To trace the source and explain the spread of the latest (seventh) cholera pandemic, next generation sequencing has been used by investigators. They also highlighted the impact of the acquisition of resistance to antibiotics on shaping outbreaks and show resistance was first obtained around the year 1982.
The particular cholera type responsible for the present pandemic can be traced back to an ancestor that first appeared in the Bay of Bengal 40 years ago, whole genome sequencing revealed. Cholera has spread from this ancestor over and over again to different areas of the world in multiple waves.
These discoveries provide a greater understanding of the mechanisms behind the spread of cholera, a diarrhoeal infection which is normally connected to unhygienic conditions and poor sanitation systems and frequently found in disaster areas, such as the Haitian earthquake in October 2010. Cholera is estimated to affect 3-5 million individuals every year, with 100,000 to 120,000 deaths.
By analyzing the genomes of the causative bacterium Vibrio cholerae taken from 154 patients worldwide over the past 40 years, the team tracked the spread of the organism. With the ability to track single DNA changes in the genome of this strain, the researchers were able to map the transmission routes of the bacteria, helping future health planning and making backtracking of the disease to its origin possible.
The current strain of bacterium (El Tor strain) first became resistant to antibiotics in 1982 by acquiring the genetic region SXT, which entered the bacteriums at that time which generated renewed worldwide transmission from the original source, researchers discovered.
Dr Julian Parkhill, a senior group leader at the Wellcome Trust Sanger Institute and a senior author of the study explained:
"Through comparing the genomes of 154 cases of cholera, we have made important discoveries as to how the pandemic has developed.
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."
The origins of the pandemic strain were crucially identified, by the investigation, to its roots four decades ago in the Bay of Bengal. Since then it has infected individuals worldwide, including Africa, South Asia and South America. Nicholas Thomson from the Sanger Institute and one of the first authors of the investigation, said:
"looking at the past 40 years of transmissions from continent to continent, we found that the Bay of Bengal acts as a reservoir for cholera, where it can thrive and spread.
By tracking how the disease is spread, our maps of transmission could influence future decisions on how to tackle this disease."
Ankur Mutreja, first author from the Sanger Institute explains, these discoveries are opening up new ways for investigators researching all fields of bacterial infection: from researching how genetic changes enable strains to build up resistance to antibiotics, to being able to track a diseases transmission and trace it back to its roots. These primary findings may be the key to unlocking several other bacterial pandemics.
Professor G Balakrish Nair, Director of the National Institute of Cholera and Enteric Diseases in Kolkata, India says:
"This is among the first investigation that merges evolutionary data with emergence of contemporary new variants of Vibrio cholerae and then uses the phylogenetic signatures to track the intercontinental spread of cholera.
These discoveries in due course will lead us to understand why cholera pandemics begin in south east Asia and then spread as a wave across the globe."
Written by Grace Rattue