In considering howS. rattisenses its immune environment, it is worth
noting the details of the environmental control of theC. eleganslife cycle.
Here the arrested third-stage dauer larvae are formed at times of environ-
mental ‘stress’.C. elegansintegrates three specific cues in its decision
between dauer and non-dauer development. These are the concentration
of available food (measured as the concentration of neutral carbohy-
drate-like substances produced byEscherichia coli), the concentration
of conspecific worms (measured as the concentration of a pheromone
produced constitutively by worms) and temperature (Riddle and Albert,
1997). Intuitively, the integration of food availability and conspecific den-
sity is a good measure of the quality of a worm’s environment. However,
this also shows thatC. elegansdetermines environmental stress from the
measurement of specific external factors. Furthermore, the use of these
specific cues may be a better and more reliable indicator of the environ-
ment that the progeny of, in this case,C. eleganshermaphrodites will have
to survive in than other measures of ‘stress’. Thus, for example, internal
measurement of quality or stress (such as hermaphrodite food stores) may
be related to environmental conditions, though these conditions: (i) are
also subject to other factors, independent of environmental conditionsper
se; and (ii) are likely to reflect past (though they may possibly reflect
recent) conditions. This would therefore argue by analogy thatS. rattimay
not use its internal physiology as a measure of the anti-S. rattiimmune
response.Other nematodes
Other nematodes have specific responses to the host immune response.
Nippostrongylus brasiliensisreacts to immune pressure by secreting large
quantities of acetylcholinesterase (Selkirket al., 2001). The reasons for
this are not clear, though it has been hypothesized to play a role in
enhancing nematode persistence by various means, including regulation
of lymphoid cell functions and modification of wider host physiological
processes that affect parasite persistence. This behaviour seems to be
regulated by the host immune response, rather than by presence within
a host, since the transfer of worms to immunologically naïve animals
results in reduced acetylcholinesterase production (Selkirket al., 2001).
However, by analogy with the argument above, it is also conceivable
that some other aspect of host physiology that correlates with anti-
N. brasiliensisimmunity or some internal measure of parasite physiology
that is an accurate measure of the detrimental effect of the immune
response could also be used as cues for acetylcholinesterase production.
Many nematode species modulate their host’s immune response as a
means of enhancing their own survival (Maizelset al., 1993). This can be
seen, for example, inHeligmosomoides polygyrusinfections. Immuni-
zation of mice with deadH. polygyruslarvae generates a high level of
resistance to challenge infections. However, mice inoculated with both122 M.E. Viney