No added salt with mine, thank you; salinity levels and palatability of drinking water.

For many (many) years, studies have been done to find associations between contaminated drinking water and cholera cases. Most studies have concentrated on primary transmission through drinking water and not the possibility of alternative transmission routes.

In Bangladesh cholera cases can be year round, however, there are peaks in cholera cases that are generally witnessed during two points of the year;  southern coastal areas peak at the end of the dry season (between late March to May), while cholera in the northern and middle-belt regions of the country peak post-monsoon (September to November). It is well documented (and has been stated here on this blog many times) that Vibrio cholerae, the causative agent of cholera, thrives in the brackish waters in coastal areas, in this case; the Bay of Bengal. Colonies of V. cholerae bacteria can also grow on tiny phytoplankton and zooplankton, which is thought to play an important role in V. cholerae replication. The infective dose of V. cholerae is 10⁶ bacteria, thus, drinking water would need to contain an infective dose ≥ 10⁶ V. cholerae per litre. However, it is now understood that V. cholerae becomes hyperinfective after passing through the human digestive tract, demonstrating a tendency for person-to-person spread.

Environmental modelling has recently been introduced to measure the abundance of zooplankton throughout the river delta in Bangladesh to explain patterns of the annual peaks in the number of cholera cases throughout the country. Despite considerable advances in predicting cholera outbreaks, it remains unclear as to why studies have been unable to find culturable, V. cholerae in surface waters between peak cholera seasons as well as what environmental changes might trigger the transformation of V. cholerae from a dormant state to a culturable state and vice versa. Whilst most cases are widely accepted to be waterborne, it is not well understood what role water plays in the transmission that leads to the biannual cholera peaks witnessed each year in Bangladesh. Maximal growth of V. cholerae occurs at 25,000 mg/l salinity. The minimum level of salinity required for survival for 1 day at 25 °C is 100 mg/l, while the optimum level for survival falls in the range 5000–30,000 mg/l. Water is generally considered palatable at salinity levels below 1000 mg/l, however, generally an individual’s tolerance of salinity in drinking water may be a matter of acquired taste and/or influenced by freshwater availability.

Grant et al., (2015) aimed to measure the salinity levels of common water sources in coastal Bangladesh and explore perceptions of water palatability among the local population to discuss the plausibility of linking cholera outbreaks in Bangladesh with ingestion of saline-rich cholera-infected river water. The methods for this study were refreshingly simple; a taste testing experiment with a hundred participants taking part was orchestrated with varying levels of salinity. Two unions (a union is comprised of multiple villages) in the coastal Upazila (subdistrict) of Shyamnagar, Satkhira, Bangladesh, were chosen for the study by randomly picking the names out of a hat. An additional, separate data set of salinity measurements was gathered from various domestic and public water sources to investigate whether the salinity levels within the local water sources were optimal for V. cholerae survival and growth. Both drinking and non-drinking sources were included in this sample selection. 50 informal group discussions (25 in the monsoon season and 25 in the dry season) were conducted to gain an in-depth understanding of water sources and water uses. Questions were aimed at understanding which water sources people use for drinking and other domestic uses and how respondents perceived the saltiness of their drinking water.

Salinity levels of non-drinking water sources suggest that the conditions for V. cholerae survival exist 7–8 days within the local aquatic environment. However, 96% of participants in the taste-testing experiment reported that they would never drink water with salinity levels that would be conducive to V. cholerae survival. Furthermore, salinity levels of participant’s drinking water sources were all well below the levels required for optimal survival of V. cholerae. Respondents explained that they preferred less salty and more aesthetically pleasing drinking water.  Theoretically, V. cholerae can survive in the river systems in Bangladesh; however, water sources which have been contaminated with river water are avoided as potential drinking water sources. Furthermore, there are no physical connecting points between the river system and drinking water sources among the study population, indicating that the primary driver for cholera cases in Bangladesh is likely not through the contamination of saline-rich river water into drinking water sources.

Despite this study being relatively simple, a lot was letting itself down. The idea itself was actually a very nice idea with the potential of answering a lot of questions, however, missing from this study were what conditions the people selected to participate living in? The fact that they may have access to cleaner, more palatable water doesn’t highlight that many others may not be so lucky with their choices of drinking water.

“Respondents also stated that although they are able to drink a certain level and volume of saline water, they do not because other drinking water options are available.”

Bangladesh is among the most highly and densely populated countries in the world and to have a study based on 100 people is, quite bluntly and harshly, preposterous to think it should contribute any weight in the argument of cholera outbreaks.

Grant, S.L., Tamason, C.C., Hoque, B.A., Jensen, P.K.M., 2015. Drinking cholera: salinity levels and palatability of drinking water in coastal Bangladesh. Tropical Medicine and International Health. 20(4), 455-461.