New Delhi, Aug. 31: The first comparative genome analysis of cholera bacteria isolated from India and several other countries over the past century has shown that the germs have swapped genes to spread fast and cause severe disease.

An international research team has found that the currently circulating Vibrio cholerae germs possess hybrid genomes that combine a relatively recent potential for efficient spread and a traditional capacity for severe disease.

Classical cholera that used to cause widespread epidemics in the Indian subcontinent until the 1960s was associated with severe disease and a large number of deaths. But a new cholera strain called El Tor that caused less severe disease surfaced in 1961 and spread around the world for more than three decades. As El Tor did not kill infected patients so easily, it had a greater opportunity to spread across communities.

The new study, published today in Proceedings of the National Academy of Sciences, compared the genomes of 23 cholera strains from four continents to understand how the germ had changed over the past 98 years.

“The cholera microbe has picked up the best attributes of El Tor and classical cholera,” said G. Balkrish Nair, a research team member and director of the National Institute of Cholera and Enteric Diseases (NICED), Calcutta.

These hybrid strains can spread as efficiently as El Tor strains but they also have the cholera toxin, a protein secreted by the classical cholera strains that is responsible for severe damage to the human intestine.

“These new El Tor variants appear to have completely replaced the prototype El Tor in India and Bangladesh,” Nair told The Telegraph in an interview.

“This might explain why cholera epidemics appear more common now than a decade ago.”

Nair was part of a team led by infectious disease expert Rita Colwell at the Johns Hopkins Bloomberg School of Public Health and other researchers in Bangladesh, Korea and the US that compared the genomes of different cholera strains.

The study has also provided fresh evidence that Vibrio cholerae germs exchange genetic elements through a process called “lateral gene transfer” — the movement of genes from bacteria to bacteria.

Public health experts estimate that cholera causes illness in 2 million people and kills about 60,000 around the world each year, mainly in areas with unclean water and poor sanitation.

Calcutta’s NICED is currently engaged in a study to produce a realistic estimate of the burden of cholera infections in India.