The findings – reported in Nature on Thursday – offer better understanding of the mechanisms behind spread of Cholera – a diarrhoeal infection usually linked to unhygienic conditions and poor sanitation systems.
Diarrhoeal diseases kill almost 400,000 children and adults in India every year, a sizable chunk of which is due to Cholera. Globally Cholera affects 3 to 5 million people each year.
“The seventh pandemic began in 1961 when a new type of Cholera strain arrived in the Bay of Bengal from Sulawasi in Indonesia. Subsequently it spread to all over the world from Bengal basin,” G B Nair, director of National Institute of Cholera and Enteric Diseases in Kolkata and one of the key scientists involved in the study, told Deccan Herald.
The outward spread occurred in three distinct waves, the first of which took place between 1961 and 1985 transporting the bacteria in Africa, Europe and South America. The second wave took place between 1992 and 2002 when the bug moved to east Africa and the third wave happened between 2004 and 2009 when the germ went to Haiti from Nepal and from Bengal basin to south east Asia between 2003 and 2007. It was not a simple single spread of a strain from the Bay of Bengal. The evidence suggests there have been at least three independent overlapping waves of intercontinental spread with a common ancestor in the 1950s, representing the original strain. These movements are strongly correlated with human activity, suggesting that the strain has been carried by human travel.
“This is the world’s largest analysis of Cholera whole genomes and gives a new dimension to our understanding of the disease. It may help predict what could be the next location where cholera outbreak may occur,” Nair said.
The retracing helps scientists pinpoint when the circulating strain of cholera bug - known as El Tor – first became resistant to antibiotics. The resistance came in 1982 triggering renewed global transmission. The team tracked the spread of the organism by analysing genomes of Vibrio cholerae taken from 154 patients across the world over the last 40 years.
The research showed importance of global transmission events in the spread of cholera that goes against previous beliefs that cholera always arises from local strains, Julian Parkhill, a senior group leader at the Wellcome Trust Sanger Institute and a co-author of the study. The most recent transmission happened in Haiti almost one and half years ago when a South Asian strain originating in Nepal struck the virgin island territory and created a havoc. The genetic analysis clearly identifies the South Asian link.
