Critical factors influencing the occurrence of Vibrio cholerae in the environment of Bangladesh

Abstract

The occurrence of outbreaks of cholera in Africa in 1970 and in Latin America in 1991, mainly in coastal communities, and the appearance of the new serotype Vibrio cholerae O139 in India and subsequently in Bangladesh have stimulated efforts to understand environmental factors influencing the growth and geographic distribution of epidemic Vibrio cholerae serotypes. Because of the severity of recent epidemics, cholera is now being considered by some infectious disease investigators as a "reemerging" disease, prompting new work on the ecology of vibrios. Epidemiological and ecological surveillance for cholera has been under way in four rural, geographically separated locations in Bangladesh for the past 4 years, during which both clinical and environmental samples were collected at biweekly intervals. The clinical epidemiology portion of the research has been published (Sack et al., J. Infect. Dis. 187:96-101, 2003). The results of environmental sampling and analysis of the environmental and clinical data have revealed significant correlations of water temperature, water depth, rainfall, conductivity, and copepod counts with the occurrence of cholera toxin-producing bacteria (presumably V. cholerae). The lag periods between increases or decreases in units of factors, such as temperature and salinity, and occurrence of cholera correlate with biological parameters, e.g., plankton population blooms. The new information on the ecology of V. cholerae is proving useful in developing environmental models for the prediction of cholera epidemics.

FIG. 1.

Four sites, Bakerganj, Chaugachha, Chhatak, and Matlab, where samples were collected every 2 weeks between March 1997 and December 2000. (Reprinted from reference 29 with permission of the publisher.)

FIG. 2.

Correlation between cases of cholera and water temperature in Bakerganj Lake 2.

FIG. 3.

Correlation between cases of cholera and detection of ctx gene in Bakerganj Lake 2.

FIG. 4.

Correlation between cases of cholera and rainfall in mm in Bakerganj area.

FIG. 5.

Correlation between cases of cholera and conductivity measurements in Bakerganj Lake 2.

FIG. 6.

Observed number of cholera cases versus number of cases predicted by the Poisson regression model and upper 95% prediction limit when using water temperature (lag, 6 weeks), ctx gene probe count (lag, 0 weeks), conductivity (lag, 0 weeks), and rainfall (lag, 8 weeks) for Lake 2 in Bakerganj.