Medical Microbiology

Nematoda(Roundworms) 581

totwodaysthefirst-stagelarvaeleavetheeggshells,molttwice,anddevelop

intoinfectivethird-stagelarvae.Sincetheshedsecond-stagecuticleisnot

entirelyremoved,thethird-stagelarvaiscoveredbyaspecial“sheath.”Lar-

vaeinthisstagearesensitivetodryness.Inmoistsoilorwatertheyremain

viableforaboutonemonth.Highertemperatures(optimum: 20 – 308 C)and

sufficientmoisturefavorthedevelopmentoftheparasitestagesoutsideofa

host.

Humansareinfectedmainlybythepercutaneousroute.Factorsfavoring

infectionincludeworkinginricepaddies,walkingbarefootoncontaminated

LifeCyclesofHookworms

1a

1b

6

5

4

3

2

Fig. 1 0. 14 1 Femaleandmalehookworms; 2 hookwormeggshedinstoolwith

blastomeres; 3 developmentoffirst-stagelarva(L1)inegg; 4 hatchedL1larva;

5 L2larva; 6 L3larvawithsheath,infectivestage.

10

Kayser, Medical Microbiology © 2005 Thieme