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Untangling the impacts of climate change on waterborne diseases: A systematic review of relationships between diarrheal diseases and temperature, rainfall, flooding, and drought Karen Levy, Andrew P Woster, Rebecca S Goldstein, and Elizabeth J Carlton Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06186 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016
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Untangling the impacts of climate change on waterborne diseases: A systematic review of
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relationships between diarrheal diseases and temperature, rainfall, flooding, and drought
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Karen Levy* 1, Andrew P. Woster2, Rebecca S. Goldstein1, Elizabeth J. Carlton2
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Atlanta, Georgia, USA
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University of Colorado, Aurora, Colorado, USA
Department of Environmental Health, Rollins School of Public Health, Emory University,
Department of Environmental and Occupational Health, Colorado School of Public Health,
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*
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University Rollins School of Public Health, 1518 Clifton Road NE, Atlanta, GA 30322.
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Telephone: 404.727.4502. Fax: 404.727.8744. E-mail:
[email protected] Address correspondence to: Karen Levy, Department of Environmental Health, Emory
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ABSTRACT
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Global climate change is expected to affect waterborne enteric diseases, yet to date there has been
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no comprehensive, systematic review of the epidemiological literature examining the relationship
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between meteorological conditions and diarrheal diseases. We searched PubMed, Embase, Web
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of Science and the Cochrane Collection for studies describing the relationship between diarrheal
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diseases and four meteorological conditions that are expected to increase with climate change:
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ambient temperature, heavy rainfall, drought, and flooding. We synthesized key areas of
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agreement and evaluated the biological plausibility of these findings, drawing from a diverse,
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multidisciplinary evidence base. We identified 141 articles that met our inclusion criteria. Key
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areas of agreement include a positive association between ambient temperature and diarrheal
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diseases, with the exception of viral diarrhea; and an increase in diarrheal disease following
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heavy rainfall and flooding events. Insufficient evidence was available to evaluate the effects of
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drought on diarrhea. There is evidence to support the biological plausibility of these associations,
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but publication bias is an ongoing concern. Future research evaluating whether interventions,
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such as improved water and/or sanitation access, modify risk would further our understanding of
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the potential impacts of climate change on diarrheal diseases and aid in the prioritization of
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adaptation measures.
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KEYWORDS
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Climate, Weather, Diarrhea, Climate Change, Ambient Temperature, Heavy Rainfall, Flooding,
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Drought, Social Vulnerability
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INTRODUCTION
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Global climate change has the potential to severely impact human health worldwide, including
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diarrheal diseases,1 which are highly affected by environmental drivers such as water
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availability.2 The Intergovernmental Panel on Climate Change (IPCC) predicts that global
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surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to the
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period from 1850-1900, and in some scenarios is likely to exceed 2°C. Changes in the global
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water cycle also are expected, with increasing contrast in precipitation between wet and dry
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regions and between wet and dry seasons, although there may be regional exceptions.3
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Because of the projected impacts of climate change on hydrological systems, waterborne enteric
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diseases are among the primary expected health impacts of climatic shifts.1, 4, 5 Understanding the
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impact of meteorological phenomena on enteric diseases is critical because even relatively small
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proportional increases in risk for diarrhea represent substantial overall impacts to the global
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burden of disease. Diarrheal diseases account for 10-12% of all deaths in children under five
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years old,6, 7 and an estimated 1.4-1.9 million deaths worldwide.7, 8 In addition to mortality,
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diarrheal disease can impair growth and cognitive development, and increase susceptibility to
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other infectious and chronic diseases,9 which may also exacerbate individual and community
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vulnerability to climate change.
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Despite the importance of this topic, to date there has been no comprehensive, systematic review
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of the literature examining the relationship between climatic variables and patterns of diarrheal
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diseases. Other authors have reviewed associations between temperature and all-cause
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diarrhea,10, 11 meteorological variables and specific diarrheal pathogens,e.g., 12, 13-16 extreme
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weather events and waterborne diseases,17 and climatic influences on pathogens in the
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environment.18 However, these have been non-systematic reviews, focused on only one
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pathogen, and/or focused on specific geographical regions only. Thus, there is a need to review
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and synthesize the body of data that has accumulated on this topic, using rigorous methods
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following established guidelines,19-21 to better understand and interpret the epidemiological
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patterns reported in the literature, inform climate change adaptation and mitigation measures, and
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direct future research on the topic.
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In this paper, we present the results of a systematic review of epidemiological associations
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between diarrhea and ambient temperature, heavy rainfall, flooding, and drought. We chose
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these meteorological conditions because of the strong evidence that these conditions are
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increasing due to climate change, and because of prior publications suggesting associations with
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diarrheal diseases. We summarize key areas of agreement, as well as gaps in the literature. In a
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related paper we carried out a meta-analysis of the subset of these studies specifically related to
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ambient temperature and diarrhea.22 Here we also evaluate the biological plausibility of each key
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finding, presenting a conceptual framework that describes mechanisms to explain the observed
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exposure-response relationships. We conclude by providing examples of how the insights from
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this conceptual framework can be applied, including in the evaluation of adaptation strategies.
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METHODS
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Systematic Search
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Articles were identified through a comprehensive search of the literature and by reviewing
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references from 54 review articles identified through the search process. Briefly, we searched
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PubMed, Embase, Web of Science, and The Cochrane Collection on 26 November 2013 for the
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health outcome terms “diarrhea*” or “diarrhoea*”, paired with climate or meteorological terms:
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“climate change”, “temperature”, “rain*”, “precipitation”, “flood*”, “drought*,” and "sea surface
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temperature".
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Two independent reviewers considered all articles for inclusion. Articles were deemed to meet
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inclusion criteria if they included: 1) human health outcome data; 2) all-cause diarrhea, pathogen-
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specific diarrhea, gastroenteritis, or diarrhea and vomiting as outcomes; and 3) temperature,
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heavy rainfall, flooding, or drought as exposures of interest related to rates of diarrheal diseases.
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Only English-language articles were included. Case reports were excluded. Meta-analyses and
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reviews were included if they provided a novel analysis of the data from the studies included.
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We decided a priori to focus on articles evaluating heavy rainfall rather than average rainfall
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because of previous work suggesting that this is the primary rainfall exposure influencing rates of
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diarrheal diseases.23-25 While our search originally included sea surface temperature, we decided
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to exclude this category because all 16 articles identified focused on cholera, limiting the
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generalizability of such findings to other diarrheal diseases. Duplicate reports were handled by
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selecting the study with the most complete dataset and analysis. Following initial article review,
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the two reviewers discussed and resolved differences in inclusion decisions, consulting with the
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other study authors as necessary. Figure 1 shows the PRISMA diagram19 of the selection process.
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For each article that met our inclusion criteria, one reviewer extracted relevant data (including
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exposure, outcome, location of study, study duration, study population) and at least one other
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person verified the extracted information.
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Evaluating the quality of the evidence
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We used three approaches to assess the quality of the evidence: 1) evaluating the weight of
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evidence, including study design and strength and consistency of observed relationships; 2)
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evaluating risk of bias; and 3) exploring biological plausibility of the relationships.
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Weight of Evidence. Studies were classified as quantitative epidemiological studies, non-
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quantitative epidemiological studies or outbreak reports. A study was classified as quantitative if
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the authors presented the results of a statistical analysis. To account for potential publication bias,
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we did not require studies to present actual point estimates, allowing us to include studies that
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reported whether or not significant associations were detected by the particular analytical method
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applied. When studies presented the results of multiple analyses, we followed the Cochrane
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guidelines20 and selected the analysis that the authors indicated as the final or main analysis
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(typically a fully adjusted model), and did not include supplementary exploratory studies.
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However, when analyses were stratified by exposure, location, or another risk factor, and a single
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unified estimate was not presented, we included each of the stratified analyses in our summary.
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We limited temperature studies to those that examined associations between temperature and
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diarrheal diseases at a one-month or finer resolution.
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Quantitative studies were classified as showing positive or negative associations if the analysis
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allowed rejection of the null hypothesis (p
