Olfaction in vector-host interactions

This chapter reviews the current understanding of odour-mediated host-seeking behaviour in mosquitoes based on laboratory (indoor) studies. Most recent studies have focused on Aedes aegypti L., Anopheles gambiae sensu stricto Giles and Culex quinquefasciatus Say, as these species are all strongly associated with human biting and disease transmission. Where relevant, reference to other mosquito species is provided. Host seeking in mosquitoes is mediated primarily by chemical cues, and therefore most studies focus on the response to host odours and identification of specific compounds or odour blends. Laboratory tools for the study of odour-mediated behaviour include Y-tube and dual-choice (or dual-port) olfactometers, wind tunnels and roomsize arenas in which mosquitoes can fly freely. Mosquitoes are observed individually or in groups, usually at a specified age or range of ages, at a pre-determined time of the day when they are considered to be naturally responsive to host cues. Aedes aegypti and An. gambiae respond strongly to natural human skin emanations, whereas Cx. quinquefasciatus shows variation in this behaviour dependent on its geographic origin. Carbon dioxide acts as a principal stimulus for each of these mosquito species although with a variable species-specific effect. Several hostderived compounds have been identified that cause behavioural responses in these mosquito species, including ammonia, L(+)-lactic acid and aliphatic carboxylic acids. In addition, acetone and dimethyl disulfide enhance the attraction of blends of these compounds to Ae. aegypti. Some other ketones, such as 6-methyl-5-hepten-2-one and geranyl acetone disrupt the host-seeking process of Ae. aegypti and An. gambiae. Several aldehydes, such as octanal and nonanal, are attractive to Cx. quinquefasciatus, and reduce upwind flight and the total number of landings in Ae. aegypti. Based on these data odour blends have been identified that are attractive to these mosquito species, albeit less so than natural human odour, indicating that additional, hitherto unidentified, semiochemicals are involved in host-seeking behaviour that are likely to improve the efficacy of blends. Behavioural research combined with studies on molecular olfaction is ongoing to discover compounds that affect the host-seeking process, and further research under semi-field and field conditions is required to explore the effectiveness of putative attractants and repellents in natural circumstances.