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[SANTIAGO] The conclusion that cholera outbreaks are linked to global warming has been challenged by a study that has found that warmer sea temperatures that correlate with the outbreaks do not cause them.

Outbreaks follow the blooming of phytoplankton which is associated with warmer seas in the Bay of Bengal — but these blooms are driven by river discharges rather than warmer seas, say authors of the study, published last week (3 August) in the American Journal of Tropical Medicine and Hygiene.

Previous studies have suggested that periods of warmer sea temperature could be one of the conditions for promoting the disease, such as in the recent outbreak in Haiti, and that such knowledge could help set up an early warning system.

But it was not clear why this happens, as phytoplankton, microscopic plants that feed tiny crustaceans to which Vibrio cholerae bacteria attach, thrive in cold, not warm, water.

Now, by examining 12 years of data from several major river basins around the world, including the Amazon, Congo, Ganges and Orinoco, the researchers have found that nutrient-rich river discharges that coincide with periods when seas are warmer can fuel phytoplankton blooms, which then drive cholera outbreaks.

"Our results suggest there is no causal association between sea surface temperature and cholera outbreaks. Consequently, such a relationship cannot be used to develop cholera warning systems," Shafiqul Islam, lead researcher of the study and engineering professor at Tufts University, United States, told SciDev.Net.

But he added that global warming could lead to more extreme climate effects, such as droughts and floods — making cholera epidemics more severe.

Earlier research by Islam and colleagues has also shown that cholera outbreaks in Bangladesh were driven by river flow. Lower water levels allow nutrient-rich seawater to seep into the river basin, spreading infection, while high levels lead to river contamination with faecal matter.

"A better understanding of macro-environmental variables like river discharge and coastal plankton bloom together with recent advances in satellite remote sensing and cholera intervention and mitigation will allow us to develop such a system," he told SciDev.Net.

According to Rita Colwell, an environmental microbiologist at the University of Maryland, United States, and one of the authors of the latest study, the findings may apply to other cholera-endemic countries where freshwater discharges are a predominant mechanism for algae production in coastal areas.

She expects to work in Haiti soon to test whether the Artibonite River, along which cholera epidemics started last October, has high river discharges that create the nutrient-rich conditions that cholera bacteria thrive in.

Peter Hotez, president of the American Society of Tropical Medicine and Hygiene, told SciDev.Net that the findings "may help refine predictive models of the disease. This is more important than ever given large and protracted outbreaks in Sub-Saharan Africa and Haiti."