were recently infected were no more attractive than uninfected mice;
however, after 4 days, infected mice were significantly more attractive.
This increase in attraction corresponded with increasing parasitaemia
(Taylor, 2001), as suggested by Kingsolver (1987), but as yet cannot be
related directly to gametocyte levels.
These experiments exclude the effect of host activity, temperature and
haemostasis mechanisms in the blood-feeding process and suggest that
parasitic infection may enhance odour-mediated host attraction of at least
some parasite–host–vector complexes at a period of infection likely to
enhance parasite transmission. Increasing attractiveness of the host at a
time when infective parasitic stages are present would be adaptively
favourable to the parasite by improving transmission prospects. It is
possible – although this hypothesis has not been tested – that host
attractiveness may even decrease during periods when non-infective
stages of the parasite are present. Clearly, this is an area that warrants
further investigation.
It has been suggested that the fever induced by parasitic infection may
make infected individuals more attractive than uninfected individuals
(Gillett and Connor, 1976; Turellet al., 1984). This would be an important
factor in the epidemiology of disease transmission; however, the role of
heat as an attractant remains to be established. The raised temperature of
malaria-infected mice has no differential attractive effect on mosquitoes
(Day and Edman, 1984b) and the experiments described above would
suggest that heat is not important over even relatively short distances for
sandflies and mosquitoes.Parasitized vector attraction to host odour
No work has been undertaken in this area as far as we are aware, although
a significant amount of work has been done to examine the changes in
landing and probing behaviour of parasitized vectors.Landing and probing
Landing is believed to be controlled by stimuli acting over a short range,
including many of the same stimuli that are responsible for other stages of
the attraction sequence (e.g. odour, visual, heat and moisture cues). It is at
this stage that the decision to alight, or not, on the potential host is made.
This stage is highly regulated by the host and its defensive mechanisms
and by the persistence and density of the vector (Edman, 1989). Probing,
the active movement of the mouth-parts into the surface of the host, the
uptake of blood and cessation of feeding are dependent on the interaction
between the host and the vector. The parasite, whether infecting the host
or the vector and depending on its developmental stage, may manipulate
the process to its own advantage. Schwartz and Koella (2001) postulated
that the malaria parasite should try to manipulate the mosquito’s feedingParasite Manipulation of Vector Behaviour 269