This study examined the influence of various sanitation facility types and flood risk levels and their effect on diarrhea prevalence among children < 5 years of age in Bangladesh. Although previous studies have explored the effects of improved sanitation on health (Clasen et al. 2014; Knee et al. 2018; Patil et al. 2014; Sinha et al. 2017), this study added another categorization to sanitation facilities, focusing on transfer or storage of excreta in the environment. The study also focused on how the interactions between flood risk and sanitation facilities affect diarrhea prevalence. Previous research has focused on the influence of flood events or extreme weather on diarrhea prevalence in low- and middle-income countries (Carlton et al. 2014; Chowdhury et al. 2018; Dimitrova and Muttarak 2020; Hashimoto et al. 2014; Hashizume et al. 2008a, b; Milojevic et al. 2012). While these studies focused on specific events and regions, the current study focused on assessing flood frequency and severity using a national-level survey. This study showed how varying flood-prone levels influence diarrhea prevalence, depending on the type of sanitation facility available.
The spatial distribution of diarrhea prevalence highlighted regional inequalities. Prevalence was high in the southern coastal area and parts of the eastern area of Bangladesh. These results aligned with those of previous studies showing high malnutrition rates in southeastern regions, including Barisal, Chittagong, and Sylhet (Akram et al. 2018; Saha et al. 2019). As diarrhea prevalence is associated with growth stunting (Guerrant et al. 2013), this study also indicates that the southeastern and western regions may require additional measures to address child malnutrition and diarrhea. The northwestern regions exhibited a relatively low diarrhea prevalence, with the exception of the western part of the Rajshahi division; however, this area is prone to a severe drought risk and was thus excluded from the study (Figs. S1, S3, and S7).
The multilevel linear probability model showed no association between the JMP ladder in Model I and diarrhea prevalence with or without interactions; however, the interaction between unimproved facilities and MFP exhibited significant positive associations with diarrhea prevalence. In Model II, diffused sanitation type showed significant positive associations with diarrhea prevalence with interactions, whereas the interaction between diffused sanitation type and SFP showed significant negative associations with diarrhea prevalence. This result may indicate that diffused sanitation type reduces the possibility of contact with pathogens in SFP areas, as excreta are more likely to be flushed away by floodwaters. This result is consistent with the control of a case study conducted in Ecuador during the wet season (Bhavnani et al. 2014). These findings imply that while environmental factors in SFP areas may affect health, pathogen concentration in such areas is low, thus reducing the risk of possible pathogen contact.
Although flood-prone areas or floodplains provide suitable soils for agriculture, they are unstable and insecure environments for people who are forced to evacuate when water inundates the region (Brammer 2016). Presumably, the risk is reduced if households do not have facilities or hanging latrines, because feces are not concentrated in their living environment. Model II showed that LFP was negatively associated with diarrhea prevalence compared with NFP. This could be explained by climatological factors and the characteristics of pathogens that cause enteric infections. For example, cholera is highly prevalent during the wet season, especially during and after flood events; however, diarrhea caused by rotavirus is more prevalent than cholera when humidity and temperature are low (Bray et al. 2019; Jagai et al. 2012; Schwartz et al. 2006). NFP areas have a higher risk of diarrhea, especially in the dry season, and exhibit a higher prevalence than that in LFP areas.
This study showed that shared sanitation facilities were positively associated with diarrhea prevalence in both Models I and II, supporting the JMP ladder, which stated that shared sanitation facilities considered limited sanitation services rather than improved services (UNICEF and WHO 2019). Studies examining the Mapsan trial have shown that improving shared sanitation facilities prevents human fecal contamination in target communities (Holcomb et al. 2020, 2021). The diarrhea prevalence rate among households with children < 5 years of age and shared sanitation facilities was significantly higher than that in households with non-shared facilities (Fuller et al. 2014). Furthermore, a previous study applied a mathematical model with fictitious pathogens and communities, demonstrating that high coverage of shared sanitation facilities increases the likelihood of contact with infected people and the risk of enteric infections (Just et al. 2018). Although numerous factors, such as socioeconomic status and caretaker education level, should also be considered (Heijnen et al. 2014), the result emphasizes that shared sanitation itself is a key risk factor resulting in adverse health outcomes.
Although the result of this study contradicted the JMP ladder criteria, some studies also showed mixed results regarding improved sanitation. For example, the Total Sanitation Campaign, one of the largest intervention projects conducted in India that aimed to end the practice of open defecation, revealed that installing sanitation facilities had no significant effect on reducing diarrhea prevalence despite decreasing the frequency of open defecation (Patil et al. 2014). Another intervention conducted in rural areas of Kenya and Bangladesh, also known as WaSH Benefits, showed different results. Contreras et al. (2022) showed that individual sanitation interventions (such as pit latrine installation, providing tools to clean child feces, and promoting behavioral change) reduced diarrhea prevalence, whereas Pickering et al. (2019) found that only integrated interventions involving drinking water, sanitation facilities, and nutrition effectively reduced the risk of helminth infection. Another community-level study, focusing on hotspot districts of diarrhea prevalence in children, reported increased rates in communities with low ratios of improved sanitation facilities (Azage et al. 2016). To summarize, community environmental and meteorological factors should also be considered.
Geological factors may be another key element. In this study, the interactions between flood risk level and sanitation type were significantly associated with diarrhea prevalence. In Model II, SFP areas had a significant positive association with diarrhea prevalence, but the interaction between SFP and diffused sanitation type had a significant negative association. This result may be explained by geological features in Bangladesh, which is located in the Bengal Delta, comprising the Ganges and Brahmaputra-Jamuna Deltas and the Sylhet Basin (Mukherjee et al. 2009; Umitsu 1987). As it is located in a coastal region and an alluvial lowland, the Bengal Delta experiences river flooding as well as flooding due to tidal surges (Umitsu 1987; Wesselink et al. 2015). As this alluvial floodplain contains sandy silt or sand, any infrastructure installed in this environment carries a high risk of inundation, destruction, or deterioration due to unstable sediments. In this environment, sanitation systems such as pit latrines, which collect and store fecal sludge in vaults for years, may not be safe, as fecal sludge can contaminate the surrounding aquifers in rural areas (Islam et al. 2016; Ravenscroft et al. 2017) or surface floods in urban areas (Jenkins et al. 2015). Unlike improved sanitation facilities, open defecation, or bucket latrines, the diffused sanitation type defined in this study may not contaminate the surrounding aquifer, because it does not collect sludge at a specific place, which in turn may avoid pathogen concentration. Similar results were observed for Model I—MFP had significant positive associations with diarrhea prevalence but a negative association when interacting with unimproved sanitation types. Further studies are needed; however, improved sanitation facilities did not exhibit a clear association with the reduction in diarrhea prevalence in flood-prone areas.
This study showed an increased risk of diarrhea in MFP and SFP areas, where prevalence was not reduced by improved or concentrated sanitation systems. However, this does not mean that unimproved sanitation, such as pit latrines without slabs or open defecation areas, presents a better option. A potential future system may involve fecal sludge emptying from storage vaults at specific intervals, followed by transfer and appropriate treatment. The effectiveness of implementing alternative sanitation systems, such as “constructed wetland system (CWS)” or “ecological sanitation toilets,” has been discussed, especially in low- and middle-income countries (Bydałek and Myszograj 2019; Jehawi et al. 2020; Langergraber and Muellegger 2005). In Bangladesh, the Reed Bed System, a CWS, has been proposed (Biswas 2014) and has recently been introduced to Rohingya camps (Kabir et al. 2020; Saeed et al. 2022). In this system, fecal sludge can be emptied, transferred, and treated appropriately and safely. Other advantages of these systems include their low cost and potential as a fertilizer source for agricultural use (Kabir et al. 2020). Examining and expanding alternative systems, including those mentioned previously, in flood-prone areas will contribute to the development of appropriate sanitation systems in Bangladesh.
This study has several limitations. Firstly, the geodata for each cluster in all BDHS results were randomly displaced from their original location to maintain respondent confidentiality. Geodata could be displaced outside the district but could not cross divisional boundaries. The choropleth map was created at the district level; therefore, some clusters might have been displaced outside their original district. Secondly, because diarrhea episodes were based on caregivers’ self-reports, they were subject to recall bias.