Packer (1996) showed that individual A. punctulatus infected with
P. falciparumorP. vivaxsporozoites in Papua New Guinea fed maximally
throughout the night, in comparison with uninfected mosquitoes, which
took increasingly large blood meals as the night wore on. They attributed
this difference to more tenacious blood-feeding behaviour, i.e. infected
mosquitoes fed for longer and several times on the same or separate hosts,
in both instances, putting themselves at greater risk from host defensive
behaviour. In a further separate study, Koellaet al. (1998b) showed that
A. gambiaeinfected withP. falciparumsporozoites engorged more fully
than uninfected mosquitoes and were more likely to bite more than one
host. These studies are somewhat contradicted by that of Bockarieet al.
(1996), who showed that, in Papua New Guinea,A. punctulatusinfected
withP. falciparumsporozoites tended to bite later in the night than
uninfected mosquitoes. Other workers have reported similar changes
in feeding behaviour of naturally infected populations in the field. For
example, Maxwellet al. (1998) showed thatA. gambiaeandAnopheles
funestusinfected withP. falciparumsporozoites in Tanzania would bite
either earlier or later in the evening than uninfected mosquitoes. Robert
and Carnevale (1991) demonstrated similar changes in the behaviour of
A. ambiaeandA. funestusin Burkina Faso and Gillies (1957) in that
ofA. gambiaein Tanzania.
Together, these results suggest that the malaria parasite may manipu-
late the behaviour of the mosquito vector to enhance transmission (Koella,
1999).
Leishmania-infected sandflies, like malaria-infected mosquitoes,
probe more often than uninfected flies, i.e. they may probe the same host
repeatedly or they may probe several hosts. As a consequence, several
hosts can be infected by one fly (Beachet al., 1985). In matureLeishmania
infections, promastigote forms (flagellatedLeishmania) secrete a gel-like
plug (promastigote secretory gel (PSG)), which occludes and distorts the
cardia region of the gut, increasing the volume of the lumen by two to
three times and holding open the stomodeal valve (Stierhofet al., 1999).
Further, it has been shown that transformation from the promastigote
to the metacyclic promastigote form occurs within the PSG plug and
infective metacyclic promastigotes (fast-swimming infective forms) are
regurgitated from a position in front of the PSG plug into the wound
caused by feeding (Rogerset al., 2002). Infected flies experience difficulty
feeding, due to the presence of PSG, and feed more frequently and/or for
longer, thus increasing the opportunity for transmission. In addition, PSG
material is egested during sandfly feeding and is able to enhance meta-
cyclic infectivity and lesion development, whereas little enhancement is
observed in the presence of sandfly saliva (Rogerset al., 2002). It would
appear thatLeishmaniaparasites produce a substance that acts to modu-
late vector-feeding success and thereby enhances their transmission.
Jenniet al. (1980) showed thatG. morsitans morsitansinfected with
Trypanosoma(Trypnozoon)bruceiprobed more frequently and fed more
voraciously than uninfected flies. The increased probing may be caused272 J.G.C. Hamilton and H. Hurd