Malaria mosquitoes utilize CO 2 from exhaled air to localize humans from afar. In the vicinity of their preferred host, they alter their course towards the human feet. Researcher Remco Suer discovered how female malaria mosquitoes use foot odors in the last meters to guide them to their favoured biting place. Suer, who is defending his doctoral thesis May 9 at Wageningen University, part of Wageningen UR, sees possibilities to disrupt the host seeking behaviour of the malaria mosquito.

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 CO 2 which forms part of exhaled air by humans. However, a malaria mosquito does not follow the CO 2 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 CO 2 . By blocking the CO 2 signal the mosquito stops orienting towards CO 2 and diverts its attention to close range foot odors.

The researcher added additional CO 2 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 CO 2 response for multiple seconds.

From dozens of olfactory neurons, only one type of olfactory neuron is capable detecting CO 2 . 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 CO 2 response.

By inhibiting the perception of CO 2 , it is possible to disrupt the host seeking behavior of the malaria mosquito. Because these bacterial foot odors block the CO 2 response and at the same time activate other olfactory neurons, it is very plausible that these odors cause the switch from the long distance CO 2 signal to the preferred biting place, the feet. Behavioral experiments show that at short range these odors block the CO 2 effect and even enhance the attractiveness of an attractive basic odor blend. This implies that these CO 2 inhibitors cannot be used as repellents and even divert the orientation of the mosquito to short-range human odors.