Smell guides mosquitoes to see a human host

Aedes aegypti

It may be largely impossible to avoid getting bitten by a mosquito unless and otherwise one can stay in an environment free of carbon dioxide for extended periods, remain nearly invisible and have a cool body temperature. This is because mosquitoes use smell, visual and thermal cues that are both sequentially triggered and act independently to locate a human host, a study found.

The smell of carbon dioxide exhaled by humans acts as a trigger and influences the attractiveness of the visual objects that are as far away as 15 metres in the case of humans. An earlier study had found that mosquitoes could detect carbon dioxide as far away as 60 metres in a riverine habitat when its concentration was above the background level.

Once close to a human host, the presence or absence of carbon dioxide becomes irrelevant as they are then guided by body warmth, volatiles emitted by the skin and humidity to land at a feeding site.

To find out how and when each of the sensory information was used by mosquitoes to home in on a host, researchers released hungry, mated female Aedes aegypti mosquitoes into a wind tunnel where the different sensory cues were independently controlled.

In a paper published recently in the journal Current Biology , Floris van Breugel, the first author from California Institute of Technology, found that a brief encounter with carbon dioxide was sufficient for the insects to get attracted by visual features. Once attracted to the object, the absence of carbon dioxide had no role in determining the continued exploration of the object thereby making it clear that continued presence of both odour and visual cues was not necessary.

Once the odour-induced visual attraction got the mosquitoes close to the potential host, other cues like warmth, humidity and volatiles emitted by human skin may help decide whether and where to land on the body.

While mosquitoes preferred warm high-contrast objects after being exposed to carbon dioxide, the preference for warm objects was seen even in the absence of the gas. The preference for warm objects was thus independent of the presence of carbon dioxide. These results showed that the cues were triggered both sequentially and independently to locate a potential host.

Another cue that played a role was the presence of humidity. Even in the case of warm objects, the attraction was enhanced when the object had humidity over it than without it.

Hence, one has to overcome many “unfortunate realities” to avoid being bitten by a mosquito. Holding one’s breath indefinitely will not help when surrounded by others and even becoming invisible will be of little use as body warmth will let one down. “The independent and iterative nature of the sensory-motor reflexes renders mosquitoes’ host seeking strategy annoyingly robust,” they write.

Published in The Hindu on July 27