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(Tuis.) #1
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.)
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