Cholera epidemics occur twice a year in Bangladesh, but the mechanisms behind these unique dual outbreaks are not fully understood.
Now, researchers from Tufts University have proposed a link between cholera and fluctuating water levels in the region's three principal rivers - the Ganges, Brahmaputra and Meghna.
"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," says Professor of Civil and Environmental Engineering Shafiqul Islam.
Cholera is an acute diarrheal disease caused by the bacterium Vibrio cholerae. It lives and thrives among phytoplankton and zooplankton in brackish estuaries where rivers come into contact with the sea.
The Bengal Delta, which scientists have considered the native land of cholera, is fed by three rivers.
Almost all of the rainfall in the region occurs during the four-month monsoon season between June and September. Water levels in the river system rise, causing floods that cover 20 percent of the land in an average year. Water levels then fall rapidly, though low-lying, depressed areas remain submerged for weeks.
The research team tracked the month-by-month incidence of cholera using data from the International Center for Diarrhoeal Disease Research, a treatment center that recorded incidences of cholera for the biggest population center of Bangladesh from 1980 to 2000.
The research team 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.
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.
Islam and his team linked the incidence of cholera cases to the level of water flow in the rivers. In order to confirm their findings, the researchers looked for a consistent pattern.
They analysed the incidence of cholera in five years of severely low river flow from 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.
The researchers found a relationship between the magnitude of cholera outbreaks and the severity of the region's seasonal low river flow and floods.
"The more severe the low river flow, the larger the spring epidemic. The same thing is true with flooding during the fall," says Islam.
Islam says that the findings will help in the development of systems to anticipate and predict cholera outbreaks based on the hydroclimate of the region.
The findings were reported in the latest issue of Geophysical Research Letters.