Environmental Control of Nematode Life Cycles

Environmental Control of 6

Nematode Life Cycles

M.E. Viney

School of Biological Sciences, University of Bristol,

Woodland Road, Bristol BS8 1UG, UK

Parasitic nematodes have complex life cycles. Many aspects of these

life cycles, including their development, are affected and controlled

by environmental conditions within and without the host. The genus

Strongyloideshas a particularly complex life cycle, which is unique

among nematode parasites of vertebrates. Many factors act and interact to

control this life cycle in a remarkably complex and sophisticated way.

The Life Cycle ofStrongyloidesspp.

The parasitic stage of theStrongyloideslife cycle consists of parasitic

female worms only, which lie embedded in the mucosa of the host’s small

intestine. InStrongyloides ratti,these female parasites reproduce by a

mitotic parthenogenesis, such that the progeny of any one female worm

are genetically identical to each other and to their mother (Viney, 1994).

The female parasite lays eggs and these or newly hatched first-stage larvae

(L1s) pass out of the host with the faeces, where the extensive free-living

phase of the life cycle occurs (Fig. 6.1).

In this free-living cycle, there are two types of development, direct

and indirect. In direct (also known as homogonic) development L1s moult

via an L2 stage into infective third-stage larvae (iL3s). These iL3s are able

to infect new hosts by skin penetration, after which they migrate through

the host body, developing via an L4 stage into adult females.S. rattilarvae

have been shown to migrate through the nasal–frontal part of the head

en routeto the gut (Tindall and Wilson, 1988).

In indirect (also known as heterogonic) development, L1 progeny of

parasitic females moult via three larval stages (L2, L3, L4) into free-living

adult male and female worms. These worms mate and reproduce by

conventional sexual reproduction (Vineyet al., 1993). The free-living

CABInternational2002.The Behavioural Ecology of Parasites
(eds E.E. Lewis, J.F. Campbell and M.V.K. Sukhdeo) 111