Medical Microbiology

Platyhelmintha(syn.Platyhelminthes) 549

Theadultparasitesliveinthelumenofveins.Table 1 0. 2 summarizesdataon

variousSchistosomaspecies.Fig. 1 0. 4 (p.550)showstheirlifecycle.

SexuallymatureSchistosomafemaleslayabout 100 – 3500 eggsaday,de-

pendingonthespecies,eachcontaininganimmaturemiracidium(=ciliate

larva),whichmaturesinthehostwithinsixto 10 daysandremainsviablefor

aboutthreeweeks(Fig. 1 0. 4 ).

Atthesiteoftheirdeposition,theeggslieinchainlikerowswithinsmall

veins.Somepenetratethroughthevascularwallandsurroundingtissueto

reachthelumenoftheurinarybladderorintestine(regardingtheeggs

thatremaininthebodyseesectiononpathogenesis).Enzymesproduced

bythemiracidiumandsecretedthroughmicroporesintheeggshellandgran-

ulomaformation(seebelow)contributetothepenetrationprocess.Theeggs

areshedbythedefinitivehostinstoolorurinewithinafewweekspostin-

fection(p.i.)(seebelow).Iftheeggsaredepositedintofreshwater,themira-

cidiahatchfromtheeggshellandbegintheirsearchforasuitableintermedi-

atehost(Fig. 1 0. 4 ).

Variousgeneraandspeciesoffreshwatersnailsserveasintermediate

hosts(Table 1 0. 2 )inwhichtheinvadingmiracidiareproduceasexually,pro-

ducingmotheranddaughtersporocysts,andfinallynumerouscercariae,

whichbegintoswarmintothewaterthreetosixweeksp.i.attheearliest.

Acharacteristicfeatureoftheapproximately 340 – 520 lmlongcercariaeis

theirforkedtail.Thecercariaeswimfreelyaboutorclingtothesurfaceof

thewater.Uponcontactwithahumanhost,enzymesecretionandvigorous

movementsenablethemtopenetratetheskinwithinafewminutes,orless

frequentlythemucosawheningestedwithdrinkingwater.Duringtheinfec-

tionprocess,thecercarialosesitstail,shedsthesurfaceglycocalyx,formsa

newtegument,andtransformsintotheschistosomulum.

SchistosomamansoniPair

Fig. 1 0. 3 Thethreadlikefe-

maleisenclosedinagroove

inthebodyofthemale.

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Kayser, Medical Microbiology © 2005 Thieme