Clonal transmission, dual peak, and off-season cholera in Bangladesh

Figures & data

Fig. 1.  Molecular fingerprinting analysis using pulsed-field gel electrophoresis (PFGE) of NotI-digested genomic DNA obtained from V. cholerae serogroup O1 biotype El Tor strains isolated during the peak of the Dhaka epidemic, 2007, and pre-existing V. cholerae hybrid El Tor strains isolated between 2004 and 2007 in Dhaka and Matlab, two inland (freshwater) cholera endemic sites located 50 km apart and Barisal, a cholera endemic coastal ecosystem of Bangladesh. The dendrogram was constructed using digital images of PFGE patterns and Dice similarity coefficient and UPGMA clustering methods.

Fig. 2.  Comparative yearly data showing weekly admissions of diarrheal patients to the ICDDR,B hospital in Dhaka (2006–2008) during the unusual flood-related diarrheal epidemic that broke out in August 2007. The graph shows number of cases per week, based on actual numbers of diarrheal patients admitted to the ICDDR,B hospital. Number of cases for 2008 are included up to the third week of August.

Table 1. Phenotypic and molecular characteristics of Vibrio cholerae Ol strains isolated during the flood-related Dhaka epidemic in 2007 and V. cholerae O1 strains isolated between 2004 and 2007 in Dhaka and Matlab. V. cholerae O1 classical and El Tor strains were included as controls

Sl. No.	Location	Strain ID#	Serotype	omp U	tox R	tcpA Cla	tcpAET	ace	wbe 01	CtxA	CCA	PolyB	rstR type	CtxB type
1	Dhaka	117514	INET	+	+	−	+	+	+	+	+	R	2	Classical
2	Dhaka	117583	OGET	+	+	−	+	+	+	+	+	R	2	Classical
3	Dhaka	117645	OGET	+	+	−	+	+	+	+	+	R	2	Classical
4	Dhaka	117644	INET	+	+	−	+	+	+	+	+	R	2	Classical
5	Dhaka	117685	OGET	+	+	−	+	+	+	+	+	R	2	Classical
6	Dhaka	117773	OGET	+	+	−	+	+	+	+	+	R	2	Classical
7	Dhaka	117633	INET	+	+	−	+	+	+	+	+	R	2	Classical
8	Dhaka	117911	INET	+	+	−	+	+	+	+	+	R	2	Classical
9	Dhaka	117935	OGET	−	−	−	−	−	+	+	+	R	2	Classical
10	Dhaka	118012	INET	+	+	−	+	+	+	+	+	R	2	Classical
11	Dhaka	118113	INET	+	+	−	+	+	+	+	+	R	2	Classical
12	Dhaka	117793	OGET	+	+	−	+	+	+	+	+	R	2	Classical
13	Dhaka	117902	INET	+	+	−	+	+	+	+	+	R	2	Classical
14	Dhaka	117750	OGET	+	+	−	+	+	+	+	+	R	2	Classical
15	Dhaka	117743	OGET	+	+	−	+	+	+	+	+	R	2	Classical
16	Dhaka	117677	INET	+	+	−	+	+	+	+	+	R	2	Classical
17	Dhaka	117675	INET	+	+	−	+	+	+	+	+	R	2	Classical
18	Dhaka	117875	INET	+	+	−	+	+	+	+	+	R	2	Classical
19	Dhaka	117865	OGET	+	+	−	+	+	+	+	+	R	2	Classical
20	Matlab	L01-511	INET	+	+	−	+	+	+	+	+	R	2	Classical
21	Matlab	L01-928	INET	+	+	−	+	+	+	+	+	R	2	Classical
22	Matlab	L01-1786	INET	+	+	−	+	+	+	+	+	R	2	Classical
23	Matlab	L01-1642	OGET	+	+	−	+	+	+	+	+	R	2	Classical
24	Matlab	L01-1773	OGET	+	+	−	+	+	+	+	+	R	2	Classical
25	Matlab	L01-1782	OGET	+	+	−	+	+	+	+	+	R	2	Classical
26		569B	CL Control.	+	+	+	−	+	+	+	−	S	1	Classical
27		N16961	ET Control.	+	+	−	+	+	+	+	+	R	2	ElTor

Cca, chicken cell agglutination; Poly B, polymixixn B; R, resistant; S, sensitive; ET, El Tor; CL, classical; IN, Inaba; OG, Ogawa; ctxB type was confirmed by MAMA-PCR followed by DNA sequencing and analysis.

Fig. 3.  Map of Dhaka city showing the circular river systems along its border that receive the sewage effluent of nearly 15 million inhabitants. The city has an extensive, but not fully effective, sewage network linked to ponds, lakes, and canals that flow into the surrounding river systems. The circular arrows at the center indicate clonal recycling of V. cholerae in Dhaka, a fresh-water ecosystem situated 250 km distant from the estuarine ecosystem of the Bay of Bengal, Bangladesh. Unlike the estuarine ecosystem, where V. cholerae comprises a portion of the autochthonous microflora (12) in association with plankton year round (13,14), the water bodies of Dhaka serve as a constant source of highly transmissible V. cholerae. During extreme climatic conditions such as flooding in the poor urban setting of Dhaka, fecal-oral transmission is enhanced leading to significant off-season outbreaks, as occurred during flooding in 2007 in Dhaka. Flooding churns the waters, bringing sediment and V. cholerae biofilm attached to particulates into the surface water, thereby enriching the V. cholerae population.

Fig. 4.  (A) Climatological analysis of Bangladesh monsoon precipitation for 1980–2009 shows the relative magnitude of combined monsoon (JJAS: June–September) rainfall in 2007. The normalized anomaly values indicate 2007 was not a record year. (B) However, anomaly analysis of monthly rainfall shows that July 2007 was the highest monthly amount (for July) recorded for the Brahmaputra catchment areas in 30 years. In addition, the Ganges catchment areas also experienced very heavy rainfall in July 2007, as shown by the anomaly calculated against the 30 year base period (1980–2009).