Human-associated factors vs environmental and climatic factors

Cholera is endemic in South Asian countries especially Bangladesh and India whereas sporadic epidemics occur in South America or parts of Africa. In Bangladesh, the cholera outbreak is usually bimodal with a dominant peak soon after the monsoon and a smaller peak during the spring. In other areas, the outbreak usually occurs once a year, following the monsoon. Cholera outbreak also occurs during the periods of drought. Thus, both the increase and decrease in rainfall can influence the outbreak. Similarly, the water temperature is found to have a correlation with the outbreak. Seasonal peaks are reported in summer months when higher water temperature along with the ‘phytoplankton blooms’ provide favorable conditions for the growth of the organism. Thus, climatic and environmental factors, especially water temperature, salinity, rainfall and the plankton availability are considered as the major drivers that determine the outbreak of cholera. Thus, as per the ‘climate theory’, the aquatic environment is the reservoir for the organism and that the favorable climatic conditions help the growth of autochthonous V. cholerae which may result in the outbreak of cholera. Extreme weather phenomena such as El Nino or La Nina may facilitate the growth of V. cholerae in the aquatic environment, which in turn may trigger cholera outbreak.

Once cholera outbreak occurs, it will be transmitted quickly by secondary transmission mainly through contaminated water used for drinking, cooking or bathing. Contaminated sea foods are another important source of infection.

           

Even though it is clear that the climatic and environmental factors influence the outbreak of cholera, the important question is whether they are the major drivers of the outbreak. In other words, is it the climate factor that initiates the outbreak or is it only facilitating the outbreak and spread of the disease? Similarly, is the outbreak resulting from the regrowth of autochthonous V. cholerae in the aquatic environment or is it due to the growth of fresh bacteria introduced into the aquatic environment from outside? For example, cholera outbreaks usually occur following the monsoon which led to the suggestion that the amount of rainfall is a direct determinant in the outbreak of cholera. However, in endemic areas such as Bangladesh and India, the unhygienic practice of passing stools in the open space is very common especially among the socially and economically backward group of people. Similarly, the sewage system in such areas is underdeveloped. Hence, heavy rain can result in the washing of stool in open land and in the sewage into rivers and other water bodies, resulting in the contamination of drinking water sources. Thus, the fresh introduction of the organism into water bodies can be responsible for the outbreak and if this is the case, the reservoir would be humans themselves rather than the aquatic environment. Thus, even though rainfall is a ‘climatic factor’ it may have only a facilitating function and the outbreak can be directly related to human factors itself rather than the climatic factors. Similarly, under drought conditions, the availability of potable water may decrease resulting in an increase in the number of people sharing the same water supply thus aggravating unhygienic conditions and providing an environment for cholera outbreak.

A serious disadvantage with the climate theory is that most of the studies that had associated cholera outbreak to environmental/climate factors are based on correlations which had ignored the causal mechanisms behind these correlations (Codeco 2001)

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Codeco, C. T. (2001). Endemic and epidemic dynamics of cholera: the role of the aquatic reservoir. BMC Infectious Diseases 1(1).