Platyhelmintha(syn.Platyhelminthes) 553&Intestinalschistosomosis(intestinalbilharziosis).Causativeagents:
mainlyS.mansoniandS.japonicum,alsoS.mekongi(rare:rectallesions
causedbyS.hematobium).Incubationfourto 13 weeks(acutephase),months
toyears(chronicphase).Thecourseofaninitialinfectionisonlyrarelysymp-
tomatic(seeabove:Katayamasyndrome),inapparentandsubclinicalcourses
beingtherule.Manifestationsinthechronicphasearerestrictedalmosten-
tirelytolargeintestinewithhyperemia,granulomatousnodules,papillomas
(“bilharziomas”),ulcerations,hemorrhages,andincreasingfibrosis,abdom-
inalpainandbloodydiarrhea.
&Otherforms:thecausativeagentsofthehepatosplenicformaremainly
S.japonicum,lessfrequentlyS.mansoni.Thisfibroticformiscausedbyeggs
depositedaroundthebranchesoftheportalveinintheliver(“pipestem”fi-
brosisaccordingtoSymmers)andresultsincirculatoryanomalies,portalhy-
pertension,splenomegaly,ascites,hemorrhagesinthedigestivetract,and
othersymptoms.Pulmonaryschistosomosisisobservedmainlyinsevere
S.mansoniinfections,morerarelyininfectionswithotherspecies(including
S.hematobium).CerebralschistosomosisisrelativelyfrequentinS.japoni-
cuminfections(2– 4 %).
&Cercarialdermatitis.Cutaneouslesions(itching,erythema,urticaria,pa-
pules)inhumans,causedby(repeated)skinpenetrationofschistosomatid
cercariaeparasitizingbirds(e.g.,Bilharziella,Trichobilharzia)ormammals
(e.g.,Schistosomaspindale).Theinfectionoccursworldwideinfreshwater
orbrackishwaterandisknownas“swimmer’sitch.”Thesymptomsgenerally
abateafterafewdays.Thecercariaeofschistosomesfromhumanscancause
similar,althoughusuallymilder,symptoms.
Immunity.TheprevalenceandintensityofSchistosomainfectionsriseinen-
demicregionsinchildrenuntiltheageofabout 1 4,followedbyadecline
usuallyalsoaccompaniedbyreducedeggexcretion.Thisacquiredimmune
status,knownas“concomitantimmunity,”ischaracterizedbytotalorpartial
protectionagainstcercarialinfection.However,theschistosomesalreadyes-
tablishedinthebodyarenoteliminatedandmaypersistforyearsoreven
decades.
Theimmunedefenseisdirectedagainstschistosomulathathavepene-
tratedtheskin,areafewhoursold,andpresenttheirownantigensontheir
surface.Youngschistosomulacanbekilledmainlybyeosinophilsandmacro-
phagesassistedbyspecificantibodiestotheseantigensand/orbycomple-
ment.Bythetimetheschistosomulareachthelungstheyareresistantto
suchcytotoxicattacks.Theexplanationforthisphenomenonisthattheolder
schistosomulaareabletoacquirehostantigens(e.g.,bloodgrouporhisto-
compatibilityantigens)andtosynthesizehostlikemacromolecules,thus
“masking”theirsurfaces(=molecularmimicry)tocircumventtheimmune10Kayser, Medical Microbiology © 2005 Thieme