Cholera Seasonality in Madras (1901–1940): Dual Role for Rainfall in Endemic and Epidemic Regions Diego Ruiz-Moreno, 1 Mercedes Pascual, 1 Menno Bouma, 2 Andrew Dobson, 3 and Benjamin Cash 4 1 Department of Ecology and Evolutionary Biology, University of Michigan, 830 North University, 2041 Natural Science Building, Ann Arbor, Michigan, 48109-1048, USA 2 Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, University of London, London, UK 3 Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey, USA 4 Center for Ocean–Land–Atmosphere Studies, Calverton, USA Abstract: The seasonality of cholera and its spatial variability remain unexplained. Uncovering the role of environmental drivers in these seasonal patterns is critical to understand temporal variability at longer time scales, including trends and interannual variability. Rainfall has been proposed as a key driver of the seasonality of cholera. To address this hypothesis, we examine the association between rainfall and cholera in both time and space using the extensive historical records for the districts of Madras in former British India (1901–1940). We show the existence of two main spatial clusters that differ not just in the effect of rainfall but also in the seasonal pattern and frequency of periods with and without cholera mortality. The results support a model of cholera seasonality with two different routes of transmission: one is enhanced by increasing rainfall (in areas with abundant water), the other is buffered by increasing water. We discuss how the dual nature of the influence of rainfall creates different temporal patterns in regions where cholera is either ‘‘endemic’’ or ‘‘epidemic.’’ Keywords: Cholera, endemic cholera, epidemic cholera, rainfall INTRODUCTION An understanding of the seasonality of cholera is still elu- sive despite the long history of descriptions of the patterns in different parts of the world and different regions of its center of endemism on the Indian sub-continent. Within regions, cholera cases can exhibit different seasonal patterns at different locations, including variations in the number of outbreaks and different delays with respect to peaks in rainfall and temperature. These patterns are not well understood because environmental drivers themselves are poorly defined for the seasonal cycle of the disease. Two peaks per year is the typical pattern described for cholera in Bangladesh and former Bengal, with a decline in the summer during the monsoons, while only one peak coin- cident with the rainy season is present in other regions of former British India and contemporary Brazil (Codeco, 2001; Pascual et al., 2002). Despite these complexities, a better understanding of choleraÕs seasonality is key to identify and understand the regional mechanisms behind the described influence of the El Nin ˜o Southern Oscillation (ENSO) (Pascual et al., 2000; Koelle and Pascual, 2004; Koelle et al., 2005b). It is also fundamental to build sce- Published online: March 1, 2007 Correspondence to: Diego Ruiz-Moreno, e-mail: drmoreno@umich.edu EcoHealth 4, 52–62, 2007 DOI: 10.1007/s10393-006-0079-8 Original Contributions Ó 2007 EcoHealth Journal Consortium