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female worms lay eggs that hatch into L1 progeny, which moult via an L2

stage into iL3s, as in direct development. InS. rattiandStrongyloides

stercoralis, only one round of indirect development can occur; however,

up to nine have been observed withStrongyloides planiceps,a parasite of

the cat (Yamadaet al., 1991).

Therefore, in theStrongyloideslife cycle, young larvae appear to have

a number of developmental potentials. The control of the development in

this life cycle has been investigated in various ways for a number of

species ofStrongyloides(Schad, 1989).This background and more recent

work has now developed a good understanding of the control of the

Strongyloideslife cycle, which is most thoroughly and best understood for

the parasite of rats,S. ratti. This has shown that there are two separate

levels of control in this life cycle: a male/female sex-determination event

and a female-only developmental choice (Fig. 6.1).

112 M.E. Viney

Parasitic

female

Infective L3s

Direct development

Free-living female

Free-living male

Female Male

Intra-host

Extra-host

1

2

Fig. 6.1. The life cycle ofS. ratti, with larval stages omitted. Parasitic females lay

eggs and these and/or first-stage larvae (L1s) pass out of the host. Sex is

determined by this stage (Box 1). Female L1s can moult though one larval stage

(L2) into infective L3s. Alternatively, L1s moult through three larval stages (L2, L3,

L4) into free-living females. The developmental choice of female larvae is controlled

by environmental conditions (Box 2). Male larvae moult through four larval stages

into free-living males only. Free-living adults reproduce by sexual reproduction, the

female lays eggs and larvae moult into infective L3s. Infective L3s infect hosts by

skin penetration, and migrate to the intestine, moulting through a fourth larval stage

into parasitic females. (From Harvey and Viney, 2001.)