Changing epidemiology of cholera due to Vibrio cholerae O1 and O139 Bengal in Dhaka, Bangladesh

Burden of typhoid fever and cholera: similarities and differences. Prevention strategies for European travelers to endemic/epidemic areas

The burden of diarrheal diseases is very high, accounting for 1.7 to 5 billion cases per year worldwide. Typhoid fever (TF) and cholera are potentially life-threatening infectious diseases, and are mainly transmitted through the consumption of food, drink or water that have been contaminated by the feces or urine of subjects excreting the pathogen. TF is mainly caused by Salmonella typhi, whereas cholera is caused by intestinal infection by the toxin-producing bacterium Vibrio cholerae. These diseases typically affect low- and middle-income countries where housing is overcrowded and water and sanitation are poor, or where conflicts or natural disasters have led to the collapse of the water, sanitation and healthcare systems. Mortality is higher in children under 5 years of age. Regarding their geographical distribution, TF has a high incidence in sub-Saharan Africa, India and south-east Asia, while cholera has a high incidence in a few African countries, particularly in the Horn of Africa and the Arabian Peninsula. In the fight against these diseases, preventive measures are fundamental. With modern air travel, transmissible diseases can spread across continents and oceans in a few days, constituting a threat to global public health. Nowadays, people travel for many reasons, such as tourism and business. Several surveys have shown that a high proportion of travelers lack adequate information on safety issues, such as timely vaccination and prophylactic medications. The main objective of this overview is to provide information to help European travelers to stay healthy while abroad, and thus also to reduce the potential importation of these diseases and their consequent implications for public health and society. The preventive measures to be implemented in the case of travel to countries where these diseases are still endemic are well known: the adoption of safe practices and vaccinations. It is important to stress that an effective preventive strategy should be based both on vaccinations and on hygiene travel guidelines. Furthermore, the emergence of multidrug-resistant strains is becoming a serious problem in the clinical treatment of these diseases. For this reason, vaccination is the main solution.

High quality reference genomes for toxigenic and non-toxigenic Vibrio cholerae serogroup O139

Toxigenic Vibrio cholerae of the O139 serogroup have been responsible for several large cholera epidemics in South Asia, and continue to be of clinical and historical significance today. This serogroup was initially feared to represent a new, emerging V. cholerae clone that would lead to an eighth cholera pandemic. However, these concerns were ultimately unfounded. The majority of clinically relevant V. cholerae O139 isolates are closely related to serogroup O1, biotype El Tor V. cholerae, and comprise a single sublineage of the seventh pandemic El Tor lineage. Although related, these V. cholerae serogroups differ in several fundamental ways, in terms of their O-antigen, capsulation phenotype, and the genomic islands found on their chromosomes. Here, we present four complete, high-quality genomes for V. cholerae O139, obtained using long-read sequencing. Three of these sequences are from toxigenic V. cholerae, and one is from a bacterium which, although classified serologically as V. cholerae O139, lacks the CTXφ bacteriophage and the ability to produce cholera toxin. We highlight fundamental genomic differences between these isolates, the V. cholerae O1 reference strain N16961, and the prototypical O139 strain MO10. These sequences are an important resource for the scientific community, and will improve greatly our ability to perform genomic analyses of non-O1 V. cholerae in the future. These genomes also offer new insights into the biology of a V. cholerae serogroup that, from a genomic perspective, is poorly understood.

Staying Alive: Vibrio cholerae's Cycle of Environmental Survival, Transmission, and Dissemination

Infectious diseases kill nearly 9 million people annually. Bacterial pathogens are responsible for a large proportion of these diseases, and the bacterial agents of pneumonia, diarrhea, and tuberculosis are leading causes of death and disability worldwide. Increasingly, the crucial role of nonhost environments in the life cycle of bacterial pathogens is being recognized. Heightened scrutiny has been given to the biological processes impacting pathogen dissemination and survival in the natural environment, because these processes are essential for the transmission of pathogenic bacteria to new hosts. This chapter focuses on the model environmental pathogen Vibrio cholerae to describe recent advances in our understanding of how pathogens survive between hosts and to highlight the processes necessary to support the cycle of environmental survival, transmission, and dissemination. We describe the physiological and molecular responses of V. cholerae to changing environmental conditions, focusing on its survival in aquatic reservoirs between hosts and its entry into and exit from human hosts.

Aquatic Ecosystems, Human Health, and Ecohydrology

This chapter treats two main topics: the relationship between human health, aquatic ecosystems, and water use; and the necessity of interdisciplinary approaches for the development of water management policies and disease control.

Main waterborne diseases, mostly affecting developing countries and relevant in terms of water management and changes in land use, such as malaria, schistosomiasis, or cholera, are discussed stressing links to the global water crisis. Also, the role of artificial and natural wetlands in influenza epidemics is treated. The effects of increasing water use and scarcity on human health are discussed considering historical and contemporary incidence of diarrheal diseases in European and South Asian megacities, relationships between dams and on waterborne diseases in Asia and Africa, and intensive agri- and aquaculture resulting in man-made ecotones, fragmented aquatic ecosystems, and pathogen mutations.

It is emphasized that the comprehension of the multiple interactions among changes in environmental settings, land use, and human health requires a new synthesis of ecohydrology, biomedical sciences, and water management for surveillance and control of waterborne diseases in basin-based, transboundary health systems. Surveillance systems should monitor changes in water management, ecotones, and hydrological cycles and shifts in, for example, the outbreak timing of strongly seasonal diseases. These indicators would provide criteria for the development of innovative water management policies, combining methods of vector control and the safe creation of water reservoirs, irrigation systems, and wetland habitats.

A comparison of clinical and immunologic features in children and older patients hospitalized with severe cholera in Bangladesh

BACKGROUND

: Infection with Vibrio cholerae induces protection from subsequent severe disease, suggesting that an effective vaccine could be an important preventive strategy. Available vaccines provide less protection against cholera than natural infection, particularly in children.

METHODS

: We examined a cohort of 121 children (2 years-12 years of age) and 276 older patients (>12 years of age) hospitalized with cholera in Dhaka, Bangladesh over a 4-year period, to compare clinical features in older patients and children and immune responses to key antigens.

RESULTS

: Older patients had more severe disease. Children with cholera were more commonly retinol deficient, while zinc deficiency was equally prevalent in both groups. Children developed higher vibriocidal and serum immune responses to the B subunit of cholera toxin (CTB). In contrast, older patients mounted higher immune responses to 2 other key V. cholerae antigens, the lipopolysaccharide (LPS) and toxin coregulated pilus antigens (TcpA). We compared immune responses following infection with those occurring after receipt of a live, oral vaccine in both children and older patients in Bangladesh, during a similar time period. The response rates for vibriocidal and LPS antibodies were higher after infection than after vaccination. Both vaccinated older patients and children responded poorly to CTB and TcpA.

CONCLUSIONS

: Although children developed vigorous vibriocidal and CTB-specific responses following infection, they had lessened responses to LPS and TcpA compared with older patients, as well as lessened responses to vaccination. More studies need to be carried out to determine factors, including micronutrient interventions that can improve responses in children to both natural infection and vaccination.

Diarrhoeal outbreak of Vibrio cholerae 01 Inaba in Delhi

SUMMARY

V. cholerae O1 Eltor serotype Ogawa has been causing most of the cholera outbreaks in India till recently. However this communication reports the occurrence of Vibrio Cholerae O1 Inaba in Delhi in 2005, as a predominant causative organism of cholera in children. All strains isolated were sensitive to gentamicin and a high level of resistance towards nalidixic acid and amoxicillin was seen. There was no case fatality.

Prediction of epidemic cholera due to Vibrio cholerae O1 in children younger than 10 years using climate data in Bangladesh

To determine if a prediction of epidemic cholera using climate data can be made, we performed autoregression analysis using the data recorded in Dhaka City, Bangladesh over a 20-year period (1983-2002) comparing the number of children aged <10 years who were infected with Vibrio cholerae O1 to the maximum and minimum temperatures and rainfall. We formulated a simple autoregression model that predicts the monthly number of patients using earlier climate variables. The monthly number of patients predicted by this model agreed well with the actual monthly number of patients where the Pearson's correlation coefficient was 0.95. Arbitrarily defined, 39.4% of the predicted numbers during the study period were within 0.8-1.2 times the observed numbers. This prediction model uses the climate data recorded 2-4 months before. Therefore, our approach may be a good basis for establishing a practical early warning system for epidemic cholera.

Reemergence of epidemic Vibrio cholerae O139, Bangladesh

During March and April 2002, a resurgence of Vibrio cholerae O139 occurred in Dhaka and adjoining areas of Bangladesh with an estimated 30,000 cases of cholera. Patients infected with O139 strains were much older than those infected with O1 strains (p<0.001). The reemerged O139 strains belong to a single ribotype corresponding to one of two ribotypes that caused the initial O139 outbreak in 1993. Unlike the strains of 1993, the recent strains are susceptible to trimethoprim, sulphamethoxazole, and streptomycin but resistant to nalidixic acid. The new O139 strains carry a copy of the Calcutta type CTX^Calc prophage in addition to the CTX^ET prophage carried by the previous strains. Thus, the O139 strains continue to evolve, and the adult population continues to be more susceptible to O139 cholera, which suggests a lack of adequate immunity against this serogroup. These findings emphasize the need for continuous monitoring of the new epidemic strains.

The mannose-sensitive hemagglutinin of Vibrio cholerae promotes adherence to zooplankton

The bacterium Vibrio cholerae, the etiological agent of cholera, is often found attached to plankton, a property that is thought to contribute to its environmental persistence in aquatic habitats. The V. cholerae O1 El Tor biotype and V. cholerae O139 strains produce a surface pilus termed the mannose-sensitive hemagglutinin (MSHA), whereas V. cholerae O1 classical biotype strains do not. Although V. cholerae O1 classical does not elaborate MSHA, the gene is present and expressed at a level comparable to that of the other strains. Since V. cholerae O1 El Tor and V. cholerae O139 have displaced V. cholerae O1 classical as the major epidemic strains over the last fifteen years, we investigated the potential role of MSHA in mediating adherence to plankton. We found that mutation of mshA in V. cholerae O1 El Tor significantly diminished, but did not eliminate, adherence to exoskeletons of the planktonic crustacean Daphnia pulex. The effect of the mutation was more pronounced for V. cholerae O139, essentially eliminating adherence. Adherence of the V. cholerae O1 classical mshA mutant was unaffected. The results suggest that MSHA is a factor contributing to the ability of V. cholerae to adhere to plankton. The results also showed that both biotypes of V. cholerae O1 utilize factors in addition to MSHA for zooplankton adherence. The expression of MSHA and these additional, yet to be defined, adherence factors differ in a serogroup- and biotype-specific manner.

Case-control study of enteropathogens associated with childhood diarrhea in Dhaka, Bangladesh

The International Centre for Diarrhoeal Disease Research, Bangladesh, is a major center for research into diarrheal diseases. The center treats more than 100,000 patients a year. To obtain useful information representative of all patients, a surveillance system in which a 4% systematic sample of all patients is studied in detail, including etiological agents of diarrhea, was installed in October 1979. The first paper on etiology for the surveillance patients was published in 1982, which identified a potential enteric pathogen in 66% of patients. In subsequent years, several new agents of diarrhea have been identified. To assess the importance of a broader spectrum of diarrheal agents including the ones identified relatively recently, we studied 814 children with diarrhea. The children were up to 5 years of age and were part of the surveillance system. They were matched with an equal number of community controls without diarrhea. The study was conducted from February 1993 to June 1994. A potential enteric pathogen was isolated from 74.8% of diarrheal children and 43.9% of control children (P = 0.0001). Even though the first study was not a case-control study, it identified rotavirus, Campylobacter jejuni, enterotoxigenic Escherichia coli, Shigella spp. , and Vibrio cholerae O1 as major pathogens. The present study identified these pathogens as being significantly associated with diarrhea. In addition, the study also identified six additional agents, including enteropathogenic E. coli, Aeromonas spp., V. cholerae O139, enterotoxigenic Bacteroides fragilis, Clostridium difficile, and Cryptosporidium parvum, as being significantly associated with diarrhea. Plesiomonas shigelloides, Salmonella spp., diffusely adherent E. coli, enteroaggregative E. coli, Entamoeba histolytica, and Giardia lamblia were not significantly associated with diarrhea. Enteroinvasive E. coli, enterohemorrhagic E. coli, and Cyclospora cayetanensis were not detected in any of the children. The major burden of diseases due to most pathogens occurred in the first year of life. As in the previous study, seasonal patterns were seen for diarrhea associated with rotavirus, V. cholerae, and enterotoxigenic E. coli, and infections with multiple pathogens were common. With a few exceptions, these findings are in agreement with those from other developing countries. This knowledge of a broader spectrum of etiological agents of diarrhea in the surveillance patients will help us plan studies into various aspects of diarrheal diseases in this population.

The Vibrio cholerae mannose-sensitive hemagglutinin is the receptor for a filamentous bacteriophage from V. cholerae O139

We previously isolated from a 1994 isolate of Vibrio cholerae O139 a filamentous lysogenic bacteriophage, choleraphage 493, which inhibits pre-O139 but not post-O139 El Tor biotype V. cholerae strains in plaque assays. We investigated the role of the mannose-sensitive hemagglutinin (MSHA) type IV pilus as a receptor in phage 493 infection. Spontaneous, Tn5 insertion, and mshA deletion mutants are resistant to 493 infection. Susceptibility is restored by mshA complementation of deletion mutants. Additionally, the 493 phage titer is reduced by adsorption with MSHA-positive strains but not with a DeltamshA1 strain. Monoclonal antibody against MSHA inhibits plaque formation. We conclude that MSHA is the receptor for phage 493. The emergence and decline of O139 in India and Bangladesh are correlated with the susceptibility and resistance of El Tor strains to 493. However, mshA gene sequences of post-O139 strains are identical to those of susceptible pre-O139 isolates, indicating that phage resistance of El Tor is not due to a change in mshA. Classical biotype strains are (with rare exceptions) hemagglutinin negative and resistant to 493 in plaque assays. Nevertheless, they express the mshA pilin gene. They can be infected with 493 and produce low levels of phage DNA, like post-O139 El Tor strains. Resistance to 493 in plaque assays is thus not equivalent to resistance to infection. The ability of filamentous phages, such as 493, to transfer large amounts of DNA provides them, additionally, with the potential for quantum leaps in both identity and pathogenicity, such as the conversion of El Tor to O139.

Heterogeneity in the organization of the CTX genetic element in strains of Vibrio cholerae O139 Bengal isolated from Calcutta, India and Dhaka, Bangladesh and its possible link to the dissimilar incidence of O139 cholera in the two locales

After a lapse of 33 months, Vibrio cholerae O139, the new serogroup associated with cholera, has re-emerged in Calcutta, India and has become the dominant serogroup causing cholera from September 1996. In neighbouring Bangladesh, V. cholerae O1 biotype El Tor continues to be the dominant cause of cholera with the O139 serogroup accounting for only a small proportion of cases. Comparison of the phenotypic traits of representative O139 strains from Calcutta and Dhaka isolated between December 1996 and April 1997 showed similar phenotypic traits with the exception that Dhaka O139 strains were susceptible to streptomycin whilst Calcutta O139 strains were resistant. The Dhaka and Calcutta O139 strains displayed identical ribotypes but showed remarkable differences in the structure and organization of the CTX genetic element. In the Dhaka O139 strains, two copies of the CTX element were arranged in tandem and this resembled the pattern displayed by the 1992 epidemic strains of O139. The Calcutta O139 strains, in contrast, carried three copies of the CTX genetic element arranged in tandem with the loss of a conserved BglII restriction site in the RS1 element and the appearance of a new HindIII site in the same region. While there may be other factors, it appears that the reorganization of the CTX genetic element in the Calcutta O139 strains may have contributed to the resurgence of this serogroup in Calcutta.

Characterization and possible functions of a new filamentous bacteriophage from Vibrio cholerae O139

The emergence and rapid rise to dominance of Vibrio cholerae O139 in India and Bangladesh in 1992 led to the consideration that choleraphage might serve as both a selective mechanism and a means for horizontal transmission of genetic information. A filamentous phage '493' from O139 strain AJ27-493 has been purified and partially characterized. The phage was inactive on classical biotype V. cholerae O1 but it was active on El Tor biotype strains isolated prior to 1994 when El Tor re-emerged in Bangladesh. More recent El Tor isolates were all resistant to the phage. The phage was also active on O139 strains. Unlike the filamentous ctx phi, the receptor for 493 is not TcpA. The phage genome was a 9.3 kb closed circular single-stranded molecule containing a 0.4 kb double-stranded stem supporting a 2 kb single-stranded loop. A 283 bp fragment was cloned and used as a probe in Southern hybridization, in parallel with total phage 493 DNA. These probes hybridized both chromosomally and extrachromosomally with most O139 strains, but not with O1 strains. Infection of hybridization-negative El Tor or O139 strains resulted in the presence of hybridizing loci (both plasmid and chromosomal), in the appearance of an 18 kDa protein, and in marked alterations in colonial morphology. Phage 493 is clearly distinct from other O139 choleraphages which have been described. Phage 493 DNA hybridized with an encapsulated non-O1 (O31) strain (NRT36S) which was isolated before O139 was recognized. NRT36S also produces a phage which can infect El Tor strains with low efficiency. Further studies may reveal whether bacteriophage play a role in the emergence and the territoriality of new choleragenic vibrios.

Phenotypic and genotypic changes in Vibrio cholerae O139 Bengal

To find reasons for the recent decline of Vibrio cholerae O139 Bengal cholera in Bangladesh, phenotypic and genotypic changes in O139 isolates obtained from patients with cholera from 1993 to 1996 were studied. The isolates were tested for the presence of ctx and tcpA genes, hemagglutinin/protease (HA/P), capsule, D-mannose-sensitive hemagglutinin (MSHA), L-fucose-sensitive hemagglutinin (FSHA), tube test (tube) and CAMP test (CAMP) hemolytic activities, resistance to 2,4-diamino-6,7-diisopropyl pteridine (O/129) and trimethoprim-sulfamethoxazole (TMP-SMX), and genotype by pulsed-field gel electrophoresis (PFGE). All isolates possessed ctx and tcpA genes, HA/P, and a capsule. Most isolates were negative for FSHA, but although the majority of the isolates were positive for MSHA, no discernible trend in the activity was found during the study period. All early isolates were CAMP hemolysin positive and resistant to the vibriostatic compound O/129 and TMP-SMX, the two properties that could be used for the presumptive diagnosis of O139 cholera. However, subsequently, isolates that were CAMP hemolysin negative and susceptible to TMP-SMX and O/129 were increasingly encountered, with all the 1996 isolates being so, which suggested that these properties can no longer be used for the presumptive diagnosis of O139 cholera. V. cholerae O139 isolates that were CAMP hemolysin positive and resistant to O/129 and TMP-SMX produced a disease of greater severity than that caused by the CAMP hemolysin-negative and susceptible isolates on the basis of the lengths of stay of the hospitalized patients. The study period witnessed the evolution of four different genotypes by PFGE. All of these data suggested that the V. cholerae O139 isolates have undergone changes in some properties. However, how these changes influenced their prevalence relative to that of V. cholerae O1 in human infection is not clear. Studies of the environmental factors will provide the key for an understanding of the relative abundance of these vibrios.

Molecular analysis of toxigenic Vibrio cholerae O139 Bengal strains isolated in Bangladesh between 1993 and 1996: evidence for emergence of a new clone of the Bengal vibrios

Vibrio cholerae O139 Bengal emerged in 1992 and rapidly spread in an epidemic form, in which it replaced existing strains of V. cholerae O1 in Bangladesh during 1992 and 1993. The subsequent emergence of a new clone of V. cholerae O1 of the El Tor biotype that transiently displaced the O139 vibrios during 1994 to 1995 and the recent reemergence of V. cholerae O139 and its coexistence with the El Tor vibrios demonstrated temporal changes in the epidemiology of cholera in Bangladesh. We studied clonal diversity among V. cholerae O139 strains isolated from cholera patients and environmental surface water since their first appearance until their transient disappearance in 1994 as well as the O139 strains that reemerged during 1995 to 1996 and were isolated in the capital Dhaka and four rural districts of Bangladesh to investigate the origin of the reemerged strains. Analysis of restriction fragment length polymorphisms in genes for conserved rRNA and cholera toxin (CT) (ctxA) or in DNA sequences flanking these genes revealed four different ribotypes and four different ctx genotypes among the 93 strains of V. cholerae O139 studied. Ribotypes I and II and ctx genotypes A through C were shared by strains isolated from the epidemic outbreak during 1992 and 1993 in Bangladesh and India, ribotype III was represented by a single CT-negative O139 strain from Argentina, and 16 of 27 (59.2%) of the reemerged strains isolated during 1995 and 1996 belonged to a new ribotype of O139 vibrios designated ribotype IV. All 16 strains belonging to ribotype IV also belonged to a new ctx genotype (genotype 4). These results provide evidence for the emergence of a new clone of toxigenic V. cholerae O139 in Bangladesh. Further analysis of the rfb gene cluster by PCR revealed the absence of a large region of the O1-specific rfb operon and the presence of an O139-specific genomic region in all O139 strains. The PCR amplicon corresponding to the rfaD gene of a CT-negative O139 strain from Argentina was smaller in length than those of the toxigenic O139 strains but was identical to those of seven non-O1 and non-O139 strains. All O139 strains except the CT-negative strain carried structural and regulatory genes for CT and toxin-coregulated pili (ctxA, tcpA, tcpI, and toxR). These results suggest that the O139 Bengal strains possibly emerged from an El Tor strain but that the CT-negative non-Bengal O139 strain might have emerged from a non-O1, non-O139 strain. Thus, strains belonging to the O139 serogroup may have emerged from similar serotype-specific genetic changes in more than one progenitor strain of V. cholerae.

The evolution of epidemic Vibrio cholerae strains

The emergence of the novel Vibrio cholerae strain, O139 Bengal, which caused a large epidemic in Southeast Asia, underlines the adaptability of pathogenic microorganisms. Recent studies reveal that horizontal transfer of cell-wall polysaccharide genes played a central role in the emergence of this strain and that its genesis may not be as unique as initially believed.

Emergence of a new clone of toxigenic Vibrio cholerae O1 biotype El Tor displacing V. cholerae O139 Bengal in Bangladesh

The emergence of Vibrio cholerae O139 Bengal in 1993, its rapid spread in an epidemic form, in which it replaced existing strains of V. cholerae O1 during 1992 and 1993, and the subsequent reemergence of V. cholerae O1 of the El Tor biotype in Bangladesh since 1994 have raised questions regarding the origin of the reemerged El Tor vibrios. We studied 50 El Tor vibrio strains isolated in Bangladesh and four other countries in Asia and Africa before the emergence of V. cholerae O139 and 32 strains isolated in Bangladesh during and after the epidemic caused by V. cholerae O139 and 32 strains isolated in Bangladesh during and after the epidemic caused by V. cholerae O139 to determine whether the reemerged El Tor vibrios were genetically different from the El Tor vibrios which existed before the emergence of V. cholerae O139. Analysis of restriction fragment length polymorphisms in genes for conserved rRNA, cholera toxin (ctxA), and zonula occludens toxin (zot) or in DNA sequences flanking the genes showed that the El Tor strains isolated before the emergence of V. cholerae O139 belonged to four different ribotypes and four different ctx genotypes. Of 32 El Tor strains isolated after the emergence of O139 vibrios, 30 strains (93.7%) including all the clinical isolates belonged to a single new ribotype and a distinctly different ctx genotype. These results provide evidence that the reemerged El Tor strains represent a new clone of El Tor vibrios distinctly different from the earlier clones of El Tor vibrios which were replaced by the O139 vibrios. Further analysis showed that all the strains carried the structural and regulatory genes for toxin-coregulated pilus (tcpA, tcpI, and toxR). All strains of the new clone produced cholera toxin (CT) in vitro, as assayed by the GM1-dependent enzyme-linked immunosorbent assay, and the level of CT production was comparable to that of previous epidemic isolates of El Tor vibrios. Further studies are required to assess the epidemic potential of the newly emerged clone of V. cholerae O1 and to understand the mechanism of emergence of new clones of toxigenic V. cholerae.