by the L3 in the external environment also affects the probability of larvae
arresting their development within a host, with lower temperatures
favouring arrested development within the host (Gibbs, 1986). However,
this effect may be limited to parasite strains obtained from temperate
regions (Gibbs, 1986). The rabbit stomach worm,Obeliscoides cuniculi,
was selected for propensity to arrest development within a host following
cold treatment of larvae prior to infection. This selection regime was
effective but appeared to require a continued selection pressure to
maintain high levels of arrested development (Watkins and Fernando,
1984).
Examination of propensity for arrested development in isolates from
different locations revealed further controls of arrested development. A
comparison of different isolates ofO. ostertagifor their arrest phenotype
compared isolates from cattle managed for beef production and cattle
managed for dairy production at the same location. This found that greater
arrest occurred in the beef compared with the dairy cattle (Smeal and
Donald, 1982). This was thought to be due to the different timing and per-
sistence of beef and dairy cattle on pasture, with the effect that there was a
selective advantage for arrested development on beef pasture, but no
selective advantage for arrested development on dairy pasture (Smeal and
Donald, 1982). Comparison of isolates ofO. ostertagifrom the northern
(Ohio) and southern (Louisiana) USA showed that the parasites varied in
the timing and extent of their arrested development when tested in either
location. This indicated that these differences in arrested development
were genetic (Franket al.,1986). In these two cases the variation is likely
to be the result of differing local selection pressures for arrested-develop-
ment phenotypes.
Arrested development has also been observed with human hookworm
infections in West Bengal, India (Schadet al., 1973). In this epidemio-
logical setting, infections acquired by a host in one rainy season delay
development until the following rainy season. This ensures that infective
stages are liberated into the environment at the onset of the monsoon
season, which gives optimal conditions for the development and survival
of infective larvae before the end of that monsoon season (Hominicket al.,
1987). The cues controlling this are not known, but are likely to be similar
to those operating in non-human species.
The overall picture that therefore emerges is that species which
undergo facultative arrested development use a number of cues from their
environment both within and outside their hosts to make this develop-
mental decision. The nature of the cues probably varies among different
species of nematode and much work remains to be done on specifying
them and defining their effects. It is clear that the host immune response
is an important cue, but, as forS. ratti, the detailed nature of the cues or
their sensation is not clear. Furthermore, it is necessary to understand
how different cues interact in their effects. For example, and analogously
toS. ratti, how do developing larvae use both the host immune response
acting against larvae within the host and the temperature experienced by124 M.E. Viney