Fall 2013

Eye in the Sky

Satellite imagery may provide better warning of cholera epidemics

By Alexander Reid

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An image of Vibrio cholerae, the bacteria that causes cholera. Photo: BSIP/Photoresearchers Inc.

In two recently published papers, researchers at Tufts School of Engineering, employing methods based on remote satellite imaging, have established new techniques for predicting the severity of seasonal cholera epidemics months before they occur. Taken together, findings from the two papers may provide the essential lead time to strengthen intervention efforts before the outbreak of cholera in endemic regions.

Cholera is an acute diarrheal disease caused by the bacterium Vibrio cholerae. It occurs in the spring and fall in the Bengal Delta of Bangladesh in southern Asia, where more than 125 million people inhabit one of the most fertile regions in the world. In earlier research, scientists have used chlorophyll, a surrogate for phytoplankton, as a rough measuring stick for the presence of cholera. The cholera bacteria live and thrive among phytoplankton and zooplankton.

In the June issue of Remote Sensing Letters, Antarpreet Jutla, then a doctoral student at the School of Engineering and now an assistant professor at West Virginia University, was lead author on a study that measured chlorophyll and other organic matter using a new, more precise approach. The team, which was led by Shafiqul Islam, professor of civil and environmental engineering at Tufts, used satellite data to measure chlorophyll and algae as well as flora that also support growth of the cholera bacteria.

With their satellite images, the researchers created a “satellite water marker” (SWM) index to estimate the presence of organic matter, including chlorophyll and plankton, based on wavelength measurements. A predominance of green, plankton-rich water—which is measured at 555 nanometers—indicated the degree to which the waters contained chlorophyll, plankton and other impurities. Clear blue water—measured at 412 nanometers—indicated low levels of these impurities, according to the researchers.

The researchers targeted the spring epidemic, which is a coastal phenomenon caused by water flowing into the delta from three rivers, the Brahmaputra, Ganges and Meghna. Unlike the spring outbreak, the fall epidemic is linked to flooding, which follows the monsoons and subsequent breakdown of sanitary conditions rather than coastal conditions. In their study, the researchers correlated cholera incidence from the International Center for Diarrheal Disease Research in Bangladesh from 1997 to 2010 with NASA satellite imaging data for the period.

They discovered a relationship between SWM index measurements taken in early winter—from October to December—and the severity of cholera epidemics the following spring. “In short, the index for chlorophyll along with readings for other biological matter in early winter indicated severity of cholera incidences in the spring,” says Jutla.

The SWM is a more accurate predictor of cholera than the algorithm that measures strictly chlorophyll levels, because it measures a broader range of organic matter, Islam explains. “The probability for error in this index-based estimate is less than 10 percent, while the error in using the chlorophyll-based algorithm is about 30 percent.”

To validate their hypothesis that the index can be used in coastal areas outside the Bengal Delta, the team applied the SWM to coastal waters around Mozambique’s capital city, Maputo, and found confirmation in the results.

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