live and dead larvae do not exhibit this resistance to subsequent
challenge. Thus, here, the liveH. polygyrusimmunomodulates the host
immune response and thus suppresses an immune response directed
against a challenge infection (Behnke et al., 1983). Host immuno-
modulation has also been described for species of filarial nematodes
(Maizelset al., 1993). It is probable that species which immunomodulate
their hosts do so in response to their presence in a host, rather than
in response to an immune response acting against them. Induction of
immunomodulation automatically on entry into a host would seem to
be a reliable mechanism for ensuring survival, on the assumption that
an infection will induce an antiparasite immune response. However, if
immunomodulation is energetically expensive and/or the host population
varies widely in the ‘strength’ of antiparasite immune responses that
may occur, it would seem more efficient for a parasite only to immuno-
modulate their hosts’ immune response specifically in response to a
specific immune response acting against them.Arrested development
Other nematodes have facultative developmental decisions in their life
cycles, which are affected by various environmental conditions. Thus, for
some species of nematodes (e.g.Ostertagia, Trichostrongylus) infective
larvae can arrest their development after having entered a host. This
phenomenon of arrested development has been most thoroughly investi-
gated in nematode parasites of cattle. In these instances, L3s are ingested
by hosts and the larvae moult to an L4 stage, at which point they can arrest
their development for several months (Gibbs, 1986). This may be an
adaptation to ensure that subsequent fecundity is most likely to coincide
with newly available hosts, later in the hosts’ reproductive year.
There has been much debate about the factors that induce arrest. It
appears that host immune effects can induce arrest, at least for some
species of nematodes (Michelet al., 1979; Gibbs, 1986). However, the
growth and arrested development ofHaemonchus contortusappears to be
affected by host factors other than the host immune response (Coadwell
and Ward, 1977). This was concluded from observations of infections in
worm-free, and thus non-immune, sheep in which the worm growth and
arrested development varied throughout the year. This host effect was
hypothesized to be due to factors present within the host which vary
through the year and which are sensed by developing worms, and that
this is the basis on which worms make the developmental decision
between arrested and continued development (Coadwell and Ward,
1977). The presence of adult worms ofOstertagia ostertagi in a host
appears to favour the arrest of developing larvae. This was shown from
experiments in which the removal of adult O. ostertagi from calves
by drug treatment stimulated the development of extant arrested larvae
into adults (Michel, 1971). The environmental temperature experiencedEnvironmental Control of Nematode Life Cycles 123