Synthesis and future challenges: the response of mosquitoes to host odours

There is ample evidence that host seeking in mosquitoes is mediated by semiochemicals emanating from the host. Olfactory cues (kairomones) are detected through an intricate pathway, beginning with sensilla located on the antennae (odour) and palpi (CO2). Age and physiological state of the mosquito determine whether detection of kairomones results in a behavioural response. Only a few kairomones have been described so far. CO2 is a kairomone for most mosquito species and signifies the presence of a potential host because of its occurrence in the volatile emissions of all vertebrates. Other chemicals are likely to play a role in mosquito-host interaction as well, notably L-lactic acid, fatty acids and 1-octen-3-ol. Species-specific host preference is thought to be olfactory based and related to the presence of specialized sensilla or perception at the central olfactory pathway. Host specificity is genetically determined, as demonstrated by inherited differences within members of the Anopheles gambiae complex. In view of the available evidence, challenging research areas are: (1) the function of olfactory receptors and the level of specialization; (2) the identification of general and host-specific kairomones; (3) demonstration of behavioural responses to laboratory-identified olfactory cues in the field; (4) studying whether mosquito host-locating behaviour can be manipulated by kairomones; (5) the genetics of the regulation of olfactory behaviour; and (6) determining whether there is an olfactory basis for the evolution of mosquito-host interactions.