Medical Microbiology

578 10 Helminths

Epidemiology.Reservoirhostsoftheparasitearehumans.Theexcretedeggs

remainviableforyearsinamoistenvironment(soil),butaresensitiveto

desiccation.InfectiveAscariseggscanbeingestedbyhumanswithcontami-

natedfoods,soil(geophagiainchildren!)and,lessfrequently,indrinking

water.Inendemicareas,theprevalenceandintensityofA.lumbricoidesin-

fectionsarehighestinchildren.

Pathogenesisandclinicalmanifestations.Mildinfectionsfrequentlyremain

inapparent.Inmoresevereinfections,larvalmigrationinthelungsprovoke

hemorrhagesandinflammatoryinfiltrations,whichpresentasdiffusemot-

AscarislumbricoidesandTrichuristrichiura:LifeCycles

1a

1b

1c

1d

1e

2a

2b

2c

Fig. 1 0. 13 1 aAdultstagesofA.lumbricoides;1bfreshlyshedegg,notyet

infective;1ccontaminationofvegetableswitheggs;1dinfectiveeggwithlarva;

1eingestionofinfectiveeggswithcontaminatedfood.2aAdultstageofT.tri-

chiura;2bfreshlyshedegg,notyetinfective;2cinfectiveeggwithlarva.(The

transmissionroutesofT.trichiuraaresimilartothoseofAscaris.)

10

Kayser, Medical Microbiology © 2005 Thieme