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Dennis Linton

Cholera is a widespread and often deadly disease. What is cholera, what causes it, and how might we reduce its incidence?

W Key words Human disease Cholera Vibrio cholerae

Cholera toxin Pandemic Virulence factor

e are currently in th e throes of a cholera pandemi c. A pandemic is a human infection that spreads over vast distances such as acr oss continents or even worldwide . The current ch o lera pandemi c is the seventh to sweep across the gl obe during the last tw o centuries. You probably aren't aware of thi s because cho le ra is extremely rare in the developed world, but in the developing world the situation is very different. Cholera is now establish ed acr oss Africa and Asia, and has more recentl y spread in Central and South America .

Cholera: disease on a global scale Cholera is characterised by successive pandemics arising out of the Indian subcontine nt where it is endemic. It spreads across th e globe and then retr eat s to the endemic region. The first documented

A woman in Haiti pouring household waste into an open sewer outsideher home. Open sewers can causeepidemics ofwaterborne di seases such as cholera.

cholera pand emic began aroun d 18 I6. It originated in the Bengal region of India and spread via established trading routes to China and as far west as southern Russia . A second pandemic began in 1826 and reached the UK around 183 I. The third pandemic (1852-60) caused over a million deaths in Russia . Further pandemics occur red in 1863-75, 1881-96 and 1899-1923. The seventh and ongoing cholera pandemic

Figure 1

Worldwide distribution of cholera 2004-05.

/

Countries with reported cholera cases Countries with imported choleracases

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BIOLOGICAL SCIENCES REVIEW


be gan in Indonesia in 1961 and from there spread to India ( 1964 ), Africa (1970) and southern Europe (1970 ), reaching South America in 1991 (see Figure I ). Accord ing to the World Health Organization, in 2005 the number of rep orted ch olera cases worldwide wa s 131 943, with 2272 deaths. Howev er, du e to und er­ rep orting, these figures are a gross underestimate and th e true figures have been put at between 3 and 5 million cases with around 120 000 deaths an nually ­ an appalling figur e. At the time of writing this articl e, there wa s a large ch olera o utbrea k in northern Iraq , inv olving an estimated 30000 cases, and last year there were massive outbreaks in Sudan and Angola . In contrast, owing to clean water supplies and h ygienic sani tation conditions, cholera cases are uncommon in the developed world . The few cases th at do occur -less than 50 reported cases per year in England and Wales­ are probably imported infe ctions.

What is cholera and how is it treated? Cho lera is an acute intest inal infect ion characterised by sudden on set of p rofuse watery diarrhoea that may be accompanied by vomiting. The diarrhoea often con tains white specks and is sometimes referred to as a 'ri ce­ water stool ' becau se it resembles dirty water in whi ch rice has been wa sh ed. In severe cases, the diarrhoea can be produced in such ma ssive volumes (up to a litre ev ery hour) that the resultant dehydration can kill within hours of th e on set of symptoms . Untreated cho lera has a death rate of up to 50 % . Other sym ptoms of cholera are th e result o f dehydration and includ e weak or ab sent pul se as a co nsequence of low blood pr essure, reduced skin elast icity and a sunken appear­ an ce to the eyes. Ultimately the se symptoms can lead to shock - where insufficient blood flow reaches the bod y tissues - and death . Fortunately, mod ern tre atments ar e highl y effective and ha ve reduced death rat es to Jess than J % . Th e mainstay of treatme nt is rapid rehyd ration th erapy to replace fluids and salts lost due to diarrhoea . Patients

with severe dehydration sh ould receive intravenou s fluid s as quickly as possible, while mildly deh ydrated pati ents can be treated with oral rehydration therapy. The development of oral rehydration solution (essen­ tially salt, sugar and water) has undoubtedl y sa ved m illions of lives. It is relati vely cheap and can be admin­ ister ed without medical exp ertise. Patients m ay also be given a sh ort course of antibiotics to reduce the len gth of their illne ss.

What causes cholera? The first clues about what causes cholera cam e from the pioneering work of John Snow (1813-58) , a quite rem arkable man. As well as being a pioneer of anaesthe­ siology (he administered ana esthetic to Queen Victoria during the birth of two of her children). he is considered th e father of modern epidemiology for his work on ch olera. Until the middle of the nin eteenth century, infectious diseases, including cholera, were thought to be spread through mia sma or 'bad air' . Through a series of careful studi es of cholera outbreaks in Lond on, Snow cam e to the conclusion that ch olera wa s spread through contaminated water. In 1849, th e first ed ition of Snow's best-known work, 'On the m ode of communication o f cholera', wa s published but it wa s not well received . An outbreak of ch olera in the Soho area of London near where Snow lived, on 31 August 1854, presented an opportunity to test his idea s. By accurately recording data on the loca­ tion of chol era (affected households) , Snow was able to demonstrate that all of them obtained their water from a com mon sour ce, namely th e Broad Street pump (see Figure 2). Following rem ova l of the pump handle on 8 September 1854, the ch olera outbreak a bated, al th ough it ma y well ha ve already been in decline . Nev ertheless, it wa s thereafter accepted that cholera was sp read through contaminated water. A replica water pump lacking a handle wa s placed ou tside th e 'J oh n Snow' public house in Broad Street, Soho as a memo­ rial a nd still sta nds th ere toda y. Figure 2 (A) John Snow and (8) a map of the Broad Street cholera outbreak ' in 1854. Bars represent cholera fatalities within each hou sehold.

SEPTEMBER 2008

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Figure 3 Coloured transmission electron micrograph of a section through a single Vibrio cholerae cell. x80000

explained Acute infection An infection that comes sharply to a crisis. Endemic An infect ion is endemi c when it is main­ tained within a population without external input. Endocytosis The uptake of substances by a cell by engulfing th em within a vesicle formed from th e plasma memb rane. Epidemiology The study of the causes, distribution and control of diseases in populations. Serogroup A group of bacter ia that produ ce a common antigen enabling their serological (using anti­ bodies) identification. Virulence factors Molecules produced by a pathogenic organism that contri bute to virulence or pathogenicity.

Robert Koch , som e 30 year s later. Koch had recentl y isolated the bac teria respon sible for causing ant hrax a nd tub erculo sis. Usin g his con siderable mi crobiol ogical expertise, h e was ab le to isolat e and gro w the ch olera bacter iu m in pure cultu re. We now kn ow th at ch olera is caugh t by ing esting th e bac te rium Vibrio cholerae (see Figure 3). Spread of cho lera occurs wh en food or water is consumed tha t is con taminated with the faeces of someone in fected w ith cholera and therefore con taining large nu mbers of Vibrio cholerae. In Western developed count ries, this kno wl­ edge has driven th e improvements in th e wa ter supply and sewerage systems necessar y to eradicate epidemic ch olera from thes e areas of th e w orld. But wh a t exactly wa s in th e wa ter? Filippo Pacini becam e in terested in cho lera durin g the third cho lera pan demi c, w hich rea che d Flore nce in 18 54. As a skilled m icro sco pist he was ab le to see the com ma -sha ped bacilli, which h e termed Vibrio, in the in testi ne s of ch olera victims follow ing au top sy. In credibly, Pacin i's work wa s la rgely ignored and the cholera bacte riu m was 'r ediscovered' by one o f th e great microbiologists,

How does Vibrio cholerae cause cholera? The pri ncipa l virulence factor of Vibrio cholerae is a toxin secreted by the ba cterium. Cho lera tox in be longs to a family of pro teina ceous to xin s produce d by other bacte rial pa tho gens. Th ese in clude d iph th eria to xin pro duced by Corynebacterium diphtheria, an d pertussis

Figure 4 Mechanism of action of chol era toxin.

. / Vibrio chole rae bacteria in the

Cholera toxin (A subunit in red and "'" pentameric B subunits in blue) Cholera toxin secreted by Vibrio cholerae in human intesti ne

~en

of the human intestine

'c:»>' CIMicrovill i CI- CI-

Loss of wa-t-er-f-ro-m-t-he gut tissues through osmosis ------, Increases efflux of chtoride ions across plasma membrane

Binds to surface of gut

epithelial cell

Taken up by endocytosis

Concentration of cyclic adenosine monophosphate increases

The A subunit activates the enzyme adenylate cyclase

• --_.......

Transported to the ER

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The A subunit is secreted from ER

BIOLOGICAL SCIENCES REVIEv.


How can cholera be reduced?

Further reading www.ph.ucla.edulepi/snow.html A co llection of mat e rial relating to the work of John Sn ow p ut tog eth e r by ep ide m iologis ts from th e Unive rs ity of Cal ifornia in Los Angeles .

www.who.intltopicslcholeralenl

World Health Organ iza tion s ite re lating to c holera .

toxi n prod uc ed by Bordetella pertussis - the causative age n t of w hoo ping co ug h . Th e cho lera toxin is know n as an AB5 to xin, so-ca lled beca use th e tox in co nsists of one molecule o f th e A su b u n it protei n with five mole ­ cules o f th e B subu ni t. Th e A and B mo lec u les of th e chole ra tox in have distin ct fu nc tions (see Figure 4) . Th e B su bu nits bi nd th e toxin to a speci fic mo lec ule o n the su rface of th e gu t epitheli al ce lls of th e hos t. Thi s trigge rs endocytosis, and th e tox in pa sses in to the cell enclosed in a ves icle form e d from th e pla sm a m e m b ra n e . The vesicle is transpo rted to th e e ndoplas m ic re tic ulu m, w ith w h ich it fu ses. Th e A sub uni t is re leased bac k in to the hos t ce ll cytoplasm where it starts a chain reaction th at activa tes an enzyme ca lled ade nyla te cycla se. This in t u rn leads to a pro longed in cr ea se in th e concen tra tio n of cyclic ade nosine rnonopho sp hat e (cA M P). Th is ha s th e e ffec t of stim u la tin g a sustained loss of ch lor ide ions from the cell and a su bse que nt osmoti c mo ve m e n t of w ater d o w n t he re su lt ing wate r po te ntia l gra die nt. This ma ssive wa te r loss from ce lls o f the human gu t ca uses th e pro fuse an d watery di arrh o e a ch a rac teris tic of cholera . Th e genes that en code ch o lera toxi n a re located on a piec e of DNA from a bac te rio phage (a vir us th a t infects ba cterial ce lls) that has become inserted into t he Vibrio cholerae chrom osome . We know that m any genes that encod e bac te ria l virul en ce facto rs can be tran sferred among bacteria and that bacterioph ages often ca rry suc h m ob ile viru lence genes in their genomes.

Only certain types of Vibrio cholerae cause epidemic cholera The nat ura l habitat of Vibrio cholerae is aq ua tic and it can be fo u nd in rivers, es tuaries and coasta l wa ters . Th e current cho ler a pa ndemic is caused by a specific typ e o f Vibrio cholerae kn own as serogroup 01. Am on g th e more th a n 200 se rogro up s of Vibrio cholerae, o n ly tw o cause epidemic cho lera : serogrou p 0 I, w h ich ca used the fifth, sixt h and sevent h pandemics (we do not know the cause o f the first four pan dem ics bu t it is likel y that th ey were a lso due to serogrou p 01 strain), and serogrou p 0139, firs t isolated in 1992 and itse lf derived fro m a serogroup 0 1 strain . Othe r serogroups of Vibrio cholerae can ca use human d isease but n ot la rge -scal e epidemics. Why are only tw o serogro ups of Vibrio cholerae ab le to cau se ep idemi c chol era? The answer see m s to be because th ese serogrou ps have acq uired se ts of ge nes th at encode impo rtant vir u lence fa ctors ena bling t hes e strai ns to col on ise h u m ans. ca use disea se an d spread. On ly some of th ese acq u ire d ge nes - fo r exa m p le, th e ge n e encodin g cholera toxin - have been identified . SEPTEMBER 2008

Chole ra is ca u sed by fae cal con tamin ation of food a n d water. P ro vid ing sa fe, clea n wa ter, im p ro vin g sa ni ta ­ tio n an d ed ucating g ro ups at risk are therefore th e most e ffective meas ur es to red uce ch o le ra . Howe ver, sign ifica nt im p ro vements to wa te r supplies a nd sanita ­ t ion fac ilities curre n tly remai n unreal ist ic fo r m an y pe o pl e in th e deve lo ping world a n d so t h e y wi ll contin u e to Jive wi th the th reat o f ep idem ic chol e ra . Vaccina tio n aga inst Vibrio cholerae is ano t he r possibl e approach . In 20 04, a n oral vaccin e call e d Du kor a l w as deve lo ped . It cons isted o f kil led w ho le ce lls of Vibrio cholerae a lo ng wit h p uri fied B s ub u n it o f the chole ra tox in. Alth ou gh th is is th e o n ly ch ol e ra vacci n e curre nt ly bei n g prod u ced, others are in va rio u s stages of develo p men t.

Summary Cho le ra is a d readful di se ase a n d it is sobe rin g th at desp ite adva n ces in health ca re it rema ins en dem ic in la rge parts o f the developing wo rld. To red uce disease d ue to cho le ra wi ll requ ire im prove me n ts in wa ter­ supply facilities and sanita tion for so me of the poorest pe o ple in the wor ld . Furth e rmore, be ca u se Vibrio cholerae is wi dely distributed in su rface w a te rs a ro u nd th e globe , th e emerge nce of ne w varian ts able to ca use epi de m ic cho lera re m ains a poss ibility. The orga nis m Vibrio cholerae will probab ly al wa ys be wit h us. Th e cha llen ge is to re duce the in cidence of ch ol er a disease. Dr Dennis Linton is a lecturer in microb iology in the

Facu lty of Life Sciences a t the University of Manc hes ter. The aim of his current resea rch is to unde rstand how the bacterium Campy/obacter jejun i ca uses dia rrhoea in hum ans .

points • Cho lera is ca us ed by th e a q ua tic bacter ium Vibrio

cholerae. • Large-scale pandemics a re a key feature of c hole ra e pide miology. • Cholera e pide mics s prea d throu gh cons u m ption of wat er and food that ha s been conta mina ted with faecal mat erial conta ining Vibrio cholerae. • The main symptoms of cholera are the produ ction of large volum e s of watery di arrhoea a nd res u lta nt d ehydration . • Cholera toxin is the prin cipal virul en c e fact or of

Vibrio cholerae. • The toxin induce s a sustained sec retion of c h loride ions by ce lls of the gut. A s u bse q ue nt os motic move­ ment of water down the resulting wat er pote ntial

gradi ent results in seve re dehydration .

• Treatment by rehydration is the key to reducing mo rta lity. • Improved sa n ita tion and provision of clean water is the most effective way to red uce the inc ide nc e of c hole ra .

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Cholera  

Article from Biological Sciences Review Vol 21

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