Medical Microbiology

Platyhelmintha(syn.Platyhelminthes) 551

MigrationofSchistosomesintheHumanBody

Infection!schistosomulapenetratesubcutaneoustissues!findvenouscapil-

lariesorlymphvessels!migratethroughthevenouscirculatorysysteminto

therightventricleoftheheartandthelungs!travelhematogenouslyintothe

intrahepaticportalveinbrancheswheredevelopmentintoadultwormstakesplace

aswellsasmale-femalepairingjustpriortosexualmaturity!retrogrademigration

ofpairsintomesentericveinsortothevesicalplexus(Table 1 0. 2 ).

DependingontheSchistosomaspeciesinvolved,theprepatentperiodlasts

aboutfourto 10 weeks.Schistosomesremaininthedefinitivehostforan

averageoftwotofiveyears,butinsomecasesforaslongas 20 – 40 years.

Epidemiology.Schistosomosisoccursasanautochthonousinfectionintrop-

icalandsubtropicalregions.Aquaticfreshwatersnailsthatpreferstandingor

slow-movingbodiesofwateraretheintermediatehostsforS.hematobium,

S.mansoni,S.intercalatum,andS.mekongi(Table 1 0. 2 ).Theintermediate

hostsofS.japonicumareamphibioussnailsalsofoundonmoistground

andplants,e.g.,inricepaddies.

AlthoughthecyclesofallSchistosomaspeciescanincludeanimalsas

hosts,humans arethemostimportantparasitereservoirs(Table 1 0. 2 ).

However,animalscontributesignificantlytothedisseminationoftheeggs

ofS.japonicumandS.mekongi.Travelerstoendemictropicalareascan

acquiretheinfectionbyasingleinstanceofcontactwithwatercontaining

cercariae.

Pathogenesisandclinicalmanifestations.Theinfectioncanbedividedinto

thefollowingphases:

&Penetrationphase:penetrationofcercariaeintotheskin,eitherwithout

reactionor—especiallyincasesofrepeatedexposure—withitchingandskin

lesions(erythema,papules),whichdisappearwithinafewdays.

&Intheacutephase,abouttwoto 10 weeksafterasevereinitialinfection,

thesymptomsmayincludefever,headache,limbpains,urticaria,bronchitis,

upperabdominalpain,swellingoftheliver,spleenandlymphnodes,in-

testinaldisturbances,andeosinophilia(=Katayamasyndrome).Duetore-

leaseofSchistosomaantigens,theserumantibodylevels(IgM,IgG,IgA)

riserapidlyandimmunecomplexesareformedthatcancauserenalglomer-

ulopathies.Thesesymptomspersistforseveraldaystoseveralweeks.Nor-

mally,Schistosomaeggsarenotyetexcretedatthebeginningofthisphase

(seeprepatentperiods).Inlow-levelinfectionsthisphaseisusuallyinappar-

entorsubclinical.

&Chronicphase:themostsignificantphaseinpathogenictermsbegins

afteranincubationperiodofabouttwomonthswithovipositionbytheSchis-

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Kayser, Medical Microbiology © 2005 Thieme