Since the beginning of the century it has been recognized that malaria mosquitoes use host odors for host-seeking. It is vital for any blood-feeding insect to discriminate volatiles emanating from a living host from residues on materials in absence of the host. Carbon dioxide present in expired air is considered to be reliable ‘life sign’ of a living host and is a proven kairomone for many blood-feeding insects, including mosquitoes. However, in addition to expired air, olfactory stimuli may also originate from the skin. Studies on the anthropophilic Afrotropical malaria mosquito, Anopheles gambiae, showed that volatiles from the human skin, specifically sweat, play an important role in host-finding. Behavioral responses of An. gambiae to freshly collected sweat samples were variable; the females were attracted to some fresh sweat samples but not to others. However, responses to sweat samples that had been incubated for two days at body temperature were stable; mosquitoes were attracted to every incubated sweat sample. The pH of the sweat samples changed from acidic (pH 5-6) when fresh to alkaline (pH 8-9) after incubation. During incubation microbes in the sweat proliferated exponentially. The pH shift was caused by the production of ammonia from urea and other nitrogen-rich components by the action of cutaneous microbes. In bioassays, An. gambiae was highly attracted to a wide range of ammonia concentrations. This is the first report of An. gambiae being attracted to a single compound other than carbon dioxide. As ammonia is continuously produced on the skin of a living host but evaporates quickly from non-living material, it may function as a reliable ‘life sign’, in addition to carbon dioxide, for blood-feeding insects.

Index terms: Anopheles gambiae, sweat, body odor

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