Self-limiting nature of seasonal cholera epidemics: Role of host-mediated amplification of phage

Abstract

Phage predation of Vibrio cholerae has recently been reported to be a factor that influences seasonal epidemics of cholera in Bangladesh. To understand more about this phenomenon, we studied the dynamics of the V. cholerae-phage interaction during a recent epidemic in Dhaka. Because the outbreak strain causing this epidemic was resistant to multiple antibiotics, including streptomycin, we used a selective medium containing streptomycin to monitor accurately the abundance of this strain in the environment. The changing prevalence in the environment of the epidemic V. cholerae O1 strain and a particular lytic cholera phage (JSF4) to which it was sensitive was measured every 48-72 h for 17 weeks. We also monitored the incidence of phage excretion in stools of 387 cholera patients during the epidemic. The peak of the epidemic was preceded by high V. cholerae prevalence in the environment and was followed by high JSF4 phage levels as the epidemic ended. The buildup to the phage peak in the environment coincided with increasing excretion of the same phage in the stools of cholera patients. These results suggest that patients toward the end of the epidemic ingested both JSF4 phage and the outbreak V. cholerae strain. Host-mediated phage amplification during the cholera epidemic likely contributed to increased environmental phage abundance, decreased load of environmental V. cholerae and, hence, the collapse of the epidemic. Thus, in vivo phage amplification in patients and subsequent phage predation in the environment may explain the self-limiting nature of seasonal cholera epidemics in Bangladesh.

Fig. 1.

Map of Dhaka showing the environmental sampling sites (•) and the location of the ICDDRB cholera hospital at the center of the city.

Fig. 2.

Dynamics of epidemic V. cholerae O1 and vibriophage concentration in surface water during a cholera epidemic in Bangladesh. Weekly mean V. cholerae O1 levels (A) and lytic vibriophage levels (B) in the aquatic environment were taken during the course of a cholera epidemic in Bangladesh. Environmental sampling was conducted from the third week of August to the third week of December 2004. The number of cholera patients are extrapolated from 2% surveillance samples.

Fig. 3.

Vibriophage excretion by cholera patients and its correlation with environmental prevalence of the same phage. Excretion of lytic vibriophages in cholera stools (A) and weekly mean vibriophage levels in the aquatic environment (B) were taken during the course of a cholera epidemic.

Fig. 4.

Schematic model for factors influencing seasonal epidemics of cholera. Events occurring in the environment are shown in circles, whereas those occurring in human cholera victims are shown in rectangles. 1, Seasonal conditions (e.g., floods or physical-chemical factors causing phage instability) lead to the reduction of viable lytic phages in the aquatic environment. 2, Other seasonal factors (e.g., elevated temperature or biological-cofactors) lead to a bloom of nonpathogenic and pathogenic Vibrio species in local water sources. 3, A favorable aquatic environment facilitates infection by a pathogenic V. cholerae strain (black vibrios), causing an asymptomatic or undetected symptomatic infection in an index case. 4, Amplification of the pathogenic clone in the index case seeds the environment. 5, An epidemic begins, triggered by the index case leading to host amplification of the pathogenic clone and massive environmental contamination. 6, Phages produced by environmental Vibrio species (e.g., through induction of lysogens or lytic growth) begin to amplify on the pathogenic clone in the environment. 7, Cholera victims increasingly ingest both the pathogenic clone and the lytic phage from environmental sources leading to amplification of the phage in vivo. 8, Environmental contamination with phage shed by cholera victims and further amplification of phage on the pathogenic clone within the environment lead to collapse of the epidemic. Factors contributing to the collapse of the epidemic include the decreasing concentration of the pathogenic clone in the environment coincident with a upsurge in environmental phage concentration that together contribute to a dramatic reduction in the rate of transmission of cholera in the interepidemic period.