Allomonal effect of breath contributes to differential attractiveness of humans to the African malaria vector Anopheles gambiae

Abstract

BACKGROUND: Removal of exhaled air from total body emanations or artificially standardising carbon dioxide (CO2) outputs has previously been shown to eliminate differential attractiveness of humans to certain blackfly (Simuliidae) and mosquito (Culicidae) species. Whether or not breath contributes to between-person differences in relative attractiveness to the highly anthropophilic malaria vector Anopheles gambiae sensu stricto remains unknown and was the focus of the present study. METHODS: The contribution to and possible interaction of breath (BR) and body odours (BO) in the attraction of An. gambiae s.s. to humans was investigated by conducting dual choice tests using a recently developed olfactometer. Either one or two human subjects were used as bait. The single person experiments compared the attractiveness of a person's BR versus that person's BO or a control (empty tent with no odour). His BO and total emanations (TE = BR+BO) were also compared with a control. The two-person experiments compared the relative attractiveness of their TE, BO or BR, and the TE of each person against the BO of the other. RESULTS: Experiments with one human subject (P1) as bait found that his BO and TE collected more mosquitoes than the control (P = 0.005 and P < 0.001, respectively), as did his BO and the control versus his BR (P < 0.001 and P = 0.034, respectively). The TE of P1 attracted more mosquitoes than that of another person designated P8 (P < 0.021), whereas the BR of P8 attracted more mosquitoes than the BR of P1 (P = 0.001). The attractiveness of the BO of P1 versus the BO of P8 did not differ (P = 0.346). The BO from either individual was consistently more attractive than the TE from the other (P < 0.001). CONCLUSIONS: We demonstrated for the first time that human breath, although known to contain semiochemicals that elicit behavioural and/or electrophysiological responses (CO2, ammonia, fatty acids) in An. gambiae also contains one or more constituents with allomonal (~repellent) properties, which inhibit attraction and may serve as an important contributor to between-person differences in the relative attractiveness of humans to this important malaria vector.

Figure 1

Apparatus used to study the response of An. gambiae s.s. mosquitoes to human breath, body odours and a combination thereof. Breath was separated from body odours using a one-way breathing valve and diverted to the exhaust of the same (1) or other tent (2), or vented out through the main air exhaust (3). Alternatively, the test person sat outside the tent and his breath was diverted to one of the tent exhausts (4). a: test person, b: tent, c,d: tent exhaust, e: mosquito release cup, f: choice chamber, g: trapping chamber, h: main air exhaust. Dimensions shown are in cm. Broken arrows depict the direction of movement of air currents. Further descriptive details of the experimental set up see [16].

Figure 2

Relative attractiveness (RA) of total emanations (TE) of test person P₁ versus a control (Expt. 1), his body odour (BO) versus his breath (BR) (Expt. 2), his body odour versus a control (Expt. 3) and his breath versus a control (Expt. 4). N: number of replicates, n: total number of mosquitoes collected by both treatments in each experiment. P: statistical significance level of (i) differences between the catches of treatment A and treatment B (RA = 0.5) or (ii) the change in the relative attractiveness of treatment A between different experiments (RA₁ = RA₂).

Figure 3

Relative attractiveness (RA) of person P₁ in choice experiments evaluating mosquito responses to total emanations (TE) of P₁versus total emanations of person P₈ (Expt. 1), breath (BR) of P₁ versus breath of P₈ (Expt. 2), body odour (BO) of P₁ versus body odour of P₈ (Expt. 3), body odour of P₁ versus total emanations of P₈ (Expt. 4) and total emanations of P₁ versus body odour of P₈ (Expt. 5). N: number of replicates, n: total number of mosquitoes collected from both treatments in each experiment. P: statistical significance level of (i) differences between the catches of P₁ and P₈ (RA = 0.5) or (ii) the change in the relative attractiveness of P₁ between different experiments (RA₁ = RA₂).