Water, sanitation and hygiene risk factors for the transmission of cholera in a changing climate: using a systematic review to develop a causal process diagram

Cholera is a severe diarrhoeal disease affecting vulnerable communities. A long-term solution to cholera transmission is improved access to and uptake of water, sanitation and hygiene (WASH). Climate change threatens WASH. A systematic review and meta-analysis determined ﬁ ve overarching WASH factors incorporating 17 speci ﬁ c WASH factors associated with cholera transmission, focussing upon community cases. Eight WASH factors showed lower odds and six showed higher odds for cholera transmission. These results were combined with ﬁ ndings in the climate change and WASH literature, to propose a health impact pathway illustrating potential routes through which climate change dynamics (e.g. drought, ﬂ ooding) impact on WASH and cholera transmission. A causal process diagram visualising links between climate change dynamics, WASH factors, and cholera transmission was developed. Climate change dynamics can potentially affect multiple WASH factors (e.g. drought-induced reductions in handwashing and rainwater use). Multiple climate change dynamics can in ﬂ uence WASH factors (e.g. ﬂ ooding and sea-level rise affect piped water usage). The in ﬂ uence of climate change dynamics on WASH factors can be negative or positive for cholera transmission (e.g. drought could increase pathogen desiccation but reduce rainwater harvesting). Identifying risk pathways helps policymakers focus on cholera risk mitigation, now and in the future. Cholera is a severe diarrhoeal disease affecting vulnerable communities.

INTRODUCTION ineffective in children under 5 years (Bi et al. ; Lessler et al. ). Also, there are insufficient supplies of vaccine for population-wide administrations (Lessler et al. ).

METHODS
The first step in our analysis was to undertake a systematic review on cholera and WASH to identify the key WASH factors associated with cholera transmission. A paucity of the literature on climate change and cholera meant it was not possible to directly review this relationship, and thus, a two-stage process was undertaken. Full details of the systematic review including detailed methods and results can be found in Supplementary file 1. In summary, using Scopus, Science Direct, Medline, and six grey literature sources, 37 search terms associated with WASH were combined with cholera (see Supplementary Table S1). Initially, 8,410 papers were retrieved; after screening and eligibility assessment, 53 papers were selected for qualitative synthesis and meta-analysis (see Supplementary Figure S1). WASH factors were categorised as follows: (1)  the key mechanisms that can drive health outcomes and the points where it may be possible to break the flow of impact. These pathways are useful for clarifying the association between risks and health impacts (Few ). In this study, a health impact pathway was developed during a 90-minute focussed discussion between three paper authors ship), as found in the systematic review is also shown. In this study, a lower odds ratio (less than one) for a WASH factor and cholera transmission from the systematic review is indicative of a more protective relationship on the causal process diagram, whilst a higher odds ratio (greater than one) is indicative of a more risky relationship. Table 1 shows the pooled odds ratios for cholera risk factors taking into account multiple studies within the same paper.

RESULTS AND DISCUSSION
The findings showed that boiling water, chlorinating water, sourcing municipal/piped water with no waterborne outbreak (water which has no cholera epidemic associated with it), and rainwater all showed lower odds of cholera transmission. Similarly, all handwashing and specific handwashing before food and after defaecation also showed lower odds, as did using soap. However, drinking untreated water, using surface water as a water source and well water as a water source, showed higher odds of cholera transmission. Open defaecation and shared sanitation facilities also showed higher odds of cholera transmission, as did storing water in containers that were not narrow-mouthed. The   Table 1) as being important in the transmission of cholera. The dynamics of climate change can potentially impact all of these in varying ways. In Water treatment: Untreated water. Within the metaanalysis, drinking untreated water was shown to have higher odds of cholera transmission (OR 2.8; 95%CI 1.82-4.29). There exists a desire to end the consumption of untreated water worldwide (United Nations ); however, there is potential for the dynamics of climate change to impact on this. Evidence suggests that improved water supplies will be negatively affected by the dynamics of climate change such as flooding, extreme rainfall, drought, increased temperatures, and sea-level rise (World Health     for which of these is more common is unclear, as some studies suggest more people wash their hands before preparing and eating food than after using the toilet (Phillips et al.

)
, while others show washing hands after using the toilet is more common than before preparing and eating food (Luby et al. ). Therefore, it is not possible to determine which may be more affected with reduced water availability.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wh.2020.088.