Guy R. Knudsen
Recent news stories, originating from a National Public Radio blog article and interview with Dr. Rita Colwell of the University of Maryland, have provided a new but weakly-documented hypothesis about the origin of the cholera epidemic that has ravaged Haiti for almost two years. The impetus for the NPR blog, “Scientists find new wrinkle in how cholera got to Haiti”, was a recent article by N. A. Hasan et al., entitled “Genomic diversity of 2010 Haitian cholera outbreak strains”. The article appears in a recent issue of Proceedings of the National Academy of Sciences (PNAS), as a paper contributed by co-author Colwell.
The take-home message of the NPR interview with Colwell and her colleague David Sack of the Johns Hopkins Bloomberg School of Public Health, is that strains of the cholera pathogen may have resided undetected in the Haitian environment since long before the reported dumping of large quantities of human sewage by Nepalese MINUSTAH (United Nations) forces into Haitian waterways. This hypothesis contradicts the widely-held belief that MINUSTAH negligently introduced the cholera pathogen into Haiti, a view which is based on several factors including a comprehensive statistical analysis of the timing and rate of disease spread (Piarroux et al. 2011), along with scientific reports (e.g., Ali et al. 2011; Chin et al. 2011; Hendricksen et al. 2011; Reimer et al. 2011) of extremely close genetic similarity between Vibrio cholerae from Haitian cholera victims and Asian isolates of the pathogen, including isolates from Nepal where cholera is endemic (and where, in the fall of 2010, a major cholera epidemic was already in progress). In her interview, Colwell provides an imaginative but strained scenario, in which a resident but previously undetected cholera pathogen was given the chance to rapidly multiply by a “perfect storm” concurrence of the 2010 earthquake (shaking up Haiti’s limestone bedrock, thereby making rivers more alkaline), an unusually hot summer that year, and a hurricane that presumably stirred up nutrients into an aquatic broth which then nourished the bacteria.
When looked at more closely, this hypothesis has several flaws and only a tenuous connection to the results reported in the PNAS paper. While it is true that Haiti’s geologic underpinnings are largely limestone, and limestone features are present in many parts of the country, it has not been shown that an earthquake whose epicenter was southwest of Port-au-Prince would have disrupted bedrock underlying the upper reaches of the Artibonite river, to the northeast and outside of the zone of highest earthquake intensity, thereby altering the pH of the river or its tributaries. Similarly, data have not been put forward to support the contention that summer temperatures in 2010 were sufficiently higher than Haiti’s typical hot summers to raise river and estuary water temperatures to more Vibrio-favorable levels. Haiti, of course, is chronically plagued with hurricanes and tropical storms, but the most significant such event of 2010, hurricane Tomas, occurred in November, after the first reports of epidemic cholera on the island. Its role as a predisposing event seems improbable. However, it is likely that the resultant flooding created conditions that favored the further spread of the disease.
Although the PNAS paper by Hasan et al. is cited by Colwell as providing evidence that “local (Haitian) strains are involved” in the epidemic, this contention is very questionable. For example, although relatively few details of the authors’ sampling methodology are provided in their paper, they identified both the O1 serotype (closely resembling the Nepalese strain) as well as non-O1/O139 Vibrio cholerae from stool samples obtained at several locations in Haiti late in 2010. The latter serogroups, according to the Centers for Disease Control and Prevention (CDC), can cause a diarrheal disease which is less severe than cholera and is not considered to have epidemic potential. In fact, non-O1/O139 serotypes have a natural reservoir in sea and coastal waters, and they cause a number of non-cholera infections in the U.S. each year, primarily through consumption of raw or undercooked seafood such as oysters. Non-O1/O139 V. cholerae have previously been reported to be associated with seawater and plankton from coastal marine areas of the Caribbean (Fernandez-Delgado et al. 2009), so their presence in Haiti does not seem especially surprising, now that someone has looked for them. It is certainly plausible that non-O1/O139 (i.e., non-cholera) strains have been present in the Haitian environment and/or human population, and perhaps have become more prevalent due to climatic change, but there is no evidence linking them to the cholera epidemic. Unfortunately, Hasan et al. provide no information, either in their manuscript or the supporting information, about the health status of individuals from whom they obtained clinical (stool) samples. Presumably, if any of those individuals from whom only the non-O1/O139 V. cholerae was isolated had actually been diagnosed with acute cholera symptoms, it would at least have been worthy of mention, since those serotypes are not presently known to cause the disease. In that light, any assertion that local (Haitian) strains are implicated in the cholera epidemic is unsubstantiated at best.
The paper by Hasan et al. does provide useful information about genetic diversity among V. cholerae isolates, and the mechanisms and rates by which genes are exchanged among strains. The phenomenon and ubiquitous nature of mobile genetic elements, or MGEs, in V. cholerae is well known, and this paper, to its credit, does provide additional data on that subject. However, the previous statistical and epidemiological analysis of the early stages of the Haiti cholera epidemic by Piarroux et al., as well as a number of molecular genetics studies which are cited above, provide much more thorough and convincing evidence about the origin of this epidemic, and strongly support the theory of foreign importation of the virulent strain that has killed so many Haitians.
Guy R. Knudsen is Professor of Microbial Ecology & Plant Pathology at the University of Idaho, a board member of Paloma Institute, a non-profit organization that supports local agriculture and public health in Haiti, and a private practice attorney with interests in environmental and human rights law.