Medical Microbiology

566 10 Helminths

Echinococcusgranulosus(DwarfDogTapeworm)

Causativeagentofcysticechinococcosis(CE)

Occurrence.E.granulosusoccursworldwide,withrelativehighprevalences

ineasternandsoutheasternEurope,theMediterraneancountries,theNear

East,northernandeasternAfrica,SouthAmerica,andvariouspartsofAsia

andAustralia.TheparasitehasbecomerareinnorthernandcentralEurope;

mostofthehumancasesofCEdiagnosedintheseareasareimported,inpar-

ticularfromMediterraneancountries.E.granulosusandE.multilocularisoc-

curtogetherinsomeareas.

Morphologyanddevelopment

&Adultstage.E.granulosusisa 4 – 7 mmlongtapewormwithascolex(bear-

ingrostellarhooks)andnormallythree(twotosix)proglottids.Anotable

characteristicistheuteruswithitslateralsacculations,containingupto

1500 eggs(Fig. 1 0. 10 ap.569).

&Definitive(final)andintermediatehosts.Themostimportantdefinitive

hostforE.granulosusisthedog,wherebyotherCanidae(jackal,dingo,and

otherwildcanids)areinvolvedincertainregions.Herbivorousandomnivo-

rousvertebratesfunctionasintermediatehosts,inparticulardomesticani-

mals(ruminants,pigs,horses,camels)andinsomeareaswildanimalsas

well.

&Lifecycle(Fig. 1 0. 9 ).Theadulttapewormsliveinthesmallintestineofthe

definitivehostforaboutsixmonths,afewforuptotwoyears(Fig. 1 0. 10 c).

Eggsareeitherreleasedfromgravidproglottidsintheintestineandshed

withfecesorpassoutofthehoststillenclosedinthetapewormsegments.

Theeggs(diameterapprox. 30 – 40 lm)arenearlyspherical,containanon-

cosphereandfeaturearadiallystripedshell.Theycannotbemorphologically

differentiatedfromtheeggsofotherEchinococcusorTaeniaspecies(see

Fig. 1 0. 1 ,p. 54 4).Infectionoftheintermediatehosts,humans,andother

accidentalhostsisbyperoralingestionofeggs,fromwhichtheoncospheres

arereleasedinthesmallintestine,penetrateintoitswallandmigratehema-

togenouslyintotheliver,aswellassometimesintothelungsandother

organs.Atfirst,theoncospheresdevelopintolittlevesicles,thengradually

intometacestodes.

&ThemetacestodeofE.granulosus(alsoknownashydatidcysts,fromhy-

datis=waterbladder)isnormallyafluid-filledcystwithoneormultiple

chambers,thewallofwhichismadeupofaninner,cellular,germinativelayer

andanouter,acellular,laminatedlayer(cuticularlayer),enclosedbyalayerof

hostconnectivetissue(Fig. 1 0. 1 0d,f).Broodcapsulesdevelopfivetosix

monthsorlaterp.i.onthegerminativelayer,eachcontainingupto 20 or

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Kayser, Medical Microbiology © 2005 Thieme