Sniff, Sniff, Bite (Mosquitoes are Hunting You)

Mosquitoes, the bane of summer, are persistent pests that can plague our outdoor activities. Not only do they produce itchy bites, but they can also carry a variety of disease-causing viruses including Zika virus, West Nile virus, dengue virus, and St. Louis encephalitis virus. There are 176 species of mosquito found in North America, with the most common types in the United States being the Aedes, Culex, and Anopheles mosquitoes. For all three types, it is only the female mosquito that bites as the female needs a blood meal to produce its eggs; males simply drink nectar and don’t bother anyone. While most of us don’t distinguish between these three common mosquito types, there are actually significant differences in their feeding behavior. Culex mosquitoes feed at night and prefer birds, but will bite humans if hungry. Anopheles mosquitoes also bite at night and are attracted to mammals, including humans. In contrast, humans are the preferred hosts for Aedes mosquitoes, and this type feeds during the day. The peak feeding times are early morning and dusk, times when humans like to be outdoors in the summer, making us often easy targets for these tiny marauders.

It is well known that mosquitoes are attracted to CO₂ (which is exhaled during breathing) and also body heat. However, animals produce CO₂ and body heat so why do Aedes mosquitoes prefer humans and how do they identify us? A paper in the journal Nature reveals at least part of the answer. Working with Aedes aegypti, a species found commonly in many parts of the world, researchers examined the mosquito brains in response to odor-producing volatile chemicals from rats, guinea pigs, quails, sheep, dogs, and humans. Using CRISPR technology, they inserted a gene for a fluorescent protein into the mosquito brains. If the brain reacted to stimuli the fluorescence would increase and could be observed and measured by the investigators. One area of the brain specifically reacted to human scents but not animal odors. To refine this further, they examined individual chemicals in human scent and found two compounds that elicit the human-specific response in the mosquitoes: decanal and undecanal. Decanal and undecanal are found in sebum which is an oily substance produced by skin glands. Both compounds have a sweet and citrusy smell and are prevalent in human scents but not in animal scents. In separate tests, Aedes aegypti mosquitoes were strongly attracted to a blend of decanal and undecanal and would actively fly towards a source of these compounds. These compounds likely account for the specificity of Aedes for humans and help these insects locate us against a background of other smells. Interestingly, people who produced near the human average amount of decanal and undecanal were the favorite target for these mosquitoes. Persons with low or high levels of these chemicals were less favored by the mosquitoes, so if you are one of these people it may explain why you are less often bitten. As someone who seems to be bitten frequently, I just hope that scientists can figure out a way to use this information to create an easier way to avoid mosquitoes.