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Whole-Genome Study Nails Haiti-Nepal Cholera Link
23 August 2011 5:36 pm
A new study has yielded the most solid evidence yet that U.N. peace-keeping forces from Nepal inadvertently brought cholera to Haiti last year, setting off an epidemic that has killed more than 6000 people so far. The paper, published today in the online open access journal mBIO, is the first to compare the whole genomes of bacteria from Haitian cholera patients with those found in Nepal around the time in 2010 when the peacekeepers left their country. It found that the genomes from the two sets of bacteria are virtually identical.
Most researchers were already convinced of the link. An on-the-ground investigation by French epidemiologist Renaud Piarroux shortly after the outbreak started found ample circumstantial evidence that it originated at a Nepalese camp with dysfunctional sanitation in Haiti's Artibonite region; several limited genetic studies of the isolated cholera bacteria suggested that the microbes originated in South Asia as well. In May, an international panel appointed by U.N. Secretary-General Ban Ki-moon concluded that the data "overwhelmingly" pointed to the peacekeepers' unknowing involvement. But none of the studies published so far included genomic data from the 2010 cholera outbreak in Nepal itself.
For the new work, researchers at the National Public Health Laboratory in Kathmandu gave bacterial samples, collected between 30 July and 1 November 2010 from 24 patients in five districts in Nepal, to Frank Aarestrup and his colleagues at the National Food Institute of the Technical University of Denmark in Kongens Lyngby. The Danish researchers teamed up with Paul Keim, a microbial detective at the Translational Genomics Research Institute in Flagstaff, Arizona.
Keim and his colleagues determined the entire DNA sequence of the 24 isolates and compared it with 10 previously published genomes of the cholera bacterium—including three from Haitian patients—and drew up a phylogenetic tree showing how the various pathogens are related. They found that the isolates from Nepal formed four distinct but closely related clusters; the three Haitian isolates fell right within one of those clusters. Indeed, the Haitian samples differed from their closest Nepalese relatives by only one or two DNA base pairs.
"They're practically identical. This is as close as you can come to molecular proof" for the Nepalese link, says Harvard University microbiologist John Mekalanos, the author of the first genomic study on the issue, who had tried in vain to get his hands on samples from Nepal himself. "The authors have to be congratulated for closing the book on this issue at the molecular-genetic level."
Piarroux says the study perfectly complements his own shoe-leather work in Haiti last year. "I am very impressed that the Nepalese scientists agreed to help the truth become known," Piarroux says. "That was very courageous. I'm sure that not everybody in Nepal will be happy." Keim says the Nepalese group knew and trusted the Danish scientists; they were well aware from the outset that the outcome might finger the peacekeepers, he says, and agreed to have the results published.
The study should have a practical upshot, Piarroux says. Now that there's evidence beyond a reasonable doubt, the United Nations should accept responsibility and make amends to Haiti, he says—for instance, by offering financial compensation or by supporting an all-out effort to make the country cholera-free again. "More than 6000 people are dead," Piarroux says. "It's our fault, as the people of the world."