Incubated human sweat but not fresh sweat attracts the malaria mosquito Anopheles gambiae sensu stricto

A dual-port olfactometer was used to quantify behavioral responses of the malaria mosquito Anopheles gambiae sensu stricto to volatiles emitted by sweat samples collected from three human volunteers. Significant attraction (P < 0.05) was found to sweat of two volunteers after two days and to one of the volunteers' sweat after one day of incubation at 37 degrees C. There was no attraction to any of the fresh sweat samples. The pH of the sweat that became attractive after one or two days had changed from acidic (5.5-5.75) to alkaline (8.25-8.75), while the pH of the sweat of the third volunteer (pH 7) did not change during incubation. Microorganisms, present in all fresh sweat samples, showed distinct growth during incubation. The concentration of lactic acid decreased an average of 23% in two days of incubation. The role of the pH, skin microflora, and lactic acid in the differential attractiveness of human sweat samples is discussed.