The association between cholera and weather: a systematic review and direction-of-effect meta-analysis

BackgroundCholera is often used as a model of the effect of climate and weather on infectious diseases. Yet the empirical evidence remains inconsistent, and the few existing reviews have not provided a synthesis of evidence nor systematically assessed sources of heterogeneity. Our objective was to address both of these gaps. MethodsWe conducted a systematic review on the association between cholera and precipitation, temperature, floods and drought. PubMed, Scopus, Global Health, and ProQuest were searched to identify studies before 18 May, 2023. We extracted data on exposures, outcomes, and analytical methods, along with contextual study factors, and assessed study reporting completeness. Evidence synthesis focused on the direction of association using the vote counting method, and we performed a direction-of-effect meta-regression analysis to attribute variability to contextual factors. FindingsWe included 60 studies spanning 2000-2020. Study characteristics were highly heterogeneous, with a geographical bias towards Asia (Bangladesh) vs. Africa. Only 23/60 studies had full reporting completeness scores with the most frequent limitation being lack of information on the epidemiological outcome. Among all studies, we found a positive association between cholera incidence and precipitation (p < 0.001), temperature (p < 0.001) and floods (p 0.021). When subsetting to studies with complete reporting no association remained statistically significant. The variability in association direction was primarily attributable to study-level variability, and not to contextual factors. InterpretationThe current landscape of evidence on the association between cholera and weather patterns is fragmented and suffers from important geographical biases and methodological limitations. Climate effects on cholera should therefore be systematically evaluated and discussed in the context of other known drivers of cholera risk. There is an important opportunity for future studies with transparent reporting, improved data quality, and explicit consideration of contextual and epidemiological factors to generate reliable evidence to guide climate-related cholera forecasts and mitigation approaches. FundingThis study was funded by the Welcome Trust. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSWe conducted a PubMed search for previous reviews on August 16, 2024, with no language or date restrictions using the search terms ["cholera*" AND ("climate" OR "weather") AND ("Systematic review" OR "narrative review" OR "science review")]. Of the 20 reviews identified, only six examined the associations between cholera and the key weather exposures (precipitation, temperature, flood, and drought). Among these, two provided global coverage, three focused on Africa, and one on Asia. Two articles assessed all four weather exposures, while others addressed only one or two. All six reviews qualitatively summarized the association effects between weather exposures and cholera, but none synthesized the association effects systematically (e.g., the direction of the association effect). Findings across reviews were heterogeneous, with variations by setting (e.g., inland versus coastal regions). Notably, none of the reviews systematically evaluated study quality nor completeness of reporting, and only one assessed the statistical methods used to estimate associations. Added value of this studyThis study addresses a critical gap by providing a comprehensive synthesis of the evidence linking key weather exposures to cholera outcomes, including a quantitative assessment of the direction of association effects. Additionally, this study investigates the sources of heterogeneity. We found that most of the included studies focused on Asia and Africa, with a bias towards Bangladesh (Asia), and spanned over the period from 2000 to 2020. Study characteristics, including the spatio-temporal coverage, cholera outcome definitions, and study reporting completeness, as well as the analytic methods varied widely across different studies. Consistent with previous reviews, we observed substantial heterogeneity in effect estimates. In evidence synthesis we found a statistically significant positive association among all studies between cholera incidence and precipitation, temperature and floods, but not in studies with full reporting completeness scores. Our meta-analysis further suggested that the observed variability in directions of associations was primarily driven by within-study error variance and between-study differences, rather than contextual or epidemiological factors. Implications of all the available evidenceCurrent evidence on weather and cholera associations is highly inconsistent due to geographical biases, methodological heterogeneities and data limitations. This review highlights the need for future studies on the association between cholera and weather in cholera-affected countries that have been relatively less investigated, using improved cholera epidemiological data based on standardized definitions following established international guidelines, with transparent reporting on exposures, outcomes and analytical methods, and accounting for epidemiological and contextual factors.