London: Scientists, who did a rapid genetic analysis of bacteria collected from Haitian patients, have concluded that the strain of cholera currently sweeping through post-earthquake Haiti originated in South Asia.
The finding supports the notion that the cholera bacteria fueling the outbreak arrived on the island via recent visitors.
"The mostly likely explanation for the sudden appearance of cholera in Haiti is transmission of V. cholera by an infected human, food, or other contaminated item from a region outside of Latin America to Haiti," concluded Howard Hughes Medical Institute (HHMI) investigator Matthew Waldor and co-authors.
While cholera is endemic in many parts of the world, including regions of Latin America, until October, Haiti had historically been spared from the intestinal disease. But in mid-October, an outbreak flared in northern Haiti and quickly swept across the country. By December 3, the bacteria had sickened more than 93,000 people, killing some 2,100. The World Health Organization anticipates that the outbreak will last a year or longer.
"The scientific question for us was, ``How did cholera come to Haiti?`` It hadn``t been there for more than a hundred years," said Waldor, a microbiologist and infectious disease specialist whose laboratory at Brigham and Women``s Hospital studies cholera and other pathogenic gut bacteria.
Waldor obtained two samples of Vibrio cholerae, the bacterium that causes cholera, from two Harvard Medical School colleagues, Stephen Calderwood and Jason Harris, who traveled to Haiti in November to assess the outbreak. Waldor then established a collaboration with Pacific Biosciences, which manufactures powerful DNA sequencing machines that can rapidly scan and identify millions of bases of genetic material.
A team of scientists there, led by Eric Schadt, sequenced the complete genomes of the cholera bacteria in the samples. Waldor received the V. cholera samples on November 8 and had the bacterial DNA sequence from Pacific Biosciences in hand by November 12.
The DNA readout showed that the two Haitian strains of V. cholera – isolated from different patients – were essentially identical, supporting the idea of a single origin of the nation-wide outbreak. The two strains were also essentially identical to three other Haitian outbreak samples that had been sequenced (but not analyzed) by the Centers for Disease Control and Prevention (CDC).
The HHMI, Harvard, and Pacific Biosciences team then compared the genome of the Haitian strain to the genomes of 23 other V. cholera strains from various parts of the world that were stored in the genetic data repository GenBank.
Surprisingly, the Haiti strain bore the strongest resemblance to strains that are currently circulating in South Asia. Conversely, the Haitian strains differed significantly from the bacteria currently circulating in Latin America. Some cholera experts had suggested that endemic Latin American V. cholera – found in Peru and elsewhere – was the most likely source of the Haitian outbreak.
"The big conclusion is that the Haiti cholera epidemic is caused by a strain that was most likely introduced into Haiti from South Asia, and not from some strain that washed up environmentally from Latin America," Waldor said.
The findings have been reported in the New England Journal of Medicine (NEJM).