doi:10.1038/nindia.2012.93 Published online 27 June 2012
Researchers have proposed a new model that could warn us of an impending cholera outbreak. By measuring the density of chlorophyll in phytoplankton in coastal waters of the Bay of Bengal, they are able to tell if an outbreak could be due1.
What does phytoplankton chlorophyll have to do with cholera? "Cholera bacteria attach to zooplankton, especially copepods, forming thin biofilms in the brackish water of coastal regions. Since copepods feed on phytoplankton, the proliferation of phytoplankton may increase the number of cholera bacteria," says principal investigator Shafiqul Islam, member of an interdisciplinary research team from the Tufts University and National Oceanic and Atmospheric Administration, US.
The research team used satellite data to measure the density of chlorophyll and track the blooming of phytoplankton. The causative agent of cholera outbreaks, Vibrio cholerae, cannot be measured from space.
However, the bacterium shows strong affinity to phytoplankton blooms which can be studied from satellite data by measuring the chlorophyll present in phytoplanktons. "Our study shows high correlation between chlorophyll and cholera. Thus coastal chlorophyll could be used as a variable in models predicting cholera outbreaks," Islam says. Such a model will be able to give warning two to three months ahead of cholera outbreaks, he claims.
Cholera burden could be significantly reduced if preventive measures are deployed ahead of time with an early warning system like this.
The researchers began by studying coastal water, the natural reservoir of cholera bacteria. Laboratory studies had already suggested a significant positive correlation between plankton abundance and pathogenic cholera bacteria. The researchers tapped satellite data for space-time measurements of plankton abundance in terms of chlorophyll content (the greenness of ocean water) over large areas. Several studies have suggested that remote sensing may be useful to study cholera outbreaks using ocean chlorophyll signatures. However, a comprehensive study linking ocean chlorophyll content and cholera outbreaks was lacking.
Islam and his team used data from the Earth-orbiting satellite Sea-viewing Wide Field-of-view Sensor (SeaWiFS). They quantified the space–time distribution of chlorophyll using 10 years of satellite data from northern Bay of Bengal, which receives discharge from the Ganges, Brahmaputra and Meghna (GMB) river system. The river system brings in a huge amount of freshwater containing terrestrial nutrients to the coast during summer and monsoon months. The study probed the implications of chlorophyll variability on two seasonal cholera outbreaks: one in the spring (defined as the average of March–April–May months) and the other in autumn (defined as the average of September–October–November months) in the Bengal Delta.
They found that chlorophyll showed distinct seasonality and chlorophyll values were significantly higher close to the coast than in the offshore regions. Cholera incidence in the Bay of Bengal region exhibits bi-annual peaks; one in autumn and the other in spring. However, coastal chlorophyll shows only one seasonal peak in autumn.
The presence of a single peak in chlorophyll and double peak in cholera suggests that chlorophyll variability may not be the only thing affecting cholera outbreaks.
"Based on our current understanding, high river discharge from the GBM river system is responsible for the autumn peak in cholera outbreaks, whereas the spring peak is related to low flow discharges and subsequent intrusion of coastal plankton laden seawater," Islam explains.
Their findings also suggest that continuous measurement of plankton over an extended period of time will be necessary to develop a meaningful time series.
"Vibrio cholerae exhibits strong association with zooplankton in coastal waters. Many investigators have used chlorophyll as a surrogate variable for plankton abundance, and subsequent cholera outbreaks in endemic regions," says T. Ramamurthy, deputy director of Kolkata-based National Institute of Cholera and Enteric Diseases (NICED).
The satellite data on chlorophyll concentration could be used to monitor coastal processes and track cholera outbreaks in endemic regions, Ramamurthy adds.