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The research, published today (4 August) in Nature, is among the first to link changes in climate to the dynamics of disease outbreaks.
A team led by Mercedes Pascual of the University of Michigan, United States, analysed 40 years of records detailing the incidence of cholera in the town of Matlab, 40 kilometres southeast of Bangladesh’s capital, Dhaka.
The team created a computer model that took into account disease transmission in the region, local people’s immunity, and climatic factors such as changes in rainfall.
They found drought or floods increased the spread of the disease.
This is because these extreme climate conditions enable the waterborne bacterium Vibrio cholerae to be spread more easily. Floods caused by heavy monsoons can contaminate drinking water with the bacterium. In droughts, the bacterium can grow more easily in stagnating water in ponds and rivers.
Pascual told SciDev.Net that the results confirm long-standing fears that climate change will affect the outbreaks of infectious diseases such as cholera.
Rainfall patterns in the region, as in other parts of the world, are influenced by ‘El Niño’ — a periodic warming of the Pacific Ocean off the west coast of South America. The phenomenon can trigger extreme floods and drought, changes in sea surface temperatures, and variations in the severity of monsoon rains thousand of kilometres away.
Climate experts expect extreme weather events to be more severe and frequent in future because of climate change caused by increased greenhouse gas emissions.
But climate is not the only factor affecting the likelihood of cholera outbreaks, says Pascual’s team.
Their computer model suggests that local immunity is also a factor. Whether or not many people become infected depends on how well the population can ward off infection.
The team found that major outbreaks of the disease tended to be followed by a lull in which fewer people were infected, despite environmental factors being present that would encourage the bacterium to spread more easily.
This, they suggest, is because once infected, people acquire long-lasting (up to three years) immunity to the cholera bacterium — a notion that has been debated by scientists for some years.
"We have also shown that it is not sufficient to consider climate alone to understand the size and timing of cholera epidemics," says Pascual, "It is also important to consider the immunity levels in the population."
"These levels interact in complex ways with climate to determine the actual size of the outbreaks," she said.
Cholera causes severe diarrhoea and if left untreated can lead to severe dehydration and death. It kills tens of thousands of people worldwide every year.
Pascual says the team’s next step will be to investigate how these results could form the basis of computer models to forecast cholera outbreaks in the short-term. The researchers will also try to produce models of how cholera would be affected by various climate-change scenarios.