportunities for disease spread. The resulting increased proximity of people will require comprehensive disease spread control measures and prevention. It is vital that during times of natural disaster and conflict, the spread of disease is mitigated among the most vulnerable communities.

Increased Rates of Antibiotic Resistance Attributed to a Warming Climate  

Multiple studies have shown that increasing local temperatures and population density can be attributed to an increase in antibiotic resistance among pathogens.¹⁰ These climatic factors are exacerbating the ongoing public health crisis of antimicrobial resistance driven by overuse of antibiotics.¹⁰ Growing resistance to antibiotics places innovative medicine and treatment options at risk, making them increasingly ineffective against deadly infections and viruses.¹⁰  Researchers estimate that mortality attributed to antimicrobial resistance could reach upwards of 10 million by 2050.¹¹

A study of three common bacterial pathogens – E. coli, K. pneumoniae, and S. aureus – examined 1.6 million bacterial pathogens spanning 41 states from 2013-2015.¹¹ The results of the study confirmed previous patterns of increased resistance among southern states.¹¹ Another study of antimicrobial resistance centered on country-level antibiotic resistance prevalence across 28 European countries, representing 4 million isolates.¹² The researchers found that between 2000-2016, antibiotic resistance against E. coli and K. pneumoniae increased more rapidly in countries with 10°C warmer relative ambient minimum temperatures.¹²

Mechanisms exacerbated by increased temperature include horizontal gene transfer, bacterial growth rates (bacteria growing more rapidly under warmer temperatures), and the environmental growth of resistant strains. Behavioral and social factors of increased gatherings could also contribute to increased antibiotic resistance.¹¹ As a result of these mechanisms, the collection of resistance genes is believed to become more diverse in warmer climates.¹² The long-term effect of increased temperatures on antibiotic resistance poses a great threat to public health worldwide, heightening the current crisis caused by overuse of antibiotics.¹²

The simultaneous rise of antibiotic resistance patterns and increased transmission of infectious diseases is exacerbated by our changing climate. As infectious diseases become more widespread due to changes in the distribution of vector-borne pathogens and human displacement, it will become increasingly important to have access to effective antibiotics. However, if the same underlying causes of increased disease spread further antibiotic resistance, the intersection of such crises will pose grave threats to public health systems.

Conclusion: Mitigating the Impacts of a Changing Climate

As the world confronts multiple health crises caused by climate change, it is vital to prepare for an increased spread of infectious diseases. For such preparation, one must understand the closely interconnected mechanisms through which disease spread will be exacerbated. While human displacement will increase proximity and disease spread, such interaction will also result in the spread of antibiotic-resistant genes. Similarly, an increase in vector-borne diseases due to warming temperatures will require the usage of antibiotics, leading to further opportunities for misuse and overuse of antimicrobials.  In order to mitigate the impacts of our changing climate, we must understand climate change and public health system differences on a geographic granular level. According to Dr. Erik Franklin, Professor at the University of Hawai’i at Mānoa, every geographic region will require a specialized priority.¹³ Future research must have a geographical approach in order to best inform localities of necessary prevention and healthcare measures.¹³ In closing, our response to the climate crisis and increased natural disasters must ensure specialized prevention and treatment of infectious diseases across geographic localities.

Lilia Potter-Schwartz is a sophomore in Pauli Murray College. She can be contacted at lilia.potter-schwartz@yale.edu.

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References

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13. Franklin, Erik C, and Lilia Potter-Schwartz. “Impact of Climate Change on Infectious Disease Spread.” 19 Dec. 2023.