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Higher temperatures and rainfall predict cholera outbreaks months in advance, allowing preventive measures to be taken
An early warning system that can predict devastating outbreaks of cholera months before they strike has been developed by scientists.
The work could transform medical care in improverished tropical zones where the gut infection is endemic and kills more than 100,000 people a year.
With a few months' warning, health services would stand a better chance of mobilising pre-emptive measures, including vaccine programmes and the distribution of medication to vulnerable communities to combat the illness.
Cholera spreads through water and food contaminated with the bacterium Vibrio cholerae. The infection can take hold in less than a day and kills through dehydration caused by severe diarrhoea. The short incubation time of the disease means health officials can only identify outbreaks once people start to die.
Researchers led by Mohammad Ali at the International Vaccine Institute in Seoul examined climate records and cholera outbreaks between 1997 and 2006 on the two main islands of Zanzibar off the east coast of Tanzania. The study revealed a strong correlation between higher temperatures and rainfall and cholera outbreaks several months later.
Writing in the American Journal of Tropical Medicine and Hygiene, the researchers describe how a 1C rise in average monthly temperature was a sign that cholera cases would double in the next four months. A rise in monthly rainfall of 200 millimetres preceded a 60% rise in cholera cases on average within two months.
The work will be fed into a forecasting system that draws on local climate monitoring and rainfall patterns to warn of impending outbreaks. Ali told the Guardian that work was ongoing to extend the forecasting system to other countries where cholera is rife, but in some regions climate records were too incomplete to verify the research.
As temperatures rise as a result of global warming, cholera is expected to become a more serious problem in countries that have historically been spared the disease.
Estimates by the World Health Organisation put the total number of people affected by the disease at between three and five million every year. The majority of cases arise in areas where warm weather, rainfall and poor sanitation provide ideal conditions for the bacteria to thrive. In the past 30 years, the burden of cholera has shifted from Asia to Africa, where more than 94% of cases are now reported.
The changing nature of cholera outbreaks, which have gone from lasting a few weeks to more than a year, is another concern, according to Peter Hotez, president of the American Society of Tropical Medicine and Hygiene. In 2008, an outbreak in Zimbabwe lasted 11 months and killed 4,288 people out of 98,592 reported cases.
"I think this is a wake-up call that cholera is not going to go away, but is likely to become a more serious problem," Hotez said. In conjunction with other preventative measures, the research "could significantly reduce the needless suffering and deaths of thousands of people", he added.
