Attraction of a vector to a host depends on two separate factors: one is
the inherent attractiveness of the host and the other is the extent to which
the vector responds. Therefore a parasite may potentially alter either
aspect of the attraction equation, depending on its stage of development
and location (in host or vector). For example, a parasite infecting a vector
might be able to alter the sensitivity of the vector’s host-odour receptors,
making the vector more or less responsive to host factors. Again, this
could be dependent on the stage of development of the parasite within the
vector. In the alternative scenario, a parasite infecting the host could make
the host more attractive by producing a greater quantity of attractive
chemicals, inducing the production of new attractive chemicals or possi-
bly reducing the repellence of an individual, thus indirectly modifying
the vector’s behaviour and making the host more likely to be fed upon by
the vector. In any scenario, the parasite would presumably improve its
chances of transmission. There is very little direct evidence for any of
these proposed manipulations and yet their significance in the epidemiol-
ogy of disease transmission must be underestimated.Parasitized-host attractiveness
Infection of a host may modify its odour profile, a fact that has been recog-
nized by physicians for hundreds of years (Penn and Potts, 1998). These
changes have been associated with exhaled breath (Davieset al., 1997;
Smithet al., 1998) or may be associated with changes in epidermal micro-
bial flora (Brakset al., 1999). Little evidence exists for the role of parasites
in altering host odour. There is further confusion as to whether any
alteration is due to direct physiological change or an alternative scenario
where the odour is altered as a secondary consequence of infection,
e.g. the sick animal may groom less, which leads to increased bacterial
activity, or alternatively pyrexia increases the volatilization of host odour
compounds. However, if the parasite were altering the attractiveness
of the host in a manner that would increase parasite transmission, we
might expect to see increased attraction at periods associated with the
maturation of the infective stages of the parasite.
Laboratory-based experiments describe the preferential selection of
rodents infected with malaria above non-infected rodents by vectors
seeking a blood meal (Day and Edman, 1983; Dayet al., 1983). In
these experiments, rats were infected with eitherPlasmodium berghei,
Plasmodium chabaudiorP. yoeliiand experiments were conducted at
periods of peak gametocytaemia (gametocytes are the malarial stage that
can infect the vector). However, studies by Burkotet al. (1989) showed
that at a field site in Madang Province, Papua New Guinea,A. punctulatus
mosquitoes did not preferentially select malaria- or filaria-infected
humans. This field site is hyperendemic for malaria, withPlasmodium
falciparum,Plasmodium malariae,Plasmodium vivaxandPlasmodium
ovaleall present. In none of the households examined were infected
individuals preferentially selected byAnopheles punctulatus. In this
natural situation, the peak gametocytaemia rarely exceeded 0.4% forParasite Manipulation of Vector Behaviour 267