Identifying the environmental conditions that foster the growth of the deadly cholera bug is key to strengthening our ability to predict outbreaks of the debilitating and sometimes fatal infection. A paper published today in the American Journal of Tropical Medicine reports that measuring nutrient-bearing river flow helps explain the seasonal patterns of cholera. Integrating river flow into models, thus, could help communities and organizations prepare for severe outbreaks.
Cholera affects an estimated 3 to 5 million people and claims the lives of 100,000–120,000 each year. And the disease can spread swiftly: The number of cases has increased by 24% between 2004 and 2008, according to the World Health Organization. The cholera-causing bacterium, Vibrio cholerae, is difficult to eradicate, says Shafiqul Islam of Tufts University in Medford, Massachusetts. The bacterium hides in the intestines of small, coastal crustaceans called copepods around the world — a host reservoir “where it can survive almost indefinitely,” he says. Islam aimed to find a way to predict the growth of the copepod’s main source of food, phytoplankton, and measure whether that in turn could predict cholera incidence.
Islam and his colleagues focused in on the Bay of Bengal outside Bangladesh, where cholera strikes often. The team used satellite chlorophyll measurements as a stand-in for phytoplankton abundance and river flow as a proxy for nutrient load. They compared this data with sea surface temperatures and cholera statistics. Traditionally researchers associated cholera with sea temperatures, but Islam noticed that the relationship decoupled during winter months. River flow measurements strengthened the association, suggesting that inland nutrient input drives the cycles of cholera around the bay.
The team also looked at three other regions worldwide—the Orinoco and Amazon Rivers in eastern South America and the Congo River Basin—and found that river flow strengthened the sea temperature-cholera assocation.
“River flow can explain about 70% of the variance of cholera outbreaks,” Islam says.
He insists that river flow can be used to represent nutrient load, especially taking the “muddiness of water, color of water, and pH” into account. But ecologist Walter Boynton of the University of Maryland on the Chesapeake Bay says that you can’t necessarily make that assumption. While many coastal rivers do sweep nutrients downstream, “the actual amount of nutrient getting into the next downstream system is dependent on the nutrient concentration in the river,” he says. Measuring nitrogen and phosphorus levels in the water would help boost their assertion, he adds. “There is a lot of variability between rivers.”
Disease ecologist Mercedes Pascual of the University of Michigan thinks that focusing on the coastal ocean oversimplifies the drivers of the cholera. “It is likely that a big part of this is through the way people live” and their sanitary conditions, she says. Just looking at coastal ecology ignores city water supplies and those non-coastal cities that are more affected by rainfall. “Other mechanisms affect the transmission of the disease,” she says.
Islam hopes to use this information to predict cholera outbreaks based on satellite records of phytoplankton combined with river flow data. “Our goal is to essentially be able to develop a cholera warning system,” he says.
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