Medical Microbiology

Nematoda(Roundworms) 577

yearsandcanbeactivatedinresponsetoimmunodeficiencyanddevelopinto

life-threateningsystemicinfections.Carefuldiagnosticexaminationsare

thereforenecessaryifaStrongyloidesinfectionissuspected. &

Ascarislumbricoides(LargeRoundworm)

Causativeagentofascariosis

Occurrence.Thehumanlargeroundwormoccursworldwide.Thenumberof

infectedpersonsisestimatedat 1 .38billion(WHO, 199 8).Themainendemic

regions,withprevalenceratesofapprox. 10 – 90 %,includecountriesinSouth-

eastAsia,Africa,andLatinAmerica.Autochthonousinfectionsarerareincen-

tralEurope.

Parasiteandlifecycle.Theadultascaridslivinginthesmallintestine(as-

caris:worm)are 15 – 40 cminlength,aboutasthickasapencilandofayellow-

ishpinkcolor(Fig. 1 0. 12 ).Thesexuallymaturefemalesproduceasmanyas

200000 eggsperday,whichareshedwithfecesintheunembryonatedstate.

Theround-to-ovaleggsareabout 60! 45 lminsize,haveathick,brownish

shellandanunevensurface(Fig. 1 0. 13 ).Atoptimumtemperaturesof 20 – 258 C

withsufficientmoistureandoxygen,aninfectivelarvaintheeggdevelops

withinaboutthreetosixweeks.

Humaninfectionsresultfromperoralingestionofeggscontaininglarvae,

whichhatchintheuppersmallintestineandpenetrateintotheveinsofthe

intestinalwall.Theyfirstmigratehematogenouslyintotheliverandthen,

fourtosevendaysp.i.,intothelungs,wheretheyleavethecapillarynetwork

andmigrateintothealveoli.Viatracheopharyngealmigrationtheyfinally

reachthedigestivetract,wheretheyfurtherdifferentiateintoadultsin

thesmallintestine.Theprepatentperiodlastsforseventonineweeks.

Thelifespanoftheseparasitesis 12 – 18 months.

Ascarislumbricoides

Fig. 1 0. 12 Maleandfemale

Ascaris.

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Kayser, Medical Microbiology © 2005 Thieme