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[NEW DELHI] The secret behind the cholera bug's ability to evolve into more robust forms has been revealed by scientists who have done the first genetic comparison of strains collected over the last century.

The researchers compared the genomes of 23 cholera strains and showed that the bacteria's trick is to continually exchange and mix genes. They published their analysis this week (31 August) in Proceedings of the National Academy of Sciences.

The team found that different strains of the bacterium Vibrio cholerae — responsible for cholera, an acute, watery diarrhoeal infection that can be fatal — are continually evolving into newer genetic combinations that cause infection but do not kill their host, ensuring better chances of survival for themselves and greater spread of the disease.

For example, classical strains of cholera caused severe illness and high death rates until the 1960s. But these were displaced by a less severe form of the disease called 'El Tor' that has become even more widespread. Newer, hybrid forms of the disease combine both strains.

The insights gained from the analysis — the first of its kind — will "impinge on all areas of cholera", including our understanding of patterns of disease spread, and vaccine development, says G Balakrish Nair, co-author of the study and director of the National Institute of Cholera and Enteric Diseases (NICED), Kolkata.

"In the past we were unable to recognise how and why one type of cholera strain replaced another in a new pandemic," Nair told SciDev.Net.

The new study not only throws light on how new strains evolved by exchanging genetic material, but could also help predict what kind of genetic changes can be expected in the future, how fast such changes could take place and the kind of cholera epidemics they could trigger, says Nair.

The team worked with the entire genetic sequences of 23 strains that have appeared worldwide over the last 98 years. "This is the first time genomes of all 23 strains have been compared," he says.

The disease is endemic in many developing countries, and a 2008 WHO report cites more than 190,000 cases worldwide, with more than 5,000 deaths.

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