Medical Microbiology

546 10 Helminths

Platyhelmintha(syn.Platyhelminthes)

Trematoda(Flukes)

General.Mostofthetrematodespeciesthatparasitizehumansaredorsoven-

trallyflattenedwithanovaltolancetshape,althoughothershavedifferent

shapessuchasthethreadlikeschistosomes.Suckers(trema:hole,opening)

serveasattachmentorgans:anoralsuckeraroundthemouthconnectedto

theesophagusandtheblind-endingintestine,andaventralsucker.Thebody

surfaceofadulttrematodesiscoveredbyacellulartegument(composedofan

outerannucleate,syncytiallayerofcytoplasmconnectedbycytoplasmic

strandstoinnernucleatedportions)throughwhichsubstancescanbeab-

sorbedfromtheenvironment.Mostspeciesarehermaphroditic,onlythe

schistosomeshaveseparatesexes.Snailsarethefirstintermediatehosts;

somespeciesrequirearthropodsorfishassecondintermediatehosts.

Schistosoma(BloodFlukes)

Causativeagentsofschistosomosisorbilharziosis

&Schistosomosis(bilharziosis)isoneofthemostfrequenttropicaldiseases

withabout 200 millioninfectedpersons.Theoccurrenceofschistosomosis

dependsonthepresenceofsuitableintermediatehosts(freshwatersnails).

Humaninfectionsresultfromcontactwithstandingorslow-movingbodies

ofwater (freshwater)when Schistosoma cercariae penetrate theskin.

Schistosomahematobiumcausesurinaryschistosomosis;S.mansoni,S.japo-

nicum,S.intercalatum,andS.mekongiarethecausativeagentsofintestinal

schistosomosisandotherformsofthedisease.Diagnosiscanbemadeby

detectionofeitherSchistosomaeggsinstoolorurineorofspecificantibodies

inserum. &

Parasitespeciesandoccurrence.Schistosomosisisalsoknownasbilharzio-

sisaftertheGermanphysicianTh.Bilharz,whodiscoveredSchistosomahe-

matobiuminhumanbloodvesselsin 1851 .Schistosomosisoccursendemi-

callyin 74 tropicalandsubtropicalcountriesofAfrica,SouthAmerica,and

Asia(Fig. 1 0. 2 ).Thenumberofpersonsinfectedwithschistosomesisestim-

atedat 200 million(WHO, 2004 ).

ThemostimportantspeciespathogenictohumansareSchistosomahema-

tobium(Africa,theNearEast,andquestionableoccurrenceinIndia),S.man-

soni(Africa,theCaribbean,thenorth-eastofSouthAmerica),andS.japoni-

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Kayser, Medical Microbiology © 2005 Thieme