Olfactory responses of host-seeking Anopheles gambiae s.s. Giles (Diptera: Culicidae).

Anopheles gambiae s.s. Giles, a highly anthropophilic mosquito species, is Africa's most important malaria vector (White, 1974). It has been shown that this species is attracted to 4.5% carbon dioxide (Knols et al., 1994a), a dose thought to be equivalent to what is emitted by humans during breathing. Also, evidence exists for an activating and attracting effect of carbon dioxide in the field (Gillies, 1980; Snow, 1970). Carbon dioxide, however, is not a human-specific kairomone and the attraction of An. gambiae to this chemical alone cannot explain its anthropophilic behaviour. We therefore decided to investigate whether chemicals present in exhaled human breath, other than CO2, might play a role in the attraction of this mosquito species. Breath has been found attractive for other Anopheles spp. (Mer et al., 1947; Laarman, 1955; Knols et al., 1994b; De Jong and Knols, 1995) and for Aedes aegypti L. (Khan et al., 1972). Breath samples from three resting male Caucasian volunteers (aged 27, 38 and 50 years) were collected in Tedlar ® gas bags and analyzed for their carbon dioxide concentrations by using an infrared COz gas analyser. A similar bag was then filled with cleaned, moistened air and adjusted to a similar CO2 concentration as measured in the breath sample, by adding COz from a pressurized gas cylinder. 10 ml of distilled water were then added to both bags which were then stored for 24 h at 25°C in order to obtain equal, near-saturated, moisture levels. Both odours were then tested against each other by pumping them at 230 ml min -I into traps fitted in a windtunnel olfactometer (for a description of the apparatus and experimental procedures see Knols et al., 1994a). The results (Table 1A) show that breath-related chemicals do not enhance the attractiveness of CO2 alone. When breath was tested against clean air (Table 1B) we found a slightly significant attraction for only one of the subjects (PH). It seems therefore that the doses of carbon dioxide present in