Researchers at the University of California, Riverside have determined that some female mosquitoes identify and target human hosts for infection using carbon dioxide trails and odors released through our skin.
The findings were built on previous studies that investigated how the mosquito Aedes aegypti travels based on odor signals in its environment. This mosquito is common in tropical regions of the world and is a vector of many diseases, most notably responsible for transmitting yellow fever, which is responsible for 30,000 deaths worldwide per year, and dengue fever, which causes an additional 12.5 million deaths in the same period.
These preliminary studies found that the mosquito is not "guided" by an odor plume, but instead exhibits "upwind flight" (movement into the plume), induced by the presence of the odor. The progress of its flight pattern is regulated by visual feedback from the insect's eyes, a process called odor-mediated optomotor anemotaxis. The main model for determining the flight of such mosquitoes are male moths, which exhibit the identical movement pattern when responding to pheromones emitted by their female counterparts.
Their experiment, described in the October issue of The Journal of Experimental Biology, employed a laminar flow wind tunnel, in which movement of air passes above and below objects in the tunnel and is uninterrupted.
To detect responses to carbon dioxide, increasing concentrations of the gas were released into the tunnel. They found a positive correlation between carbon dioxide concentration and the percentage of Aedes aegypti mosquitoes that were able to locate the source of the carbon dioxide. When the percentage of carbon dioxide in the air was at its experimental maximum of 4 percent, the source finding frequency also topped out.
However, they found changes in behavior depending on the length of exposure to carbon dioxide. Upwind motion was brief when only a whiff of carbon dioxide was released. Sustained upwind motion resulted from several exposure patterns, including fluctuating carbon dioxide levels, turbulent plumes of gas and concentrations mimicking those of an average human. Further, they found that at low concentrations, the mosquitoes tended toward a crosswind motion – that is perpendicular to the flow of carbon dioxide.
To detect the response to human skin exposure, a live human arm was stuck into the tunnel. They found that skin exposure produced a markedly less accurate source finding result in mosquitoes, and it required a much longer exposure time for a measurable comparison with the results of carbon dioxide exposure. Also, unlike in the carbon dioxide exposure experiments, there was no sustained upwind flight when the odor plume was turbulent, only when it was constant.
Further investigation showed that response to body odor was heightened when the odor plume was broad, which is consistent with when the insect is in close proximity to the odor source. This suggests that responses to body odor activate only when the insect is close to a potential human host for the virus it carries.
After combining the two experiments and exposing the insects to both carbon dioxide and human body odor they found that a response to body odor increased between 500 and 2500 percent, if it was exposed to it after being "primed" with exposure to carbon dioxide.
Acknowledging the limitations of their study, the researchers noted in their article that successful odor source location could also depend on visual cues in the environment, as well as the speed and orientation of the wind. They also propose that their methods can be used to investigate the flight patterns of other mosquito vectors, including Culex quinquefasciatus, which transmits the roundworm that causes lymphatic filariasis, and Anopheles gambiae, which is the most efficient vector of malaria in the world.
