Malaria and Leishmania infections represent the best studied examples of parasite determinants of vector transmission. Altered feeding behaviour by infected vectors can increase pathogen transmission, such as an increase in host contact (Molyneux & Jefferies, 1986; Moore, 1993). Mosquitoes infected by malaria parasites exhibit an increase in probing behaviour during blood feeding. Only when the sporozooites have infected the salivary glands are these changes apparent; therefore, the mosquito can take several blood meals before this affected behaviour is apparent. One cause for the increase in probing behaviour may be altered levels of apyrases, ADP-degrading enzymes that inhibit platelet recruitment, counteract host haemostasis, and promote longer blood feeding. Apyrase and sporozooites are both found in the distal regions of the mosquito salivary gland lobes (Sterling et al., 1973). P. gallinaceum-infected Ae. aegypti has a 25 % reduction in apyrase activity and a threefold increase in the mean time to locate a blood vessel before feeding (Ribeiro et al., 1985; Rossignol et al., 1984). Koella et al. (1998) determined that 22 % of P. falciparum-infected An. gambiae mosquitoes collected from field conditions had bitten more than one person in a night, compared with 10 % of uninfected females, suggesting that this infection increases multiple host contacts and enhances transmission in nature. The mechanism for the decrease in apyrase levels in infected salivary glands is unknown.

The observation that Leishmania parasite transmission is enhanced by the feeding behaviour of infected sandflies was made very early (Shortt & Swaminath, 1927). This phenomenon was termed the 'blocked fly hypothesis' to describe the prevention of blood flow into the midgut (Jefferies et al., 1986). Difficulties in feeding lead to an increase in probing behaviour and thereby increased pathogen transmission via increased salivation. The occlusion of the stomodeal valve is caused by a filamentous, gel-like matrix full of infectious metacyclic promastigotes (Rogers et al., 2002). Filamentous promastigote proteophosphoglycan (fPPG), a parasite secretion that forms a major component of the gel-like plug, has been found in at least 10 Leishmania-sandfly associations. The fPPG is also thought to prevent unattached promastigotes from being swept into the midgut with the blood meal, enhancing parasite fitness by promoting transmission (Stierhof et al., 1999).

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