Inland cholera in freshwater environs of north India

In the freshwater environment of north India, cholera appears seasonally in form of clusters as well as sporadically, accounting for a significant piece of the puzzle of cholera epidemiology. We describe a number of cholera outbreaks with an average attack rate of 96.5/1000 but an overall low case fatality (0.17). Clinical cholera cases coincided with high rainfall and elevated temperatures, whereas isolation of V. cholerae non-O1 non-O139 from water was dependent on temperature (p < 0.05) but was independent of rainfall and pH (p > 0.05). However, isolation from plankton samples correlated with increased temperature and pH (p < 0.05). A lag period of almost a month was observed between rising temperature and increased isolation of V. cholerae from the environment, which in succession was followed by an appearance of cholera cases in the community a month later. Our results suggested that the aquatic environment can harbor highly divergent V. cholerae strains and serve as a reservoir for multiple V. cholera virulence-associated genes that may be exchanged via mobile genetic elements. In agreement with PFGE, AFLP data also proved that the V. cholerae O1 population was not clonal but was closely related. Our investigation did not support the concept that seasonal cholera outbreaks occur by movement of a single clonal strain across the region, as the clinical isolates from the same years were clearly different, implying that continuous evolution of V. cholerae O1 strains occurs in the cholera endemic area. Interestingly, the viable but non-culturable (VBNC) V. cholerae O1 cells were demonstrated in 2.21% samples from natural water bodies in addition to 40.69% samples from cholera-affected areas respectively. This suggests that aquatic environs do harbor the pathogenic O1 strain, though the isolation of culturable V. cholerae O1 is a rare event in the presence of relatively abundant non-O1 non-O139 isolates.