December 2, 2009

Forecasting Cholera Outbreaks

Shafiqul Islam hopes that satellite images will allow health professionals to stay one step ahead of a ruthless disease

By Marjorie Howard

Tufts researchers have discovered a link between the cholera outbreaks that strike up to a quarter million people in Bangladesh each year and the fluctuating water levels in the three rivers that feed the Bengal Delta. The findings may allow them to forecast outbreaks of the acute diarrheal infection so people could be treated before they get really sick.

“Can we tell two or three months in advance that there will be cholera in South Africa or Bangladesh or Mozambique?” ask Shafiqul Islam. “I think we can.” Photo: Alonso Nichols

Cholera breaks out in Bangladesh twice a year, in spring and fall. This had puzzled Shafiqul Islam, a professor of civil and environmental engineering at Tufts, since in Africa and Latin America, where the disease is also prevalent, there is typically just one outbreak a year. But the fluctuation in the three rivers that flow into the Bengal Delta, which scientists consider a hot spot for cholera, explained the discrepancy, the researchers reported in a recent issue of Geophysical Research Letters.

The bacteria that causes cholera cannot be eradicated because it can survive in the environment, so knowing how to predict its arrival is especially valuable, says Islam, who also holds an appointment as a professor of water and diplomacy at the Fletcher School at Tufts. “It’s not a microbiological explanation,” he says. “The key is the river discharge and regional climate.”

Although the government does not report figures, it is estimated that thousands of Bangladeshis get sick each year from the disease, which they contract by drinking water or eating food contaminated with the bacterium Vibrio cholerae. The bacterium releases a toxin in the gut that causes severe diarrhea. Left untreated, the infection kills its victims within a day of the onset of symptoms.

Vibrio cholerae is a robust bug: it lives not only in humans, but in the environment, especially in brackish water near warm oceans, where it feeds off plankton.

Ebb and Flow of a Disease

Islam and the Tufts team, including graduate students Ali Akanda, E11, and Antarpreet Jutla, E10, tracked the month-by-month incidence of cholera using data from the International Centre for Diarrhoeal Disease Research, a treatment center that recorded incidences of cholera for the biggest population center of Bangladesh from 1980 to 2000. When they correlated their findings with measurements of the water discharged from the Bengal Delta’s three main rivers, the Ganges, Brahmaputra and Meghna, they discovered two distinctive patterns.

A spring outbreak of cholera typically occurs in March, during the period of low river flow in Bangladesh, because the low water levels allow seawater from the Bay of Bengal, laden with bacteria-carrying plankton, to move inland. The second epidemic happens in September and October, after the monsoons have raised water levels. The floodwaters mix with water from sewers, reservoirs and rivers. As the floods recede, Vibrio cholerae bacteria are left behind.

And indeed, the researchers found a relationship between the magnitude of cholera outbreaks and the extent of the region’s seasonal variations in water level. “The more severe the low river flow, the larger the spring epidemic,” says Islam. “The same thing is true with flooding during the fall.”

Islam’s ambitious second project, dubbed COP (Cholera Outbreak Prediction) from Satellites, will use images from NASA satellites to locate blooms of plankton in coastal areas where cholera is known to proliferate. Such blooms indicate the presence of cholera-causing bacteria, so knowing where and when they bloom could help predict outbreaks.

“Can we tell two or three months in advance that there will be cholera in South Africa or Bangladesh or Mozambique?” says Islam. “I think we can, because it lives on phytoplankton and zooplankton, the base-level food chain we have in the ocean. With satellites, we can see these planktons from the sky. If there is a significant bloom that we detect from a satellite, [we know that] two to three months down the road there will be an outbreak.”

In another cholera study, Islam is working with researchers from the University of Maryland and the Institute of Water Modeling in Bangladesh to examine the effect of climate change on disease outbreaks. The research team was just awarded a $500,000 National Institutes of Health Challenge Grant in Health and Science Research from federal stimulus funds to examine how rises in sea level and variations in precipitation might affect transmission of the disease. Islam’s project was one of slightly less than 2 percent of the proposals chosen from over 21,000 submitted.

Marjorie Howard can be reached at

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