Identification of olfactory stimulants for Anopheles gambiae from human sweat samples

The behavioral and electroantennogram (EAG) responses of female Anopheles gambiae mosquitoes to pooled samples of freshly collected human sweat and human sweat incubated for 42-52 hr were tested. No behavioral or EAG response was obtained to pooled fresh sweat samples, whereas incubated pooled sweat samples produced a behavioral as well as an EAG response. GC-MS analysis of the headspace composition of the fresh sweat revealed ethanol (15.1% of the total amount of volatiles trapped), acetic acid (10.9%), and 3-hydroxy-2-butanone (9.5%) as the most abundant compounds; a wide range of ethyl esters was present as well. None of the ethyl esters was detected in the headspace collections from incubated sweat, while the relative amounts of ethanol, acetic acid, and 3-hydroxy-2-butanone were strongly reduced. In the latter collections, indole (27.9%), 1-dodecanol (22.4%), and 3-methyl-1-butanol (10%) were present in high amounts, while they were absent or present in only minor amounts in the headspace collections from fresh sweat. Geranyl acetone (6%) and 6-methyl-5-hepten-2-one (1.9%) were relatively abundant in both the fresh and incubated headspace samples. EAG responses were observed in response to indole, 6-methyl-5-hepten-2-one, and geranyl acetone.