L3s in the environment to make the developmental decision between
arresting and continuing their development?The Sensory Biology of Parasitic Nematodes
It is clear that many nematode species, includingStrongyloidesspp.,
sense aspects of their environment both within and outside the host
and that this information is used in making developmental decisions.
However, it is also clear that we know relatively little about the details of
the cues used by these species. InC. elegans, some of the neuronal and
genetic bases of temperature and chemosensation are beginning to be
understood (Bargmann and Mori, 1997) and this information is likely to
readily apply to parasitic nematodes. However, our lack of knowledge is
especially apparent when considering how parasitic nematodes sense an
immune response that is acting against them. It often seems to be assumed
that nematodes use the generally ‘poor conditions’ of their within-host
environment as their means of sensing an immune response. This may be
so. However, with the perspective of studies on the sensory biology of
C. elegans, it seems more probable that there are very specific cues used
by nematodes in sensing all aspects of their environmentper seand
therefore, by extension, that parasitic nematodes use very specific cues to
assess the host immune response acting against them, or at least its
consequences. Indeed, given the importance of the immune response in a
parasitic nematode’s environment, it seems unreasonable that they would
do otherwise. Furthermore, it seems likely that in natural infections in
wild hosts there will potentially be significant variation between hosts in
the quality and quantity of the immune response that a host will mount
against a parasitic nematode; i.e. there will be significant variation in the
immune environment in which a parasitic nematode may find itself. For
this reason, it seems more reasonable still that parasitic nematodes will
sense or otherwise assess the host immune response acting against them,
and use this information in developmental decisions.Summary
S. rattiresponds in a complex and sophisticated way to its environment
within and outside its host. Of particular note is its developmental
response to host anti-S. ratti immune responses. Other nematodes
also have specific responses to host immunity. In trying to understand
these developmental responses, it is clear that we need to determine the
specific cues used by nematodes and to determine how parasites sense
these cues. Here I have suggested some possibilities for how nematodes
may do this, largely by analogy with environmental sensation by other
nematodes, such asC. elegans. It is clear that understanding this area of
parasitic nematode biology is a research challenge for the future.Environmental Control of Nematode Life Cycles 125