Medical Microbiology

584 10 Helminths

Givencertainconditions,thefirst-stagelarvaecandevelopintoafree-living

(nonparasitic)generationofmalesandfemales.Thefertilizedeggslaidbythe

femalesofthisgenerationdevelopintoinfectivethird-stagelarvae.Thisca-

pacityforexogenousreproductionexplainstheenormouspotentialforcon-

taminationofagivenenvironmentwithStrongyloideslarvae.Third-stagelar-

vaearehighlysensitivetodesiccation,butremainviablefortwotothree

weeksinthepresenceofsufficientmoisture.

Theparasiticpartofthelifecycleissimilartothatofthehookwormsin

thatStrongyloidesalsopenetratethehost’sskinandthelarvaereachtheir

targetlocalizationinthesmallintestinebywayoflungandtrachealmigra-

tion.Theprepatentperiodisatleast 17 days.Strongyloideslarvaecanalsobe

occasionallytransmittedviamother’smilk.

Thepotentialforautoinfectionwiththisorganismisworthyofmention.

Thefirst-stagelarvaecantransformintoinfectiouslarvaeduringtheintes-

tinalpassageorintheanalcleftandpenetrateintothebodythroughthelarge

intestineorperianalskin.Continuousautoinfectioncanmaintainanunno-

ticedinfectioninanimmunocompetentpersonformanyyears(seebelow).

HumansarethemainreservoirhostsofS.stercoralis;infectionscanalsobe

transmittedfrommonkeysordogs,butthisrouteofinfectionisinsignificant.

Pathogenesisandclinicalmanifestations

&SkinlesionsareobservedwhenthelarvaeofStrongyloidesspeciespene-

tratetheskin,inparticularinsensitizedpersons.LarvaeofStrongyloidesspe-

ciesfromanimalscancause“cutaneouslarvamigrans”(p.602).

&Inthelungs,themigratinglarvaeprovokehemorrhagesandinflamma-

toryreactionsthatmanifestclinicallyaspneumonicsymptomsandcoughing.

&Duringtheintestinalphase,heavyStrongyloidesinfectionscausecatar-

rhal,edematous,orulcerativeformsofenteritisaswellascolitis.

&Systemicinfection. AStrongyloidesinfectioncanpersistinalatentstate

formanyyearsduetocontinuousautoinfection.Ifimmunedefenseiscom-

promised,forinstancebyAIDSorimmunosuppressivetherapy,parasitere-

productioncanbestimulated,resultinginmassivesystemicinfections(hy-

perinfections)inwhichStrongyloideslarvaearefoundinthewallsofthecolon

andmesentericvessels,inthebileductsandinotherorgans.Insuchcases

sexuallymaturefemalewormsarealsofoundinthelungs,andlessfrequently

inotherorgansaswell.Abroadspectrumofsymptomsisassociatedwith

systemicinfection.

Diagnosis.Larvae(Fig. 1 0. 1 , p. 5 44 ) ofS.stercoraliscanbedetectedinfecal

sampleswiththeBaermannmethodand/orlarvalcultureinabout 60 –

70 % of infected persons (egg detection with flotation technique for

S.fuelleborni).Betterresultscanbeexpectedifduodenalfluidisexamined.

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Kayser, Medical Microbiology © 2005 Thieme