Malaria mosquitoes accurately find their way to smelly feet

African malaria mosquitoes, Anopheles gambiae, use their olfactory organs, two antennae, two mouthparts (maxillary palps) and the proboscis, to search for their hosts to obtain a bloodmeal. From a distance of several tens of meters mosquitoes detect CO₂ which forms part of exhaled air by humans. However, a malaria mosquito does not follow the CO₂ trail to its source, the mouth, but at a certain point close to the source is diverted toward the feet, which is the preferred biting place for this mosquito species.

PhD candidate Remco Suer from the chair group Entomology of Wageningen University has uncovered a mechanism for this behavior. Previous research within this project, funded by the Bill and Melinda Gates foundation, showed that bacteria living on the human foot produce various odors and identified ten bacterial foot odors that, when offered as a blend, were attractive to malaria mosquitoes. Remco Suer now shows that nine out of these ten foot odors are detected by olfactory neurons present underneath hair-like structures on the mouthparts of the malaria mosquito. More importantly, he discovered that 5 of the 10 microbial odors are capable of blocking the response to CO₂. By blocking the CO₂ signal the mosquito stops orienting towards CO₂ and diverts its attention to close range foot odors.

The researcher added additional CO₂ to the experiments to simulate exhaled air. A short stimulation of 1 second with the highest concentration of the five foot odors separately resulted in complete inhibition of the CO₂ response for multiple seconds.

From dozens of olfactory neurons, only one type of olfactory neuron is capable detecting CO₂. This olfactory neuron is co-compartmentalized together with two other olfactory neurons underneath the capitate peg sensilla, hair-like structures, present on the mouthparts of the mosquito. By registering the responses of these olfactory neurons, Suer was able to determine which human odors the female malaria mosquito detects. From the ten microbial odors previously discovered nine elicited responses from all three olfactory receptors on the mouthparts and 5 of them inhibited the CO₂ response.

By inhibiting the perception of CO₂, it is possible to disrupt the host seeking behavior of the malaria mosquito. Because these bacterial foot odors block the CO₂ response and at the same time activate other olfactory neurons, it is very plausible that these odors cause the switch from the long distance CO₂ signal to the preferred biting place, the feet. Behavioral experiments show that at short range these odors block the CO₂ effect and even enhance the attractiveness of an attractive basic odor blend. This implies that these CO₂ inhibitors cannot be used as repellents and even divert the orientation of the mosquito to short-range human odors.

Odors that block the CO₂ receptor but activate other olfactory neurons, thereby diverting the orientation of the malaria mosquito to other odor sources, have potential applications in odor trapping systems as a barrier. By placing a barrier releasing these CO₂ inhibitors, it might be possible to lure malaria mosquitoes towards odor traps containing a mixture of other attractive human odors.