AsianScientist (Aug. 30, 2011) – Researchers have used next-generation sequencing to trace the source and explain the spread of the latest (seventh) cholera pandemic. They have also highlighted the impact of the acquisition of resistance to antibiotics on shaping outbreaks and show resistance was first acquired around 1982.
In a paper published in the journal Nature, the use of whole-genome sequencing revealed that the particular cholera type responsible for the current pandemic can be traced back to an ancestor that first appeared 40 years ago in the Bay of Bengal.
From this ancestor, cholera has spread repeatedly to different parts of the world in multiple waves affecting three to five million people each year, corresponding to 100,000-120,000 deaths.
The team tracked the spread of the organism by analyzing the genomes of the causative bacterium Vibrio cholerae taken from 154 patients across the world over the last 40 years.
Using the ability to track single DNA changes in the genome of this strain, they were able to map the transmission routes of the bacteria, aiding future health planning and enabling ‘backtracking’ of the disease to its origin.
They discovered that the current strain of the bacterium – known as the El Tor strain – first became resistant to antibiotics in 1982 by acquiring the genetic region SXT, which entered the bacterium’s genome at that time, triggering renewed global transmission from the original source.
“Our research shows the importance of global transmission events in the spread of cholera. This goes against previous beliefs that cholera always arises from local strains, and provides useful information in understanding cholera outbreaks,” said Dr. Julian Parkhill, senior group leader at the Wellcome Trust Sanger Institute and a senior author of the study.
The analysis shows that there was not a simple single spread, but that there have been at least three independent overlapping waves of intercontinental spread of Vibrio cholerae with a common ancestor in the 1950s, representing the original El Tor strain. These movements are strongly correlated with human activity, suggesting that the strain has been carried by human travel.
“This is among the first study that merges evolutionary information with emergence of contemporary new variants of Vibrio cholerae and then uses the phylogenetic signatures to track the intercontinental spread of cholera,” explains Professor G. Balakrish Nair, Director of the National Institute of Cholera and Enteric Diseases in Beliaghata, India.
These findings offer much better understanding of the mechanisms behind the spread of cholera – a diarrheal infection which is usually linked to unhygienic conditions and poor sanitation systems often found in disaster areas, such as the Haitian earthquake in October 2010.
The article can be found at: Mutreja A et al. (2011) Evidence for several waves of global transmission in the seventh cholera pandemic.
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Source: The Wellcome Trust Sanger Institute.
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