True vector manipulation will occur via an extension of the pheno-
type of the parasite (Dawkins, 1982, 1990) such that manipulator
molecules are produced that directly affect vector behaviour patterns
(Hurd, 1998). Manipulative effort is costly for the parasite; thus the
degree of manipulation is likely to be constrained and manipulation will
be optimized, not maximized (Poulin, 1994). A survey of the current
literature on vector–parasite interactions suggests that we appear to be a
very long way from identifying manipulator molecules in any vector-
transmitted parasite. Thus, at the present time, the issue of pathology
versus adaptive manipulation is difficult to assess and will remain so
until substantive studies of several associations provide evidence with
which to weigh the alternative hypotheses.
Blood feeding provides the point of contact between the vector and
the next host, but this is not the only factor that is a candidate for adaptive
manipulation. The activity and longevity of the vector will also affect
chances of parasite success. Vectors with reduced lifespan may have
fewer encounters with their hosts and thus fewer chances for trans-
mission. This is particularly so if the parasite has a long developmental or
extrinsic latent period in the host prior to becoming a patent infection, as
do malaria parasites and filarial worms. If vector lifespan is compromised
by infection during this developmental phase, there will be no trans-
mission. It is difficult to generalize concerning the effect of parasites on
vector lifespan, because conclusions resulting from survivorship studies
are contradictory, even when the same parasite–vector association is
being assessed (Lineset al., 1991; Lyimo and Koella, 1992). In addition,
studies need to be conducted in the field, where stressors in addition to
infection operate and parasites are associated with their natural vectors.
One of the major problems that we face here is the difficulty in deter-
mining the age of insect vectors. Methods for assessing age or, more
importantly, parous status (number of blood meals taken) that are simpler,
quicker and more reliable than the present age-grading techniques
(Detinova, 1962; Sokolova, 1994) need to be devised.
Parasites that develop and reproduce within the vector inevitably
utilize host resources. If the vector does not replenish these resources
more often than an uninfected organism, activity and/or longevity are
likely to be compromised. In addition, many parasites evoke a defence
response in their vector that, even if it does not eliminate the parasite, will
be costly to the vector in metabolic terms (Ferdiget al., 1993; Richman
et al., 1997; Luckhartet al., 1998). There is growing evidence to support
the view that vector resource allocation may be altered by infection such
that the balance between reproduction and growth and maintenance
is changed, due to a curtailment of reproductive effort (Hurd, 2001).
Evolutionary theory suggests that delayed or depressed reproduction will
result in increased lifespan (Price, 1980); thus, if fecundity is reduced,
activity levels and longevity may remain unchanged by the demands
imposed by the infection, and fecundity reduction may be a strategy that
minimizes the effects of infection (Hurd, 2001). Although parasites from262 J.G.C. Hamilton and H. Hurd