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论文翻译

合肥学院

数学与物理系

毕业设计（论文）翻译资料

学生姓名：

江海波

学号：

0707021037

系部：

数学与物理系

专业：

数学与应用数学（师范）

题目：

一个钉螺控制模型的研究

指导教师：

胡雁玲

英语原文

Modellingandsimulationofaschistosomiasisinfectionwith

biologicalcontrol

E.J.Allen*,H.D.Victory,Jr.

DepartmentofMathematicsandStatistics,TexasTechUniversity,Lubbock,TX79409-1042,USA

Accepted27January2003

1.Introduction

Oneofthemostseriouspublichealthproblemsinthetropicsandsubtropicsishumanschistosomiasis,aparasiticinfectionthatises-

timatedtoaffectover200millionpeople.Inthepresentinvestigation,astochasticmodelofschistosomiasisdynamicsisdeveloped.Themodelisbasedon,butgeneralizesinsomeways,previousmathematicalmodelssuchasthosedescribedbyAndersonandMay（1985）;Kimbir（1997_/1998）;WuandFeng（2002）.Inparticular,themodelallowscompetitionbetweentheintermediatehostsnailsandaresistantsnailspeciestostudytheadvantagesofbiologicalcontrol.Furthermore,asecondmammaliandef-

initivehostisincludedinthemodel.Thestochasticmodeltakesintoaccounttherandomnatureofbirths,deaths,andtransmissions.Math-

ematicalanalysesandanumberofcomputationalsimulationsarecarriedouttostudytheepidemiologicalconsequencesofcontrolstrategies.Interpretationsofthemathematicalresultsleadtoanimprovedunder-

standingoftheimpactofcontrolprograms.

Schistosomiasisischaracterizedbylong-termdisabilityandisthoughttosignificantlyimpedetheadvancementofmanyunderdevelopedcountrieswherethediseaseisendemic.Duetosuccessfulcontrolprojects,thedistributionofschistosomiasishaschangedinthelast50years.

However,thetotalnumberofpeopleinfectedoratriskofinfectionhasnotchanged（Chitsuloetal.,2000;Ghibodaetal.,2000）.ThepublichealthimpactandmagnitudeoftheproblemareevidentfrominformationavailablefromtheWorldHealthOrganization（WorldHealthOrganization,1993）:

morethan600millionpeoplein74countriesareatriskandmorethan200millionpeopleareinfected（increasedfrom114millionin1974）.Mortalityexceeds100,000annually.Schistosomiasisremainsformidabletohumansbecauseofthecomplexitiesofparasiticadjustmenttotwoormoredifferenthosts（Garrett,1994;McNeill,1977）.

Thepersistenceofaschistosomiasisinfectioninalocalitydependsonacomplexcycleinvolvinghumansandpossiblyadditionalmammalianspecies（definitehosts）,certainparasiticflatworms（schistosomes）,andparticularspeciesofsnails（intermediatehosts）.Theadultsch-

istosomewormsmateheterosexually.Thepairedadultsreproduceinthebloodvesselsofahumanormammalianhost.Thefertilizedeggspassfromthebloodintotheintestineorbladderandarevoidedwiththefecesorurine.Becauseoflowhygienicstandards,someoftheeggsaredepositedinfreshwaterwheresmallciliatedlarvae（miracidia）emergeandenteramolluscanhost.Byasexualreproductioninthesnail,thousandsofasecondlarvalform,cercariae,areproduced.Whenmature,thecercariaeenterafree-swimmingstageandoncontactwithahuman,theyrapidlypenetratetheskinandmatureintojuvenileschistosomes.Thisisfollowedbypairingwiththeoppositesex,copulation,andovipositionwhichbeginsthecycleoveragain.

Theepidemiologicalmapofschistosomiasisischanging.Activitieslinkedtothedevelopmentofwaterresourcesandpopulationdisplacementshavespreadthediseasetopreviouslynon-endemicareaswhilemanyco-

untrieshavesucceededineliminatingtheinfectionfromallorpartoftheirterritories（Chitsuloetal.,2000）.Forexample,SchistosomajaponicumhasbeeneradicatedfromJapanandiseffectivelycontrolledinmanypartsofChinaandthePhilippines.PuertoRicoisofspecialinterestinthattheinfectionhasdeclinedwithoutsubstantialhumaneffortthroughtheaccidentalintroductionofThiaragranifera,asnailwhichcompeteswiththeintermediatehostsnailBiomphalariaglabrata.Ontheotherhand,inmuchofAfrica,therehasbeenfewattemptstocontrolschistosomiasiswherethehumanpopulationhasincreasedfrom344millionin1976to577millionin1995.Itisestimatedthat85%ofthetotalnumberofpeopleinfectedwithschistosomiasisliveontheAfricancontinent（Chitsuloetal.,2000;HeesterbeekandRoberts,1995）.Inaddition,large-scalewaterimpoundmentprojectsforelectricityandirrigationhaveintroducedschistosomiasisintonewareas.Recently,geographicinformationsystem（GIS）riskmodelsforschistosomiasishavebeendevelopedthatarebasedonclimateandsatellite-retrieveddataontemperatureandvegetationcover（see,e.g.Kirstensenetal.,2001orZhouetal.,2001）.Thesemodelswillhelpidentifythechangingge-

ographicdistributionofthedisease.

Humanschistosomiasisiscausedbyfivespeciesofflatworms:

S.mansoni,S.intercalatum,S.japonicum,S.mekongi,andS.haematobium.ThethreemostwidespreadareS.japonicumwhichisfoundinseveralAsiancountries,S.haematobiumwhichispresentinmuchofAfricaandtheMiddleEast,andS.mansoniwhichisprevalentinmuchofAfrica,theCaribbean,andSouthAmerica.ItiswellknownthatS.japonicumhasmultiplema-

mmaliandefinitehosts（Lietal.,2000;WuandFeng,2002）.Cattle,horses,sheep,waterbuffalo,rodents,andothermammalsmaybecomeinfectedandtransmitS.japonicum.ItisgenerallyregardedthatmanistheonlysignificantvertebratehostforS.mansoniandS.haematobium.However,inanextensiveBraziliangovernmentprogramtoeradicateschistosomiasis（S.mansoni）inBrazilinthe1970s,researchersinagovernmentlab-

oratorynearNatalreportedaschistosomiasisinfectioninwildratsandmice（Behrman,1980）.Asaresult,itwasconcluded,astherewasanothercycleofinfectioninadditiontothehumancycle,thatitwasimpossibletoeradicatethediseaseinBrazilwithoutkillingallthecarriersnails.TheBrazilianprogramobjectivethenchangedfromeradicationofsch-

istosomiasistocontrolofschistosomiasis.TheBrazilianexperienceindicatesthatmammalianhostsotherthanhumanmaybequiteimportantforS.mansoniaswellasforS.japonicum.

Controlmethodsforschistosomiasisrangefromenvironmentalmo-

dificationtoeliminatethehostsnails,chemicalmolluscicides,che-

motherapy,andmorepermanentmethodssuchastheprovisionofsafewaterandsanitaryfacilities.Formanyendemicsituations,chemotherapyisamajoroperationalcomponentandisfocusedonschoolagechildrenandotherhigh-riskgroups.However,itisacknowledgedthatthepriceofchemicalantischistosomalcontrolisbeyondthehealthbudgetsofmanydevelopingcountries（Chitsuloetal.,2000）.Oneeffectivemethodofcontrolwhichmayrequirerelativelylittlefundingisbiologicalcontrol.Inpar-

ticular,trematodeparasitesorcompetitivesnailsoftheintermediatesnailhostshaveprovedtobeeffectiveincontrollingschistosomiasisintheCaribbeanarea（PointierandJourdane,2000）.Inparticular,theriversinthecentralpartofVenezuelawereactivetransmissionsitesforschistosomiasisbutseveralmolluscicidetreatmentsdestroyedthesnailhostcoloniesandlaterdensecolonizationoftwocompetitorsnailshaveconstitutedagoodbarrieragainstthereappearanceofthesnailhosttherebypreventingschistosomiasistransmission.

Since1973,therehavebeenmanymathematicalmodelsofthetran-

smissiondynamicsofschistosomes（see,e.g.AndersonandMay,1985,1992;Fengetal.,2001;HeesterbeekandRoberts,1995;HerbertandIsham,2000;Hirsch,1982;Hirschetal.,1985Kimbir,1997_/1998Luchsinger,2001a,b,Murray,1993;Nasell,1976;NasellandHirsch,1973;PellegrinelliandGabriel,1993;Woolhouse,1991,1992;WuandFeng,2002）.Althoughthemodelshavebroughtinsightintoschistosomebiology,theyhavehadrelativelylittleimpactonfieldstudiesoronthedesignofschistosomecontrolprograms（Woolhouse,1991,1992）.Thedecisionofwhethertoallocatefundstochemotherapy,molluscicides,orothercontrolmeasuresmustinvolvepredictionsoftheeffectsofdifferentcontroloptions.Manyofthepresentmodelsofschistosomiasisdynamicsconcentrateones-

timatingpopulationlevelsofschistosomes,miracidia,andcercariae.Equationsareformulatedthatrelatethepopulationofschistosomesandlarvaeinthehumanhosts,snails,andwaterbody.Asaresult,thesemodelsareifficulttostudyintermsofevaluatingcontrolstrategiessuchaschemotherapy,molluscicidetreatments,sanitationimprovements,andbiologicalcontrolsuchasacompetitivesnailspecies.

Inthepresentinvestigation,thepopulationsofinfectedanduni-

nfectedhumans,hostsnails,competitorsnails,andasecondmammalianhostareexplicitlymodelled.Adeterministicsystemofdifferentialequationsisderivedthatdescribestheinteractiondynamicsofthedifferentpopulationsandtakesintoaccountthetimeintervalbetweeninfectionofasnailbyamiracidiumandthereleaseofcercariae.Byincludingtermsthataccountfortherandomnatureofthebirth,death,andtransmissionprocesses,asystemofItostochasticdifferentialequationsisobtained.Themathematicalsystemisusefulinevaluatingtheeffectivenessofcontrolstrategies.Computationalsimulationsofthestochasticmodelareperformedtodeterminetheeffects,forexample,ofchemotherapyandsnailcompetitors.（Itisassumedinthepresentin-

vestigationthatasnailspeciescanbefoundthatisresistanttoin-

fectionandisabletooutcompetetheintermediatehostsnailspecies.）Adisadvantageofthemodelmaybethatschistosome,miracidia,andcercariaepopulationsarenotexplicitlycalculated.Forexample,com-

plicatedrelationsbetweennumbersofovipositingworms