Zooming In On the Cholera Tree of Life (And Death)

In the wake of last year’s earthquake in Haiti, cholera arrived on the island for the first time in 60 years. According to the World Health Organization, 419, 511 Haitians got sick with cholera as of July 31, of which 5,968 died. The infection rate is dropping right now, but the arrival of Hurricane Irene could change that.

As I wrote in December, scientists applied evolutionary biology to find clues to how cholera–or, more precisely, the bacteria Vibrio cholerae— came to Haiti. They compared the DNA in the strain in Haiti to ones that have been found in other parts of the world. From this analysis, they drew a tree, which I’ve reprinted below.

The bacteria in Haiti was more closely related to strains in South Asia than ones from South America. So it was unlikely that cholera came to Haiti floating by water from a nearby country. The evolutionary tree led credence to idea that U.N. peacekeeping troops, some of whom came from Nepal, brought it with them by plane. An outbreak of cholera hit Nepal in September 2010, shortly before a battalion of Nepalese peacekeepers left for Haiti.

This analysis was a bit like a picture taken from 10,000 feet in the air. The bacteria that the scientists analyzed were just a small selection of the many strains that have made people sick over the past few decades. Notably missing from the tree were any bacteria from Nepal. That’s because those strains had not made their way into bacteria collections.

To get a picture up close–and to test the idea that U.N. peacekeepers brought cholera to Haiti–a team of Nepali, American, and Danish researchers collected 24 samples in Nepal at the end of last year. They sequenced the entire genomes of bacteria and compared them to the genomes of Haitian cholera. They reported their results today in the journal mBio.

And here’s their close-up tree. It clearly shows that the Haitian cholera strain evolved from one of four related lineages of V. cholerae circulating today in Nepal. It differs from the Nepalese strain by a single mutation.

It’s amazing that genome-sequencing methods have gotten so powerful that scientists can now use entire genomes to reconstruct an intercontinental outbreak. Yet ten months passed from the outbreak to the publication of this paper. In a blog at the mBio site, co-author Paul Keim explains why: politics. Governments can be reluctant to give up samples that might make them look bad. Building an evolutionary tree of a deadly outbreak takes more than data: it can take a lot of diplomacy, too.

Update: Martin Enserink, writing in Sciencewriting in Science, raises the question of whether the United Nations should compensate Haiti for the outbreak that this study now clearly lays at their doorstep. Hoo boy!