Seasonal and interannual cycles of endemic cholera in Bengal 1891-1940 in relation to climate and geography

Unravelling the epidemiology of cholera has been one of the most elusive problems in tropical medicine. The early contest between 'contagionists' and 'localists' in the last century finds a modern equivalent in the on-going debate on whether the epidemiology of cholera has significant environmental determinants. This study investigates the role of climate and geography on seasonality and interannual variations of cholera, using historical mortality data between 1891 and 1940 from 24 districts of Bengal. The coastal districts exhibit a bi-annual seasonal cycle, as previously described for cholera in this region, but with a dominant peak in the spring. Spring mortality, particularly in the coastal region, shows significant correlations (r = 0.50. p < 0.001) with sea surface temperatures in the Bay of Bengal. During post-Niño years (a total of 13 events) when coastal sea temperatures rise, spring epidemics with excess mortality (over 15 000 deaths on average) occur, resulting in a shift away from the usual seasonal pattern. Significant correlations (r = 0.58, p < 0.001) between sea temperatures in the Pacific (an El Niño index) and cholera's spring deaths, and a peak at a frequency of approximately 1 /4.2 years in the power spectrum, support the influence of this climatic forcing. Cholera's winter peak, dominant further away from the estuary, appears less consistently associated with water temperatures in the Bay of Bengal, and accounts for most of the interannual variation in the geographical spread of cholera in Bengal. The differences in our results for spring and winter suggest the possibility of different aquatic reservoirs for the pathogen. As sea water temperature can only explain part of the historical variability in cholera deaths, and the seasonality of temperature is similar to that of other environmental parameters, further studies to elucidate causal pathways are warranted. Here, we provide geographical and climatological support for the hypothesis that environmental factors are important in the dynamics of endemic cholera.