Diseases and climate change

Climatic conditions that support the successful colonization of geographical locations by human societies also tend to support the populations of pests and pathogens associated with human diseases. Abrupt climate change can destabilize trends in the distribution of diseases in populations, as well as society’s ability to cope with emerging pathogens and shifting demographic patterns.

Background

The most important factor in the emergence and proliferation of pathogens is the availability of susceptible hosts. Therefore, many pathogens have co-evolved, not only with human biological constraints against disease, but also with socially developed constraints such as climate-controlled domiciles and disinfection. In the event of abrupt climate change, leading to excessive fluctuations in extreme weather conditions, there occurs a selection process that affects the microbial diversity of ecosystems, with some organism declining, whereas other organisms increase in population density. In addition, long-term climate change may forge new interactions among different organisms. When these biodiversity changes coincide with increasingly dynamic relocation of people in response to climatic events, epidemics can result from the resurgence of old diseases, emergence of new diseases, or exacerbation of preexisting disease conditions.

Emergence of Climate Change as a Threat to Public Health

Seasonal trends in morbidity and mortality have long been understood by human societies. Such understandings have formed the basis of preventive health care plans in many countries. For example, preparation for the influenza (flu) season means massive vaccination campaigns during the months of September, October, and November. Similarly, respiratory conditions with no clear involvement of pathogenic agents, such as asthma and allergies, are known to follow seasonal patterns. Humans have, more or less, adapted to such seasonal inconveniences until they become so extreme to the extent that population migrations can occur. Outbreaks of contagious disease associated with scarce water supplies can destabilize communities or force local extinctions in human habitats. However, it has not been straightforward to project health impacts as part of the consequences of anthropogenic climate change.

In 1990, a World Health Organization (WHO) task group issued one of the early reports on the potential health effects of climate change. The group based its assessments on the scenario that global average temperature could increase by 3° Celsius by the year 2030; that sea level could rise by 0.10-0.32 meter; and that ultraviolet radiation, mainly UV-B, is expected to increase by a maximum of 20-25 percent in the same period. Based on these conditions, the task group anticipated both direct and indirect effects of climate change on human health.

The direct effects include those associated with thermal factors (heat disorders) and the effects of UV radiation on the incidence of skin cancer, immune response, eye function, and air quality. Indirect effects of climate change on human health are expected to include impacts on food production and nutrition, on wildlife and biodiversity, and on communicable diseases through effects on disease vectors and the incidence of infectious diseases that are not associated with specific vectors. In addition, indirect impacts of climate on health include the repercussions of human migration.

In 1997, responding to a request from the Subsidiary Body for Scientific and Technological Advice (SBSTA) of the United Nations Framework Convention on Climate Change (UNFCCC), Working Group II of the Intergovernmental Panel on Climate Change (IPCC) published a special report on the assessment of vulnerability in the regional impact of climate change. The Conference of the Parties (COP) to the UNFCCC needed information on the degree to which human conditions and the natural environment are vulnerable to the potential effects of climate change, but the regional assessment approach adopted by the IPCC revealed wide variation in the vulnerability of different populations, especially in the health sector. Different levels of vulnerability exist under similar climate and pathogen distribution patterns because of local economic, social, and political conditions, as well as the level of dependence on resources sensitive to climate variability. Therefore, instead of producing quantitative predictions of the impacts of climate change for each region, the IPCC took the approach of assessing regional sensitivities and vulnerabilities.

The adoption of “Weather, Climate, and Health” as the theme of the 1999 World Meteorological Day signified the convergence of global issue-framing strategies with the health impacts of climate change. This event emerged after more than a decade of policy formulation and scientific assessment activities by the WHO, the IPCC, and the World Meteorological Organization (WMO). Following the progress made by WHO researchers during the 1990s toward the development of quantitative methods for assessing the global burden of disease, it became possible to compare or project into the future the disease burden associated with specific risk factors such as climate change. Composite measures of disease burden such as disability-adjusted life years (DALYs, or the sum of years of life lost to premature death and years of life lived with disability) account for both mortality and morbidity, and are particularly suitable for evaluating risk factors with a broad range of disease end points. For example, in 2004, the WHO estimated that global climate change accounts for approximately 5.5 million DALYs lost directly, but exacerbation of disease conditions associated with the creation of unsanitary conditions could result in a lot more DALYs lost. Not surprisingly, children younger than five years are particularly vulnerable.

To cap the evolution of health effects as a dominant frame of reference for the threats associated with climate change, in May 2008, the 193 member countries represented at the World Health Assembly adopted a resolution to protect public health from impending global climate change. This event signaled a much higher level of commitment from the health sector to strengthen the evidence for anthropogenic climate change and to better characterize the risks to public health at the regional and global levels.

Diseases Associated with Climate Change

COMMUNICABLE DISEASES. Vector-borne diseases, such as malaria, have dominated research on the impacts of climate change on public health. The rationale behind these studies is that increases in temperature and rainfall would support the proliferation of mosquito vectors and their ability to incubate disease-causing protozoa, leading to more infections. According to the WHO, there were 212 million new cases of malaria reported worldwide in 2015, with an estimated 429,000 deaths. Although the rate of morbidity and mortality associated with malaria in endemic zones might intensify, the real fear associated with climate change is that malaria zones will expand toward the temperate regions that have hitherto been free of the parasite. There is spotty evidence of recent incidences of malaria in Europe and North America, but it is not clear that these cases are not associated with population migration, which has led to the coining of the phrase “airport malaria.” Nonetheless, a Roll Back Malaria initiative was launched in 1998 by the WHO, the United Nations Children’s Fund (UNICEF), the United Nations Development Programme (UNDP), and the World Bank to provide a coordinated global approach against malaria, including scenarios associated with the influence of climate change. As a result of efforts to fight malaria, incidence rates fell 21 percent from 2010 to 2015. However, the IPCC predicted in 2007 that under certain climate change scenarios, the global population at risk from vector-borne malaria will increase by between 220 million and 400 million in the next century.

Other vector-borne diseases that are of concern with respect to climate change include lymphatic filariases, which are also transmitted through tropical mosquitoes, typically in urban slums. The geographical zone of these diseases may expand with increasing average global temperature, but good urban planning and hygienic conditions can limit the impact of the diseases on society. This scenario is expected to be similar for other climate-sensitive vector-borne diseases such as onchocerciasis (vector: African black fly, Simulium damnosum), schistosomiasis (vector: water snails such as Biomphalaria glabrata), African trypanosomiasis (vector: tsetse flies, Glossina palpalis gambiensis), leishmaniasis (vector: sandfly, Phlebotomus species), and dracunculiasis (vector: waterborne copepods such as Mesocyclops leuckarti).

Incidences of bacterial diseases that are transmitted through ticks and body lice (tick-borne relapsing fever caused by several species of spiral-shaped bacteria; tularemia, caused by Francisella tularensis; and louse-borne relapsing fever, caused by Borrelia recurrentis) are also considered to be sensitive to climate change, primarily because of the well-defined ecological conditions that support the proliferation of the vectors. Arboviral diseases also represent a major category of potentially climate-sensitive communicable diseases that can change from endemic to epidemic forms, given favorable environmental conditions. These diseases include dengue/hemorrhagic fever (caused by Flavivirus), Rift Valley fever (Phlebovirus), and Japanese encephalitis and St. Louis encephalitis (caused by viruses in the family Flaviviridae).

Finally, waterborne diseases that are not clearly associated with vectors have also been linked to climate change. Diarrheal diseases are at the forefront in this category that includes bacterial (for example, Vibrio cholera) viral (for example, Norwalk virus), and protozoan (for example, amebic dysentery) causes. Together, these diseases account for a large portion of the global burden of diseases that disable or kill children younger than five years in developing countries. The association with climate change is that in cases of drought, people tend to use contaminated sources of water, and in the absence of reliable disinfection programs, the incidence of these diseases will increase. In addition, natural disasters such as floods, hurricanes, and earthquakes can damage water supply and sewage treatment infrastructures in developed countries, leading to the contamination of potable water supplies that can increase the incidence of waterborne diseases. Hence emergency public health preparedness is a major category of planned adaptation to climate change.

Scientists warned by 2024 that the weather-related changes from global climate change were beginning to show a significant increase in vector-borne diseases. Warmer winters allowed vector-borne diseases a greater portion of the year to multiply, while the shift to warmer, wetter weather in many environments expanded the diseases' range. This effect has already been observed in babesiosis and Lyme disease, and scientists expect it to grow more severe in the coming decades.

Heat-related diseases are the most researched category of noncommunicable diseases that have been associated with climate change. During the summer season, the frequency of extreme heat waves is predicted to increase. For example, the IPCC predicted in 2007 that the cities of Chicago and Los Angeles will experience up to 25 percent more frequent heat waves and a fourfold to eightfold increase in heat wave days by the year 2100. Based on current estimates of morbidity and mortality associated with prolonged periods of extreme heat, people with preexisting conditions such as heart problems, asthma, the elderly, the very young and the homeless will be more vulnerable. In contrast, it is also likely that under certain climate change scenarios, warmer temperatures will prevail during the winter months, leading to fewer cases of death and disability from hypothermia.

Climate change is also expected to adversely affect air quality, especially in urban areas where higher temperatures may increase the concentrations of respirable (smaller than 2.5 micrometers) and the concentration of tropospheric ozone, which can be especially dangerous for people suffering from asthma and other chronic pulmonary diseases.

Context

Motivating action around climate changes requires framing the issue in ways that command attention. Linking climate change to public health impacts continues to be one of the most cogent framings that have engendered research and policy questions about societal preparedness and adaptation. Ultimately, morbidity and premature mortality represent the crucial end points of most scenarios of climate change impacts. Most people are afraid of contracting diseases that were previously unknown in their communities, or for which there are no known cures. Many of the tropical diseases associated with climate change fall in these fearsome categories. However, it is also becoming increasingly clear that many of the diseases associated with climate change are preventable through well-known public health approaches, but these approaches require economic resources that may not be available to the most vulnerable populations across the world. Therefore, the roles of international organizations such as the WHO and its supporting agencies are crucial in global assessments of disease burden and future projections of climate-sensitive diseases, and in building capacity for adaptation in vulnerable societies.

Key Concepts

• air quality: normal atmospheric constituencies, such as levels of particulate matter, elements, and toxins
• disability-adjusted life years (DALYs): a time-based quantitative measure of the burden of disease in a population that combines years of life lost to premature mortality and years of life lost to poor health or disability
• heat wave: a long period of exceedingly hot, uncomfortable weather
• pathogens: viruses, bacteria, protozoa, or other chemical or biological agents that can infect a human host to produce disease
• vector-borne diseases: illnesses associated with pathogenic microorganisms whose transmission from an infected host to a new host is mediated by an insect or other agent (vector)
• waterborne diseases: illnesses caused by pathogens that are transmitted through contaminated drinking water or contact with environmental waters

Bibliography

Campbell-Lendrum, Diarmid, and Rosalie Woodruff. Climate Change: Quantifying the Health Impacts at National and Local Levels. Environmental Burden of Disease 14. Geneva: World Health Organization, 2007.

Intergovernmental Panel on Climate Change. Climate Change, 2007—Impacts, Adaptation, and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Martin Parry et al. New York: Cambridge University Press, 2007.

Ogunseitan, Oladele A. “Framing Environmental Change in Africa: Cross-Scale Institutional Constraints on Progressing from Rhetoric to Action Against Vulnerability.” Global Environmental Change 13 (2003): 101-111.

Yehya, Nadine A. "Experts Warn Climate Change Will Fuel Spread of Infectious Diseases." UC Davis Health, 20 Mar. 2024, health.ucdavis.edu/news/headlines/experts-warn-climate-change-will-fuel-spread-of-infectious-diseases-/2024/03. Accessed 12 Dec. 2024.