[NEW DELHI] The severity of cholera outbreaks can be linked to the rate at which rivers flow, scientists have found.
Cholera, caused by the aquatic bug Vibrio cholerae, spreads through contaminated food and water.
It has re-emerged as a major killer in recent decades, with the number of cases up ten per cent between 2007 and 2008, at 200,000, and the number of deaths up by more than a quarter at 5,000.
The team, from Tufts University, United States, analysed Bangladesh's two seasonal cholera outbreaks — one around March and a second in September–October — using cholera data from the International Centre for Diarrhoeal Disease Research (ICDDR,B) between 1980 and 2000, and water records from the country's hydrology department.
They found a link between the severity of the two outbreaks and the volume of water flowing in the deltas of Bangladesh's three major rivers: the Ganges, Brahmaputra and Meghna.
The March outbreak coincides with low water levels in the rivers. The lower the water level, the more seawater seeps in from the Bay of Bengal, carrying the microscopic plants and animals that harbour Vibrio cholerae, spreading infection, they suggest.
Severe October outbreaks are linked to high flood years, when faecal contamination in the rivers enters drinking-water sources.
The team says its findings can be used to develop an early warning system for cholera.
Shafiqul Islam, professor of civil and environmental engineering at the university and lead author, says it is unlikely that cholera will ever be eradicated because the germs thrive in sea water — where they cannot be controlled — and newer types continually emerge.
"We need a cholera warning system to control outbreaks and minimise their impact by prior planning and implementing effective interventions," he told SciDev.Net.
Environmental indicators provide advanced warning and can be applied to any region, he says.
Recent research by the ICDDR,B has also linked cholera outbreaks in Bangladesh to hours of sunshine and temperature in spring and autumn.
Increases in sea surface temperature and river height also influence outbreaks in Vietnam, says Mohammad Yunus, senior scientist at the ICDDR,B's public health sciences division.
Yunus points out such insights will be meaningful only if scientists monitoring environmental indicators inform public health scientists dealing with outbreaks on the ground.
Some cholera outbreaks have more to do with public health infrastructure breakdown, he adds — as in Zimbabwe, where the ICDDR,B scientists are helping build local capacity in clinical examination and detection.
Even in coastal countries such as Nigeria, where outbreaks can be linked with climate and water variables, collecting environmental data well in advance will be a challenge, he says.
The research was published last month (October) in Geophysical Research Letters.
Link to abstract in Geophysical Research Letters