South Asia instead of Nepal may be the origin of the Haitian cholera outbreak strains.

I read with interest the article “Population Genetics of Vibrio cholerae from Nepal in 2010: Evidence on the Origin of the Haitian Outbreak” by Hendriksen et al. (1), which was published in the July-August 2011 issue of mBio. They used whole-genome sequence typing of Nepalese strains which were isolated in 2010 from different parts of the country and found that the Nepalese strains were almost identical to the Haitian strains, concluding that Nepal was the origin of the Haitian outbreak.

An unexpected outbreak of cholera occurred in mid-October 2010 in Haiti, and the origin of the strain has become a topic of great debate. In a previous study, Chin et al. (2) showed that the Haitian strains were more closely related to the Bangladeshi CIRS 101 and M4 (MDC126) strains, and it has been hypothesized that the strains might have been introduced into Haiti by human activity from a distant geographic source, probably South Asia. Meanwhile, speculation has mounted that United Nations (UN) peacekeepers from Nepal could be the source of the cholera outbreak in Haiti. Indeed, UN peacekeepers from other countries, including Nepal and Bangladesh, were deployed during September and October 2010 in Mirebalais, Haiti. Perhaps based on this rumor and the previous study, Hendriksen et al. compared the genome sequences of the Haitian and Nepalese strains which were isolated in 2010 and a Bangladeshi strain isolated in 2002 using a phylogenetic tree and demonstrated that the Nepalese strains were more closely related to the Haitian outbreak strains than the Bangladeshi CIRS 101 strain. I, however, believe that the Bangladeshi strain may have been demonstrated to be distantly related to the Haitian strains than the Nepalese strains owing to the difference in isolation time. A high level of genetic diversity, for example, was observed among cholera strains isolated elsewhere within the year (3). Therefore, the included genome sequences of recent Bangladeshi strains (2010) may also be likely to appear almost identical to the Haitian strains because the strains found in Nepal might be concurrently circulating throughout the South Asian countries, including Bangladesh, due to the close geographic location. On that ground, it is worthwhile to compare the genome sequences of recent isolates not only from Nepal but also from Bangladesh and India in order to get a more accurate picture of the origin of the Haitian strains. Most importantly, the UN has reported that all Nepalese soldiers had tested negative for the strain and none of the Nepalese soldiers had been sick during the outbreak in Haiti (4), meaning that Nepalese peacekeepers may not be the source of the Haitian strains; hence, South Asia, instead of Nepal, may be the origin of the Haitian cholera outbreak strains.