Vibrio cholerae: Epidemiology, Ecology, Evolution and Climate Change
On Lee Lau
Abstract. With the advent of molecular techniques, such as PCR and genomics, it has been possible to identify specific pathogenic agents and their characteristics; consequently, the available literature on epidemics has increased exponentially. Here, I provide a basic guide through the expanding literature and our understanding of the non-linear properties of cholera emergence. The discovery and investigation of active cholera genes and toxins, bacterial evolution and diversity, and molecular properties reveal mechanisms that we may use to improve detection and population modeling of epidemics. In the case of cholera, biological and mathematical information is critical in directing future policy for improving health resources and education in specific areas, as new cholera biotypes continue to evolve, invade, and establish in populated areas.
15 September 2004
Research sponsored by DARPA Grant DAAD19-02-1-0288, P00001
1 September 2004
Reed College, Portland, OR
DARPA Grant
Introduction
Vibrio cholerae bacterium has evolved in various coastal areas and is likely to become infectiously active as temperatures increase during climate change. As the global temperature increases, up to 2.6 C in the next 50 years as projected by the IPCC (1), the distribution and ecology of many organisms are likely to shift according to their optimal performance. The mysterious activation of the seventh pandemic, accompanied by the molecular change in a new virulent biotype and persistence during recently prolonged El Nino years, suggests that cholera is an active threat to challenge heath care capacities (2, 3).
The impacts of cholera in its second pandemic motivated modern health care in 1854, with blocking the use of contaminated wells as directed by John Snow (3). Cholera is treatable with rehydration and antibiotics. The history of cholera provides a source of biological information to determine the mechanisms of transmission, persistence, and virulence. Recent findings on molecular biology and ecology of Vibrio cholerae and clinical information of cholera are herein reviewed in perspective of the historical knowledge that is basic to epidemiology.
During World War II, Japan released fleas infected with cholera and other pathogenic organisms over China as a biochemical agent (10). The continued virulence of cholera throughout human history attests to the flexibility of this gram-negative bacterium. Common molecular elements producing CT, TCP, and RTX enterotoxin proteins and other observed microbial characteristics help identify a population in water samples as the classical biotype V. cholerae 01, the agent of previous pandemics. V.cholerae 01 El Tor was isolated as the seventh pandemic. Recently, isolated in 1992, V. cholerae 0139, a non-01 pathogenic strain from Indonesia, varies from El Tor strains in producing a distinct capsule and demonstrates the bacterium’s ability to express pathogenic elements. It threatens to be the next pandemic. The populations of these three toxigenic strains fluctuate with respect to each other, as emerging new strains replace previous populations or cohabit (4). Over 206 serogroups are classified by the identity of the somatic O-antigen.
Clinical evaluations of Cholera
Vibrio cholerae attaches to and colonizes intestinal lining. Cells affected by the cholera toxin expel fluids. The infectious dose ranges from 103 – 105 cells, with a high dose being 108 (2, 3). Symptoms of cholera, namely watery diarrhoea and vomiting, generally occur abruptly after an incubation period of between 18 hours and 5 days. The severity of cholera symptoms may increase with a higher infectious dose, low host/local intestinal immunity, compromised gastric-acid barrier, and blood group O. Severe cholera patients may lose 500-1000mL/h and 10% of their body weight, and may die from volume shock (50% case-fatality) without treatment or sufficient rehydration. Deaths from cholera usually occur in the first day. More commonly, patients may fall into the other categories of none and some (or moderate) symptoms. Clinical signs of dehydration become apparent after 5% loss of body weight. (3)
Many rehydration products have been developed to facilitate efficient treatment of cholera symptoms. Emergency intravenous polyelectrolyte solution, commercially Ringer’s lactate containing 4 mmol/L potassium and oral rehydration solution (ORS) containing 20 mmol/L potassium or 75 mmol/L sodium are given to prevent potassium deficiency as metabolic acidosis is corrected. The same amount of fluid lost should be replaced within 2-4 hours. With hydration, a patient normally recovers in 4-5 days. (3)
Antibiotics are effective in shortening recovery time to 2-3 days. However, cholera resistance to tetracycline, ampicillin, kanamycin, streptomycin, sulphonamides, trimethoprim, and gentamicin has been reported. A chromosomally integrating SXT element in V. cholerae 0139 may allow for resistance genes from neighboring organisms. Antibiotic resistance undergoes fluctuating periods of sensitivity. (3) Antibiotics are not cost effective and demonstrate the adaptability of the microbial community as well.
Improvements in Public Health
Transmission of cholera occurs through a combination of factors: seasonal bloom of bacteria, contaminated water supply, inadequate sanitation, and contaminated seafoods. Water filtration is an effective prevention of cholera as the bacteria inhabit copepods at an infectious dose, 105 cells (2). A low cost filtration of folded sari cloth was introduced in a cholera-endemic area with dramatic 50% reductions in seasonal cases (9). Rehydration products, various ORS, are commercially available and widely used. Vaccines continue to be developed, with the latest targeting identified virulent proteins, but vaccination is limited to 6 months effectiveness and is therefore not practical for populations continually exposed to the evolving bacterium. Fluorescence detection techniques are available for pre-infectious levels of V. cholerae. In the 1900s the US was successful in curbing cholera cases through the provision of safe municipal water, especially in the New York area. Cholera is still endemic to areas in the southwestern United States. There are seasonal outbreaks in those areas, especially when the surrounding waters are compromised by pollution (8).
Genes and Toxins
Intensive study of toxigenic strains of 01 and 0139 yield a set of virulence genes most notably belonging to TCP pathogenicity island, tcpA, tcpI, and acfB encoding colonization factor TCP, and CTX prophage, ctxA encoding CT toxin (5,6). The investigation is far from complete as more genetic variants to the proteins are found and bacteria continue to transfer genetic components (4,6). The emergence of V. cholerae 0139 likely originated from a homologous recombination event of environmental strain 022 O-antigen specific genes, which becomes the 0139 wbf region, with a 01 El Tor strain. V. cholerae 0139 and 01 El Tor strains share all their virulence factors and similar ribotypes, from analysis of highly conserved rRNA (4). V. cholera 0139 strains are genetically highly diverse, with all combinations of nontoxigenic strains with various virulence genes. Those with TCP+ genotypes may undergo toxigenic conversion by CTX phage (4). Horizontally transferable conjugative transposons in V. cholerae or the presence of integrons converting antimicrobial resistance of other organisms into functional operons may explain the fluctuating antimicrobial character and genomic diversity of 0139. Genomic instability probably contributes to the bacterium’s evolution and success.
Genetic diversity of proteins within individual V. cholerae explains the versatility of the bacterium in different environments. V. cholerae contains two circular chromosomes (3, 6). Chromosome II contains genes necessary for adaptation and growth in unique environments with genes found to be active during human infection (5). In addition to gene clusters associated with toxigenic strains, there are environmental alleles of virulence genes, which include some environmental strains that are able to colonize and/or induce fluid accumulation by unknown factors (6).
Life History and Persistence
In a severe cholera patient, up to 1013 bacteria per day may be released into the environment (7); this release contributes to the continuance of an outbreak. V. cholerae can survive in seawater for more than 50 days and may enter a viable but non-culturable state under unfavorable conditions and cells may be resuscitated by heat shock (2). Association and attachment with zooplankton, algae, and other aquatic organisms as biofilms is an important aspect of V. cholerae life history to explore as developing policy may seek to control or eliminate natural populations as a method of reducing incidence of cholera (11). As governments and organizations develop policies to control the biological impact of previous policies that have contributed to global climate change, it remains that V. cholerae is a part of the natural and large oceanic ecosystem, and further understanding of the human-microbe interaction will yield medically motivated public health changes and discoveries (12) that will benefit human populations.
References
1. Gitay H, Suarez A, Watson R (Coordinating Lead Authors and Ed.) 2002. Climate Change and Biodiversity. Intergovernmental Panel On Climate Change. Tpbiodiv.Pdf. Pp.86
2. Colwell RR. 1996 Global Climate And Infectious Disease: The Cholera Paradigm. Science 274:2025-2031
3. Sack DA, Sack RB, Nair GB, And Siddique AK. 2004 Cholera (Seminar). The Lancet. 363(9404):223
4. Faruque SM, Sack DA, Sack RB, Colwell RR, Takeda Y, and Nair GB. 2003. Emergence and Evolution of Vibrio cholerae 0139. Proceedings of the National Academy of Sciences of the United States of America 100(3):1304-9
5. Hang L, John M, Asaduzzaman M, Bridges EA, Vanderspurt C, Kirn TJ, Taylor RK, Hillman JD, Progulske-Fox A, Handfield M, Ryan ET. 2003 Use of in vivo-induced antigen technology (IVIAT) to identify genes uniquely expressed during human infection with Vibrio cholerae. Proceedings of the National Academy of Sciences of the United States of America
6. Faruque SM, Chowdhury N, Kamruzzaman M, Dziekman M, Rahman MH, Sack DA, Nair GB, Mekalanos JJ. 2004. Genetic diversity and virulence potential of environmental Vibrio cholerae population in a cholera-endemic area. Proceedings of the National Academy of Sciences of the United States of America 101(7):2123-8
7. Cottingham KL, Chiavelli DA, Taylor RK. 2003. Environmental microbe and human pathogen: the ecology and microbiology of Vibrio cholerae. Frontiers in Ecology and the Environment. 1(2);80-86
8. Kaiser J.1999 Battle over a dying sea. Science 284(5411):28-30
9. Colwell RR.Huq A.and Islam MS.2003 Reduction of cholera in Bangladeshi villages by simple filtration. Proceedings of the National Academy of Sciences of the United States of America 100(3):1051-5
10. Hadfield P.2001 Lethal legacy. New Scientist 169(2276):5
11. Perez ME, Macek M, Galvan MT. 2004 In Situ Trop Measured Elimination Of Vibrio Cholerae From Brackish Water. Med Int Health (1):133-40
12. Boyce N.2000 Cholera toxin opens up the brain. New Scientist 165(2221):12-13
Resources (cited and uncited)
Reviews
Sack DA, Sack RB, Nair GB, And Siddique AK. 2004 Cholera (Seminar). The Lancet. 363(9404):223
Faruque SM, Albert MJ, and Mekalanos JJ. 1998. Epidemiology, Genetics, And Ecology Of Toxigenic Vibrio Cholerae. Microbiology And Molecular Biology Reviews 62(4):1301–1314
Holmgren J, Svennerholm AM. 1977 Mechanisms Of Disease And Immunity In Cholera: A Review. J Infect Dis.136 Suppl:S105-12.
Mekalanos JJ, Rubin EJ, Waldor MK. 1997 Cholera: Molecular Basis For Emergence And Pathogenesis.EMS Immunol Med Microbiol. 18(4):241-8.
Climate change
Colwell RR. 1996 Global Climate And Infectious Disease: The Cholera Paradigm. Science 274:2025-2031
Harvell CD.Mitchell CE.Ward JR.2002 Climate warming and disease risks for terrestrial and marine biota. Science 296(5576):2158-62
Pascual M.Rodo X.Ellner SP.2002 Cholera dynamics and El Nino-Southern Oscillation. Science 289(5485):1766-9
Epstein PR.2000 Is global warming harmful to health?. Scientific American 283(2):50-7
Anselmo JC.1997 Ocean-monitoring satellite generates first images. Aviation Week & Space Technology 147:83
Policy/Cases
Ashbolt NJ. 2004 Microbial Contamination Of Drinking Water And Disease Outcomes In Developing Regions. Toxicology 198(1-3):229-38.
Perez ME, Macek M, Galvan MT. 2004 In Situ Trop Measured Elimination Of Vibrio Cholerae From Brackish Water. Med Int Health (1):133-40
Hadfield P.2001 Lethal legacy. New Scientist 169(2276):5
Kaiser J.1999 Battle over a dying sea. Science 284(5411):28-30
Kalson DJ.Baker R. 1998 Fighting cholera in Ecuador: building a public health system that works.Journal of Environmental Health 60(6): 24-6
Colwell RR.Huq A.and Islam MS.2003 Reduction of cholera in Bangladeshi villages by simple filtration. Proceedings of the National Academy of Sciences of the United States of America 100(3):1051-5
Evolution And Diversity
Faruque SM, Sack DA, Sack RB, Colwell RR, Takeda Y, and Nair GB. 2003. Emergence and Evolution of Vibrio cholerae 0139. Proceedings of the National Academy of Sciences of the United States of America 100(3):1304-9
Faruque SM, Mekalanos JJ 2003 Pathogenicity Islands And Phages In Vibrio Cholerae Evolution.Trends Microbiol (11):505-10
Faruque SM; Chowdhury N; Kamruzzaman M 2004 Genetic Diversity And Virulence Potential Of Environmental Vibrio Cholerae Population In A Cholera-Endemic Area.Proceedings Of The National Academy Of Sciences Of The United States Of America. 101(7): 2123-8
Genes And Toxin
Peterson KM. 2002 Expression Of Vibrio Cholerae Virulence Genes In Response To Environmental Signals.Curr Issues Intest Microbiol. 3(2):29-38.
Davis BM, Lawson EH, Sandkvist M, Ali A, Sozhamannan S, And Waldor MK. 2000. Convergence Of The Secretory Pathways For Cholera Toxin And The Filamentous Phage, CtxPhi. Science 288: 333-335
Hang L, John M, Asaduzzaman M, Bridges EA, Vanderspurt C, Kirn TJ, Taylor RK, Hillman JD, Progulske-Fox A, Handfield M, Ryan ET. 2003 Use of in vivo-induced antigen technology (IVIAT) to identify genes uniquely expressed during human infection with Vibrio cholerae. Proceedings of the National Academy of Sciences of the United States of America
Boyd EF, Moyer K, Shi L And Waldor MK. 2000. Infectious CTX And The Vibrio Pathogenicity Island Prophage In Vibrio Mimicus: Evidence For Recent Horizontal Transfer Between V. Mimicus And V. Cholerae. Infection and Immunity, 60(3):1507–1513
Boyce N.2000 Cholera toxin opens up the brain. New Scientist 165(2221):12-13
Pennisi E.1998 Versatile gene uptake system found in cholera bacterium. Science 280(5363):521-2
Lencer WI. and Tsai B. 2003The intracellular voyage of cholera toxin: going retro. Review Trends in Biochemical Sciences 28(12): 639-45
Vaccine (TCP)
Du Y, Jia Wx, And Liu L. 2004. Toxin-Coregulated Pilus-Loaded Microparticles As A Vaccine Against Vibrio Cholerae O139. Chinese Medical Journal 117(4):618-620 [
Stewart-Tull DE, Lucas C, Bleakley CR. 2004 Experimental Immunisation And Protection Of Guinea Pigs With Vibrio Cholerae Toxoid And Mucinases, Neuraminidase And Proteinase.Vaccine. 22(17-18):2137-45.
Bernardi A.Checchia A.and Brocca P.1999 Sugar mimics: an artificial receptor for cholera toxin. Journal of the American Chemical Society 121(10):2032-6
Virulence
Islam MS, Ahsan S, Khan SI, Ahmed QS, Rashid MH, Islam KM, Sack RB. 2004 Virulence Properties Of Rough And Smooth Strains Of Vibrio Cholerae O1. Microbiol Immunol. 48(4):229-35.
Valeva A, Walev I, Weis S, Boukhallouk F, Wassenaar TRUDY M, Endres K, Fahrenholz F, Bhakdi S, Zitzer A. 2004 A Cellular Metalloproteinase Activates Vibrio Cholerae Pro-Cytolysin.J Biol Chem. Apr 5
Butler SM, Camilli A. 2004 Both Chemotaxis And Net Motility Greatly Influence The Infectivity Of Vibrio Cholerae. Proc Natl Acad Sci U S A. 101(14):5018-23.