After the 2010 earthquake in Haiti there was a sudden outbreak of cholera which infected over 600,000 people and around 8,000 lives were lost. This unexpected epidemic caused alarm and scientists started searching for the causes of the outbreak. In a study published in the journal Infection, Genetics and Evolution, scientists at the National Institute of Cholera and Enteric Diseases (NICED), Kolkata, claim that bacterial strains similar to those found in Haitian cholera were present in Delhi as early as 2004, showing that the disease may have originated in the Indian subcontinent.
They examined different Vibrio cholerae strains from a hospital repository in Delhi. Genomic studies were carried out on 175 strains collected from 2004 to 2012. They studied the genes ( tcpA -responsible) for colonisation of bacteria and those ( ctxB, rtxA and gyrA ) responsible for virulence.
All the representative strains were identical to the sequence of the Haitian reference strain. “We observed point mutation in tcpA at the gene and protein level,” says Dr. Priyanka Ghosh, from NICED and first author of the paper. The first appearance of Haitian variant of ctxB gene (ctxB7) was found in Delhi in 2006 and by 2012 almost 80% of the Delhi strains carried mutant ctxB7 gene. Haitian variant rtxA and gyrA also first appeared in 2004 in Delhi.
“The mutations in cholera bacterial genes started long before the Haitian outbreak. All these mutations together gathered and led to the rise of a new and more virulent Haitian cholera strain,” she adds.
Resistance and resilience
Previous studies have shown that the Haitian strains were identical and most closely related to the Indian sub-continent strains. With the help of single nucleotide polymorphism (SNP) analysis, scientists have claimed that the seventh cholera pandemic (1960s to 1970s) was transmitted worldwide via Ganges Delta, which is the home to Asiatic cholera. This study further strengthens the theory that Haitian cholera originated in Asia.
“India is a cholera-endemic zone and many Indians are immune to the bacteria. But Haiti has not experienced cholera for more than a hundred years and so had no pre-existing immunity to it. It is possible that UN peace-keeping forces from Nepal could have brought the disease to Haiti,” explains Dr. Asish K. Mukhopadhyay from NICED and corresponding author of the paper in an e-mail.
“Though we cannot say with certainly that Indians could have also transmitted the disease, our study shows that mutations similar to that of Haitian cholera bacteria were prevalent in India. The V. cholerae strains currently in India are different and more virulent,” explains Prosenjit Samanta, PhD scholar at NICED and a co-author of the paper.
