Longevity
A number of studies have been carried out to investigate the effect of
parasitic infection on the longevity of the vector. Decreased longevity of
infected vectors would reduce the proportion of infected vectors and thus
reduce parasite transmission. The results suggest that, when the infection
is atypical, the infection is pathological and the lifespan of the parasite
is significantly reduced. On the other hand, when the parasite–vector
association is natural, there is no apparent effect on longevity. The effect
on longevity and daily survival rates for wild-caughtA. gambiaeand
A. funestusnaturally infected withP. falciparumwas investigated by
Chege and Beier (1990), who showed thatPlasmodiuminfections did
not reduce vector survival. Laboratory studies on the same vector and
parasites either show similar results to the field results (e.g. Freier and
Friedman, 1987) or the parasite reduces the longevity of the vector (e.g.
Kleinet al., 1986). These conflicting results may be due to the intensity of
the infection in wild mosquitoes, which may not be sufficiently intense to
decrease survival (Chege and Beier, 1990). Bates (1949) reported that there
was no evidence that, whilePlasmodiumor viral infection did not have
any ‘unfavourable’ effect on mosquito longevity, nematodes, particularly
filariae and mermithids, cause very heavy mortality.Conclusions
During the course of the preparation of this chapter, it has become
apparent that there is an acute lack of recent studies on the effect of
parasites on vector behaviour. Of those that exist, malaria/mosquito
interactions dominate the field. This may result from a commonly held
misconception that parasites induce few changes in their vectors. Both
old and very recent studies refute this concept. Studies may be technically
difficult and time-consuming and definitely require verification in field
conditions, but the rewards that they would bring in terms of a greater
understanding of vector transmission and vector control will make them
worth the effort.References
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