Predicting Cholera Epidemics in the Bengal Delta

A new study proposes a link between the biannual cholera epidemics and fluctuating water levels in Bangladesh's three principal rivers - the Ganges, Brahmaputra, and Meghna.

Researchers from Tufts University (Boston, MA, USA) tracked the month-by-month incidence of cholera in Bangladesh from 1980 to 2000, using data from the International Center for Diarrheal Disease Research (Dhaka, Bangladesh), which records incidences of cholera in the biggest population centers of Bangladesh. The researchers then correlated these cholera incidence statistics with an analysis of water discharges from the three rivers. Their findings suggested two distinctive epidemic patterns that are associated with the seasonal cycles of low river flows and floods. A spring outbreak occurs in March, during the period of low river flow in Bangladesh; the low river flow allows seawater from the Bay of Bengal to move inland, transporting bacteria-carrying plankton. A second epidemic occurs in September and October, after monsoon rains have raised water levels. Here, a different dynamic takes place; floodwaters have mixed water from sewers, reservoirs, and rivers. As the floods recede, contamination is left behind.

The researcher analyzed the incidence of cholera in five years of severely low river flow between 1980 to 2000 and compared it with five years of average and below average river flow. The same analysis was done for extreme, average, and below average floods to study the fall epidemic. They found a relationship between the magnitude of cholera outbreaks and the severity of the region's seasonal low river flow and floods, thus linking the incidence of cholera cases to the level of water flow in the rivers. The study was published in the October 10, 2009, issue of Geophysical Research Letters.

"The more severe the low river flow, the larger the spring epidemic. The same thing is true with flooding during the fall,” said lead author Professor Shafiqul Islam, Sc.D., of the department of civil and environmental engineering. "What we are establishing is a way to predict cholera outbreaks two to three months in advance. It's not a microbiological explanation. The key is the river discharge and regional climate.”

Cholera, an acute diarrheal disease caused by the bacterium Vibrio cholerae, has reemerged as a global killer. Outbreaks typically occur once a year in Africa and Latin America, and twice a year in Bangladesh. V. cholerae lives and thrives among phytoplankton and zooplankton in brackish estuaries where rivers come into contact with the sea, such as the Bengal Delta.

Related Links:
Tufts University
International Center for Diarrhoeal Disease Research