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Cholera: Death by Diarrhoea
2 CHOLERA: DEATH BY DIARRHOEA
Cholera-anoverviewCholeraisaninfectiousdisease,causedbythebacteriumVibrio cholerae,whichaffectsthetransportofwaterinthesmallintestine.Thebacteriumsecretesatoxin,choleratoxin(CT),whichcausesseverefluidlossfromthebodyintothedigestivetract,leadingtodehydrationandultimatelydeathbydiarrhoea.
V. choleraehascausedsevenworldwidediseaseoutbreaksorpandemicssince1817,killingmillionsofpeopleandinfectingmillionsmore-includingfourepidemicsintheUKaccountingforapproximately55,000deaths.
SoutheastAsiawitnessedthefirstcholeraepidemicin1817,thoughtheconditionof‘dehydratingdiarrhoea’wasmentionedaroundthetimeofHippocrates(460-377BC).Today,choleraisprevalentinCentralandSouthAmerica,AfricaandAsia,thoughnotintheUKasitisusuallyconfinedtocountrieswithapoorsanitationinfrastructure.Worldwide,100,000-300,000casesofthediseasearereportedeachyearwithmorethan94%ofcasesinAfrica.However,thisislikelytobeamassiveunderestimateasmanycountriesinthe
Cholera-ahistoryPriorto1850,itwasthoughtthatcholerawascausedbybreathinginbadair-miasma-andthatprotectionwasofferedbystrongsmellingsubstancessuchasherbsandcamphor.JohnSnow,aprominentLondonphysician,washeraldedastheFather of Cholerawhenhequestionedthemiasmatheoryaftertreatinganumberofcholerapatients
IndiansubcontinentandSoutheastAsiadonotreporttheincidenceofcholera;forexample,nocholeracaseshavebeenreportedinBangladesh,althoughestimatessuggestthattheremaybeasmanyas1millioncasesperyear.Realisticworld-wideestimatessuggest120,000annualdeathswith3-5millionpeoplesufferingfromthedisease.
Haitiangirlsufferingfromcholera
Cartoonofdoctorsvisitingapatientwithcholera
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butnotsuccumbingtotheconditionhimself.Snowarguedthatdiseasemusthaveenteredthebodythroughthemouthoninfectedfoodordrinkandnotviathelungs,afterobservingthatpatientsbecameillwithsymptomsoriginatinginthegutsuchasvomitingandstomachache.Hehypothesizedthata‘poison’wasingestedwhichreproducedinthedigestivetractand,in1849,hepublishedhisideas,athisownexpense,inapamphletcalledOn the Modes of Communication of Cholera.Snowwasabletoprovehistheoryin1854whenhecarefullymappedtheincidenceofcholeraduringtheLondon1853-1854outbreak.BelievingthatthesourcewasawaterpumpinBroadStreet,Soho,heexaminedwaterfromthepumpunderthemicroscopeanddescribedtheoccurrenceof‘whiteflocculentparticles’.Convincedthattheseparticleswerethesourceofthediseasehepersuadedtheauthoritiestoremovethepumphandle;atthistime,thenumberofcholeracaseslocallywasalreadyindecline,thoughitisthoughtthatSnow’sconclusionsprovidedthebasisforfurtherpreventativemeasures.JohnSnowneveridentifiedthebacterialsourceofinfection;however,throughdatagatheringandstatisticalanalyses,hesuggestedthatthewaterwasthereservoirofinfectionandthemodeoftransmission.Hisrecommendationsofimprovedhygieneandboilingofdrinkingwatermostlywentunheededbythosewhodidnotsupporthisclaimsasthe‘germ’responsibleforcholerahadnotyetbeenobservedmicroscopically.
Whatcausescholera?Choleraiscausedbythecurved,rod-shapedbacteriumVibrio cholerae.Therearemorethan100speciesofVibrio,onlyafewofwhicharepathogenictohumans.Theyarefree-livingbacteriathatarecommonlyfoundinbrackishwaters.Vibriocellsaremotile,propelledbyasinglepolarflagellumand,astheynormallyliveinneutralor
InthesameyearthatSnowmadehisobservationsanItaliananatomist,FillipoPacini,publishedapaperentitledMicroscopical Observations and Pathological Deductions on Choleradescribingthecausativeagentofcholeraandsuggestingtreatmentoptions.However,thepreferredtheoryinItalyatthattimewasthatofmiasmaandassuchPacini’sworkwentunnoticed.
AGermanbacteriologist,RobertKoch,suggestedin1882thatcholerawascausedbyabacteriumandthatthisbacteriumsecretedatoxinwhichcausedrapidwaterloss.Kochobservedfaecalsamplesandidentifiedacomma-shapedbacillus,whichhecalledVibrio choleraeafteritsvibratingwiggles.Havingalreadymadehismarkonbacteriologywithhisworkonanthraxandtuberculosis,andhispostulates(seepage11),whichgenerateaformalproofthatacertainmicro-organismcausesadisease,hisideasandobservationsoncholerawerewellreceived.PaciniwasrecognizedforhisworkposthumouslyastheorganismwasofficiallynamedVibrio cholerae Pacini1854.
Artworkofthespreadofcholerainthe1830s
RobertKoch
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alkalineconditions,theyaresensitivetoacidicpH.Thebacterialcellsaretiny,aboutonefivehundredthofamillimetrelong,whichmeansthat20ormorecellswouldfitacrossthediameterofahumanhair.
Getting there…Cholerainfectionresultsfromtheingestionofcontaminatedfoodorwater.Tosurvivethejourneythroughthedigestivetract,thebacteriumhastobewelladapted.
Passagethroughthedigestivetract:Entrytothebody:V. choleraecellsenterthebodyandmaketheirwaythroughthedigestivesystem.Approximatelytwo-thirdssurvivetheacidicconditionsofthestomach;survivorsconserveenergyuntiltheyenterthesmallintestinewheretheybeginproductionoftheirflagella.Theflagellum,whichisalongtail-likestructure,propelseachVibriocellforwardthroughthemucuslayertotheintestinalwall.V. choleraealsoproducesenzymesthatdigestthelayersofmucus,whichhelpswithaccesstotheepithelialcellsliningtheintestine,aswellasdetachmentfromcellsthatarebeinglostduetothebody’sdefencemechanisms.Toconserveenergy,onreachingtheintestinewall,thebacterialcellsstopflagellumproduction,as
propulsionisnolongerrequired,andenergyisrefocusedontheproductionofhairlikeappendagescalledfrimbriaeorpiliwhichareformedonthebacterialcellsurface.Thesestructuresaremadeofproteinandallowthebacteriatoattachtotheliningoftheintestine.
Growth:Forsymptomstopersist,thebacteriamustcontinuetomultiplyintheintestine.
Colonisationandinvasion:Whentheyreachthesmallintestine,thebacteriamust‘holdon’andresistthenormaltransitmotionsofthisregion.Theydothisusingtheirfimbriaeorpili.Onceattached,thebacteriainvadethehostcell.ProductionofCTisthefinalstageinpathogenesis.
Causing trouble…Theperiodbetweeningestionandapatientshowingsymptoms(incubationperiod)isusuallyveryshortandcanbeaslittleas2hours,althoughitcantakeupto5days.Mostpeople(75%)infectedwithV. choleraedon’thaveanysymptoms(asymptomatic);however,theyremaincarriersofthedisease,excretingthebacteriaintheirfaeces,usuallyfortwoweeksbutoccasionallyforseveralyearsasinthecaseof‘CholeraDolores’.
Illustrationofthebacterialcellsthatcausescholera
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CholeraDoloresAsymptomaticcholerainfectionisunlikelytolastmorethantwoweeks;however,long-termcarriershavebeenreported,thoughtheyareextremelyrare.DuringthePhilippineepidemicof1962,Dolores,ahousewife,sufferedamildattackofcholerawhilepregnantwithhersixthchild,andwashospitalisedthoughnottreatedwithantibiotics.ItisthoughtthatshecontractedcholerafromeatingcontaminatedseafoodwhichherhusbandbroughtfromnearbyBacolodcity.SeveralmembersofDolores’familyshowedsymptomsofcholera,thoughnoneneededhospitalization.Doloresintermittentlyexcretedthebacteriumover11years,sohadthepotentialtocausefurtherinfectionalthoughthiswasneverobservedinherlocality.Thisispossiblyduetothefactthatshelivedinanareawherecholerawasendemic.LittlemoreisknownofthiscaseexceptthatV. cholerae wasresidentinherbiliarytract;itisunknownwhatfactorscontributedtoDoloresbeingacarrierforsuchalongperiodoftime.In1973,Dolores’carrierstatewasresolvedspontaneously.
DifferenttypesofV. choleraeinfectionOver100serogroups(organismsgroupedonthebasisoftheircellsurfaceantigens)ofV. cholerae exist;however,onlytwocauseepidemiccholera.TheyareserogroupO1andserogroupO139.SerogroupO1canbeclassifiedfurtherbasedonphenotypeintoElTorandClassical;thesearereferredtoasbiotypes.ElToristheinfectiveagentresponsibleforthecurrentpandemic;Classicalhasnotbeenidentifiedsincethemid-1990s.V. choleraeO1biotypescanbefurtherclassifiedintoserotypesbasedontheresultsofaserumagglutinationtest;therearethreeO1serotypes,Inaba,OgawaandHikojima.V. choleraeO139,arelativelynewserogroup,hasnotreachedpandemicpotentialsofar,thoughitsoccurrenceisbeingcloselymonitoredasitspatternofinfectionandcapacitytosurviveinwater,suggestthatitmaybebothmoreinfectiousandmorevirulentthanV. choleraeO1.
ItisworthnotingthatthegeneforCTiscarriedonabacteriophage(avirusthatinfectsbacteria)insideV. cholerae.So,intheory,ifthisvirusweremoremobileitmightbepossibleforanewserotypetoemergeagaininthefuture,justasO139hasdonequiterecently.
Bacteria V. cholerae
Serogroup O1 O139
Biotype ElTor Classical
Serotype Inaba Hikojima Ogawa Inaba Hikojima Ogawa
Molecularmodelofthesecondarystructureofcholeraenterotoxin
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LaboratorymethodsfordiagnosisofV. choleraeManydiseasescausediarrhoea;however,theproductionoffrequentwaterystoolsrequiresfastertreatmentduetothehighriskofdehydration.Lookingatastoolsampleunderthemicroscopecanhelpthemedicalpractitionermakeadiagnosis,however,thisisoflittleusetopatientsasthosewithanydiarrhoealdiseasereceivethesametreatment,i.e.fluidandelectrolyte(saltsandsugars)replacement.
Conventionalculturemethodsarethe‘goldstandard’fordiagnosisofV. cholerae.StoolsamplesorrectalswabsareplatedonTCBS(thiosulphatecitratebilesalts)agar(V. cholerae coloniesappearyellowastheyfermentglucose).Suspectedcoloniesareselectedforfurtheranalysiswithbiochemicalandserologicaltestssuchslideagglutination.
Incholeraendemicareas,wherelaboratoryfacilitiesareoftenlimited,rapidimmunochromatographicdipsticktestcanbeusedtodiagnosecholera.Itisverysimpletouse,thedipstickisdippedintoastoolsample.Iftworedlinesappearonthedipstick,thenthepatienthascholera,ifonlyoneredlineappears,thetestisnegative.Ittakesbetween2-15minutesforthetesttodevelop.
SymptomsMostpeoplewithcholeraareasymptomaticand,ofthosethatdodevelopsymptoms,80%havemilddiarrhoeawhichisdifficulttodistinguishfromthatcausedbyotherpathogenssuchasviruses.However,allofthesepeoplecarryV. cholerae,excreteitandcanspreadittoothers.Severecholera(choleragravis),whichaccountsforfewerthan10%ofcases,ischaracterizedbythe uRapidonsetofviolentwaterydiarrhoea.Thediarrhoeais‘strawcoloured’withflecksofmucus andisoftendescribedasresemblingricewaterVomitingExtensivedehydrationLegcrampsAsfluidislost,thebloodthickensandtheskinbecomesblue-greyincolour;patientsalsobegintosufferlethargy,alackofconsciousness,confusionandoccasionallyfever.Itisthisrapiddehydration-asmuchas1litreoffluidcanbelosteveryhour-whichcanprovefatalwithin24hoursofdevelopingthedisease.Inchildren,therapidlossoffluidcanleadtoseveresaltimbalance,whichcancauseconvulsionsandcardiacarrestinadditiontothesymptomsdescribed.Thefatalityrateinuntreatedcasescanbeashighas30-50%.Ifpatientsaregivenrehydrationtherapy,thenthedeathrateisbelow1%.
Womanwithcholeragettinghereyesexamined
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TreatmentTreatmentofthesymptomsofcholerafocusesonfluidreplacementaswateraloneisnoteasilyabsorbedbythebody.TheWorldHealthOrganization(WHO)recommendscleanwaterandrehydrationsaltstorestoreelectrolytebalance.Oralrehydrationtherapy(ORT)employsrehydrationsalts,whicharereadilyavailableinpharmacies;theseareofteninshortsupplyinareasofhighinfectionrate.ORThasbeenrevolutionaryincholeratreatmentand,ifproperlyused,ithugelyreducesboththeneedforhospitalizationandmortality.Somecasesmaybetreatedwithantibiotics,althoughgenerallytheindividual’simmunesystemislefttodestroythepathogen.Inextremecases,Hartmann’ssolution(asolutionofsodium,chloride,lactate,potassiumandcalciumions,whichisisotonicwithblood)isadministeredbyintravenousinjectiontoreplacebodyfluidsandmineralsalts.
TransmissionofcholeraCholerainfectionrequiresalargedoseofbacteriaandistransmittedbythefaecal-oralroute,forexamplebydrinkingfromawatersupplycontaminatedwithinfectedexcrement.Thebacteriacanalsobespreadtofoodifinfectedpeopledon’twashtheirhandsthoroughlyaftergoingtothetoiletandbeforefoodpreparation.Infectionisoftennotedafterthefuneralofacholeravictimwherecontaminationoccursasaresultofpoorhygieneinfoodpreparation.
Theenvironmentcanalsobeareservoirofinfectionasthebacteriaarepresentinbrackishwaters,ofteninassociationwithaquaticorganismsatalowlevel,andsometimesinanun-culturablestate.Shellfishlivingincontaminatedwatercantransmitcholeratoo.Theyarefilterfeedersand,astheystrainthewaterforfood,thebacteriabecomeconcentratedinsidethem.Anyoneconsumingshellfishthatarenotproperlycookedcanbecomeill.
Twogirlswashingdishesinmuddyriverwater
RetrospectivediagnosiscanbeperformedbyanalysisofthebloodforantibodiesagainstV. choleraeandCT.AlthoughidentificationofV. choleraeisnotoftenusedintreatment,thisinformationhelpstopreventfurtheroutbreaks.
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Physiologicaldifferencesaffecting infectionCholerainfectioncanbespectralinthatsymptomscanvarywithdose,serogroup,biotypeandserotype,thoughphysiologicalvariationbetweenpeoplecanalsoplayapart.Itisthoughtthatthosewhohavereducedornonexistentstomachacidduetodiseaseorailmentaremoresusceptibletocholera.Inahealthyindividual,thestomachacidactsasafirst-lineofdefenceagainstinfectionandkillsmanyV. choleraecellsbeforetheytravelto
PreventionGoodsanitation,cleandrinkingwaterandimprovedhygienepractices,suchaswashinghandsaftervisitingthetoiletandbeforepreparingfood,areallstrategiesthatneedtobefollowedtopreventtheoutbreakofcholera.Belowaresometipsfortravellersonhowtoavoidinfection,fromfoodanddrink,whenvisitinganareawherecholeraispresent.
FOOD DRINK
CookallfoodsthoroughlyUsebottled,boiled,orchemicallytreatedwatertowashdishes,cleanteethandpreparefood
Rawfruitandvegetablesshouldbeavoidedunlessyouthoroughlywashandpeelthem
yourself
Onlydrinkbottledwaterthathasthesealstillintact
Icecreamfromdoubtfulsourcesmayalsobecontaminatedthereforeshouldbeavoided
Avoidicecubesindrinks
Foodmustbeproperlypreparedandstillhotwhenserved
Boilunpasteurisedmilkbeforeyoudrinkit
Becarefulwheneatingfoodfromstreetstalls
Avoidsaladsastheymayhavebeenwashedincontaminatedwater
Avoidrawfishandshellfish
A simple rule is: Boil it, cook it, peel it or forget it!
VaccinesandimmunityOralvaccinesshowlong-lastingprotectionagainstcholerawithfewside-effects,althoughtheyprovideinsufficientprotectionforchildrenunder2yearsofage.Vaccineshoweverareonlyadvisedforpreventativeuseratherthanasamethodfor
theintestine.AnotherpossibilityforphysiologicalvariationamonginfectedindividualsmaybetheavailabilityofsurfacereceptorsforCTonthehostcellsurface,thoughthishasnotbeenproven.
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controllingoutbreaksandshouldbeusedintandemwithstandardpreventionandcontrolmeasures.Alsoimmunity,naturalorartificial,isserogroup-specific;forexampleimmunityprovidedagainstV. choleraeO1doesnotprotectfromO139.WHOrecommendsvaccinationforpeopleoccupyingslumsorrefugeecampsduetohighdensitypopulationsandincreasedriskofdiseasespread.
In1894,soonafterKoch’sidentificationofcholera,aninjectable,whole-cell,killedvaccine,wasdevelopedthatinduced48%protectionforthreemonthsagainstV. cholerae, serotypes InabaandOgawa.Thevaccine,initiallytrialledinIndia,wasneverendorsedbyWHO,althoughitmaystillbeavailableinsomecountries.
Protectiveimmunityinthoseexposedtocholeraisinducedalmostexclusivelybyantibodiesproducedintheintestine.Theseantibodiespreventbacterialcolonisationandmultiplication,andtheyinactivatetheCT.ImmunoglobulinsIgA,IgGandIgMhaveallbeendetected,althoughIgAisthemostimportant.TheantibodiespreventtheCTfrombindingwithreceptorsonthecellsurface.NaturalimmunityisprovidedbyIgMfollowedbyaswitchtoIgG.Thereisa3-yearperiodpostinfectionwherepatientsremainimmunetocholeraasaresultofacquired,naturalimmunity.
InareaswherecholeraisendemicsuchasBangladesh,infectionratesarelowamongadultswhencomparedtothechildreninthesamearea,whereasinareaswherenewepidemicsariseratesarehigherintheadultpopulation.Thisdiscrepancyillustratesaresistancelinkedtothepresenceofcirculatingvibriocidalantibodiestocholera.
Effective oral vaccines available against cholera:
uWC/rBS(Dukoral)–thisisawhole-cell,killedV. cholerae O1vaccinewithpartoftheCT protein.Trialshaveshownhighlevelsofprotection(85-90%)overaperiodof6monthsfor 2dosesofthevaccineinallagegroups,thoughprotectiondeclinesafter6monthsinyoung childrenandremainsatabout60%inolderchildrenandadultsafter2years.Protectionis providedagainstV. choleraeO1serotypesInabaandOgawa,andbiotypesClassicalandEl Tor.Dukoralalsoprovidesshort-termprotectionagainstEscherichia colienterotoxin,whichis ofaddedbenefittotravellers.
uVariantWC/rBS–acheaperversionoftheWC/rBSvaccinewhichdoesnotcontainanyCT; ithasbeenshowntohaveanefficacyof66%after8monthsinallagegroups.Thisvaccineis onlylicensedinVietnam.
uCVD103-HgR(Orochol)–thisvaccineconsistsofanattenuated,live,geneticallymodified V. choleraeO1InabastrainthathasbeenengineeredtoproducepartoftheCT.Itconfers highprotection(95%)againstO1(ClassicalandElTor)invirginvolunteers,i.e.thosenot previouslyexposedtothebacterium.Protectioninendemicregionshasnotbeenshownin aneffectivetrial,thoughretrospectivestudieshaveshownprotectioninanongoingoutbreak inMicronesia,agroupofislandsintheWesternPacific.Orocholistheonlyvaccineavailable asasingledose,which,duetoadministrationlogistics,ismoreviableforpre-emptiveand long-termoutbreakcontrolinongoingfieldconditionsaccordingtoWHO.However,due toalackofevidenceforitseffectiveness,thevaccinewaswithdrawnin2004.
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Furtherinformation uCDC-www.cdc.gov/nczved/divisions/dfbmd/diseases/cholera/
uECDC-www.ecdc.europa.eu/en/healthtopics/cholera/
uNetDoctor-www.netdoctor.co.uk/travel/diseases/cholera.htm
uTextbookofbacteriology-www.textbookofbacteriology.net/cholera.html
uWHO-www.who.int/topics/cholera/en/
EducationInordertocontrolandultimatelystopthespreadofacholeraoutbreak,communityeducationisparamount.Manyofthebasichygienemessages,whileperhapssimpletorelate,areoftendifficulttoimplementduetocost.Therefore,alternativesolutionsarerequiredtolimittransmission,forexampleWHOsuggeststheadditionoflimejuicetofoodandwatertoinactivateV. cholerae.Ideallyincommunitiesmostaffectedbycholera,awarenesscampaignscontinuethroughouttheyearincreasinginfrequencyasthecholeraseasonapproaches.Clearinformationispresentedwithmessagesadaptedforcultural,socialandeconomiccircumstances,andtheinformationisdeliveredgraphically,byradiobroadcastorastalksinareaswherepeoplearewaitingorcongregated.
FutureCurrentresearchintocholeraisfocusedonunderstandingtheCTitself.Anotherareaofanti-diarrhoealresearchisdevelopmentofapillthatwillpreventthediarrhoeapost-infection.Twodrugs,chlorpromazineandnicotinicacid,havebeenshowntobeeffectiveinanimalmodels,thoughthemechanismisyettobeunderstood.ResearchcontinuesintovaccinedevelopmentagainstbothO1andO139serogroups;severalpotentialnewvaccinesarecurrentlyinclinicaltrialsinBangladesh,IndiaandThailand.
Choleraresearch
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Koch’spostulatesRobertKochdevelopedasetofpostulatesforprovingthataspecificmicro-organismcausesaspecificdisease. uPostulate1:Thesuspectedpathogenic micro-organismshouldbepresentin allcasesofthediseaseandabsentfrom healthyindividuals[thispostulateisoften disregardedas,inthecaseofcholerafor example,itispossibletobeacarrierand notshowsymptomsofthedisease (asymptomatic)]. uPostulate2:Thesuspected micro-organismshouldbegrownin pureculture(notethecorrectmedium mustbeselectedforsuccessfulculture). uPostulate3:Cellsfromapurecultureof thesuspectedmicro-organismshould causediseaseinahealthyanimal. uPostulate4:Themicro-organismshould bereisolatedandshowntobethesame astheoriginal.
PaintingofRobertKochworkinginhislaboratory
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Cholera: Death by DiarrhoeaWrittenbyVicki Symington
DesignedbyDariel Burdass
EditedbyDariel Burdass
ProofreadbySusan Andrews
AcknowledgementsThanksareduetoProfessor Charles Penn (University of Birmingham)forhishelpfulcommentsonthistext.Everycarehasbeentakentoensurethattheinformationiscorrect,buttheauthorwillbepleasedtolearnofanyerrorsthathaveremainedundetected.
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