Medical Microbiology

Platyhelmintha(syn.Platyhelminthes) 555

Fasciolaspecies

Fasciolahepatica(CommonLiverFluke)andF.gigantica

(LargeorGiantLiverFluke)

Causativeagentsoffasciolosis

&FasciolahepaticaandF.giganticaarefrequentbileductparasitesofdo-

mesticruminants.Intheirlifecyclefreshwatersnailsactasintermediate

hosts.Humansbecomeaccidentallyinfectedwhentheyeatplants(e.g.,

watercress)towhichinfectiousparasitestages(metacercariae)adhere. &

Occurrence.Fasciolahepaticaoccursworldwideasanimportantparasitein

domesticruminantsthatcanalsoinfectotheranimalspecies.Sporadicoren-

demicF.hepaticainfectionsinhumanshavebeenreportedfromabout 50

countriesorregionsonallcontinents(WHO, 1 999).InAsiaandAfrica,human

infectionswiththe 7 .5cmlonggiantliverfluke(F.gigantica)arealsoreported.

ThenumberofpersonsinfectedwitheitherF.hepaticaorF.giganticais

estimatedat2.4million(WHO, 1 995).

Parasites,lifecycle,andepidemiology.F.hepaticaisaflattened,leaf-shaped

parasiteabout 2 – 5 cmlongandatmost 1 cmwide.Thecephalicconewiththe

oralsuckerissomewhatdemarcatedfromtherestofthebody.Afurtherchar-

acteristicfeatureisthepronouncedbranchingofvariousinnerorgans(Fig.

10 .6a).

Adultliverflukesparasitizeinthebileducts.Theyproducelarge(approx.

130! 85 lm),golden-brown,operculatedeggs(Fig. 1 0. 1 ,p.543)thatareshed

bythebileduct-intestinaltractroute.Underfavorableconditions,aciliate

larva,themiracidium,developsintheeggwithinafewweeks.Themiracidia

thenhatchandpenetrateintofreshwatersnails(LymnaeatruncatulainCen-

tralEurope),wheretheytransformintosporocysts.Afterformationoffurther

asexualreproductivestages(rediae),tailedcercariaedevelopandswarmout

ofthesnailsintotheopenwater.Theysoonattachtoplantsandencyst,i.e.,

transformintoinfectivemetacercariae,whicharetheningestedwithvege-

tablefoodoftheirdefinitivehosts.Eatingwatercresscontaminatedwithme-

tacercariaeisoneofthesourcesofinfectionforhumans.

Thejuvenileliverflukeshatchfromthecystinthesmallintestine,pene-

tratetheintestinalwall,andmigratethroughtheperitonealcavitytothe

liver.Aftermigratingthroughthehepaticparenchymaforaboutsixtoseven

weeks,theparasitesfinallyreachthebileducts,inwhichtheydevelop

tosexualmaturity.Eggexcretionbeginstwotothreemonthsp.i.atthe

earliest.

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Kayser, Medical Microbiology © 2005 Thieme