Genetic Variation of Vibrio cholerae during Outbreaks, Bangladesh, 2010–2011 - Volume 20, Number 1—January 2014 - Emerging Infectious Disease journal - CDC
Volume 20, Number 1—January 2014
Genetic Variation of Vibrio cholerae during Outbreaks, Bangladesh, 2010–2011
Shah M. Rashed, Andrew S. Azman, Munirul Alam, Shan Li, David A. Sack, J. Glenn Morris, Ira Longini, Abul Kasem Siddique, Anwarul Iqbal, Anwar Huq, Rita R. Colwell, R. Bradley Sack, and O. Colin Stine
Author affiliations: icddr,b, Dhaka, Bangladesh (S.M. Rashed, M. Alam, A.K. Siddique, A. Iqbal); Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA (A.S. Azman, D.A. Sack, R.B. Sack); University of Maryland, Baltimore (S. Li, O.C. Stine);University of Florida, Gainesville, Florida, USA (J.G. Morris, Jr., I. Longini); University of Maryland, College Park, Maryland, USA (A. Huq, R.R. Colwell)
In many areas of the world, cholera remains a major public health problem; it affects millions of persons each year and causes a substantial number of deaths (1,2). In Bangladesh, cholera transmission is seasonal; 2 annual peaks are initiated by emergence of Vibrio cholerae from environmental reservoirs (3,4). The infectious dose of V. cholerae is estimated to be 105 to 108CFU; the lower estimates are associated with a buffered stomach (5). After the organism enters the body, a physiologic change is induced, which alters the expression of most V. cholerae genes (6). One outcome of this alteration is a brief hyperinfectious state, during which V. choleraeexiting the colon are infectious at a reduced dose (7). After returning to the water for 24 hours,V. cholerae reverts to a standard infectious state (7,8). The relative contribution of the recently shed hyperinfectious strains and the strains from the environment to propagation of an outbreak of cholera remains controversial. The terms “slow” and “fast” have been used to distinguish between these 2 modes of transmission; slow refers to the human-to–aquatic environment–to-human pathway (which does not have time constraints), and fast refers to presumed person-to-person or person-to–household environment–to-person transmission (in which transfer is more likely immediately after fecal shedding, when strains are hyperinfectious) (9).
To determine the relative contribution of the hypothesized slow and fast routes of transmission during outbreaks, researchers have undertaken microbiological, genetic, and modeling approaches. However, a major problem with the first 2 approaches has been a lack of genetic diversity to track strains. Many methods, including pulsed-field gel electrophoresis (often used for outbreak analysis), detect too few genetic differences between isolates to be useful in tracking the microdynamics of strains. This problem was resolved, in part, by identification of loci containing a variable number of tandem repeats, which provided enough genetic variability to permit tracking of specific strains (10–12). However, initial studies that used multilocus variable tandem repeat analysis (MLVA) did not sample intensively enough to optimally distinguish between slow and fast transmission. One study, conducted in rural Bangladesh, in which isolates were collected every 2 weeks, showed that isolates from different geographic locations collected during different seasons and from clinical and environmental sources had only a few genotypes in common (12). Genotypes tended to be similar to one another when isolates were collected during the same season and came from the same geographic location as opposed to coming from different seasons or sources. Although this finding could be interpreted as evidence in support of fast transmission, most environmental isolates were not collected during the same month as most of the clinical isolates, making interpretation of data difficult. In another study, isolates from 100 index case-patients and their household relatives were analyzed (13). Remarkably, isolates from persons within a single household were often genetically unrelated, implying either different sources of infection or a single source with multiple genetic lineages. The unexpected variability was reinforced by the observation that unrelated genetic isolates were often isolated from a single fecal sample. However, the study design of sampling 3 households per month did not provide sufficient resolution to address transmission pathways. Mathematical modeling of incidence data from outbreaks has been used to estimate the contribution of fast and slow transmission (14,15). Although promising, these estimates have been based on clinical surveillance data without more detailed underlying epidemiologic information. Furthermore, it might not be possible to estimate the contribution of 2 transmission mechanisms from incidence data alone when the time scale of slow transmission is similar to that of fast transmission (16).
In this study, we used MLVA to characterize 222 clinical and environmental V. cholerae O1 isolates from outbreaks in 2 geographically distinct locations. Our objective was to determine genetic relatedness among the isolates, notably clonal relationships between environmental and clinical isolates, and to further explore the relative contribution of different transmission pathways to disease occurrence.