This study shows that there was significant association of hospital visits due to non-cholera diarrhoea with high and low rainfall and with high temperature in Dhaka, Bangladesh. The effect of temperature on the incidence of non-cholera diarrhoea was higher for people with lower educational attainment, those living in the household with non-concrete roof and unsanitary toilet users. The effects of rainfall were not differential by any socio-economic status or hygiene and sanitation practices.
The high rainfall association is broadly consistent with a study in Fiji reporting that monthly diarrhoea incidence in infants increased with increased rainfall after allowing for the effects of long-term trends and seasonal patterns.13 In that study, high rainfall was associated with significant increases in diarrhoea in the same month but decreased in the following month suggesting that initially high rainfall flushes faecal contaminants from pastures and dwellings into water supplies, but continued rain leads to a subsequent improvement in water quality. In the present study, however, no consistent protective effect of high rainfall was observed in any lag periods by detailed analysis of lag structure. The current study is also broadly in accordance with a US study reporting that waterborne disease outbreaks were preceded by heavy rainfall within a 2-month lag.14 However, these studies were conducted in regions which are climatologically and geographically very different from Dhaka, and careful interpretation is needed. Causative agents of diarrhoea are also likely to be different in Dhaka as compared with these regions.
This study found that the river level explained nearly all the associations between high rainfall and the incidence of diarrhoea, suggesting that factors associated with the river level are on the causal pathway between high rainfall and diarrhoea. Another study in Dhaka indicated adverse effects of flood on the number of non-cholera diarrhoea cases that was higher for tube well users, those using distant water sources and unsanitary toilet users as compared with tap-water users, those using a close water source and sanitary toilet practices (unpublished data). These findings suggest that heavy rains leading to excessive flooding break down water and sanitation systems and promote the intake of contaminated drinking water, although this study did not find any evidence for the modification of the rainfall effect by water source. Investigations on detailed pathways of the rainfall–diarrhoea relationship, particularly the role of drinking water quality are warranted.
The low rainfall effect found in this study was also broadly consistent with results of the time-series study in Fiji, which found that low rainfall was significantly associated with increases in diarrhoea in the same month and the following month.13 A possible explanation is the lack of dilution of sewage effluent and increased contamination of pond and lake water scattered amongst the communities in Dhaka and water used for washing and bathing, through stagnation during low rainfall. Transmission of enteric pathogens through the faecal–oral route could increase in overcrowded areas with relatively unsanitary practices due to low rainfall.
The positive linear relationship between non-cholera diarrhoea and temperature in this study is broadly consistent with previous studies in Peru and Fiji.3,13 This finding is also biologically plausible through higher temperatures promoting the growth of bacteria, although some enteric viruses have been suggested to increase survival and transmission under lower temperatures.15,16 Consistent with this is the finding in our analysis without the inclusion of rotavirus diarrhoea of a slightly larger effect of temperature.
A new finding of this study is that the effect of temperature on non-cholera diarrhoea incidence is higher for people with lower educational attainment, those living in the household with non-concrete roof and unsanitary toilet users. Weak evidence for modification of the temperature effect by distance to drinking water source from the kitchen was observed. Educational attainment and household construction material can be robust indicators of socio-economic status in Bangladesh, and people in lower socio-economic status are, in general, at a higher risk of suffering from diarrhoea.17
We have reported results for all non-cholera diarrhoea, though associations between rainfall or temperature would not necessarily be the same for different pathogens, as indicated by their different seasonal patterns. Separate analyses proved problematic for two main reasons. Firstly, the number of cases for any specific pathogen was small, so patterns were unclear. Secondly, a high proportion (29.5%) of cases had more than one pathogen identified, making classification of the underlying cause of the visit difficult. Therefore, the results of this study should be interpreted with caution in this regard, although the analyses without the inclusion of rotavirus diarrhoea, which is most likely to differ from the rest of the pathogens in its relationship with climate, provided the results largely unchanged. Further studies for the effect of climate on pathogen-specific diarrhoea are warranted.
Less severe cases would be less likely to be included, but this does not pose a threat to validity of the comparisons over time, which is the subject of this study. More problematic would be cases missing because of limitations in the capacity of the hospital to receive the patients, in particular during epidemics of diarrhoeal diseases. However, in principle, the hospital accepts all patients visiting the hospital and has never refused patients due to over capacity. Thus, the capacity of the hospital should not be an important threat to this study. If a bias remained, it would act to ‘blunten’ peaks, and thus most likely bias associations towards the null.
In this study, rainfall and temperature were found to explain departure of the number of diarrhoea cases from the usual seasonal pattern. This does not mean that these factors can explain the usual seasonal patterns themselves. Further work could clarify the role of weather in the seasonality of diarrhoea in Bangladesh and would be of interest.
The results of this study can contribute to development of early warning systems to predict epidemics of diarrhoea. The vulnerable groups identified in this study should have particular focus in the use of such a warning system. Expected increases in temperature, changes in precipitation patterns and increased flooding in Bangladesh18 also give particular relevance to the results, though the short-term associations reported here should not be directly extrapolated to changes in climate over decades.
In conclusion, this study found evidence that the number of non-cholera diarrhoea cases increases both above and below a threshold level with high and low rainfall in the preceding weeks. Most of the effects of high rainfall can be explained by the effect of high rainfall on river levels. Ambient temperature was also positively associated with the number of non-cholera diarrhoea cases, particularly in those individuals at a lower socio-economic and sanitation status.