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Thursday, September 12, 2013

Why mosquitoes attack at night

We’ve all made two personal observations about mosquitoes. First, we don’t like them, and for good reason. At best, mosquitoes are a minor irritation and at worst, they give you a deadly disease. For example, in Africa, the mosquito Anopheles gambiae is a major vector for malaria. Second, mosquitoes tend to come out in force as the sun is setting. Thanks to work by Samuel Rund and his colleagues from the University of Notre Dame, we now know why that might be. Mosquito feeding behavior is tied to the creatures’ circadian rhythms.

Circadian rhythms are internal biological patterns that follow a twenty-four hour period. For example, if you tend to get hungry to sleepy at about the same time of day regardless of what’s going on, you can thank your circadian clock. Virtually all living things have them, including plants and fungi. 

For most organisms, these patterns of behavior are linked to the cycle of light and dark created by the sun. In the lab, scientists who work on circadian rhythms prefer to exert their own controls over what is considered ‘day’ or ‘night’. This is much more convenient than having to come in at odd hours to run experiments. Therefore, they train their test subjects to an artificial 24 hour pattern (made up of zeitgebers) that begins and ends at the time of the humans’ choosing. For the study organisms, this is no different from acclimating to a new time zone.

Once female A. gambiae mosquitoes (only females take blood meals) had successfully recovered from jet lag, the researchers pulverized the mosquitoes’ heads and extracted and quantified the proteins made in their antennae. In particular, the scientists were comparing the levels of odorant binding proteins (OBPs, molecules that bind to and transport signature odorant molecules) at different times of ‘day’.

It turns out that these OBPs wax and wane in expression throughout the day. The peak of protein abundance occurred four hours after lights-out, which would correspond to the middle of the night in the real world. However, it wasn’t just the light (or lack thereof) that was causing the spike in protein production. OBPs did not peak earlier in the day when mosquitoes were exposed to four hours of darkness in the middle of their ‘morning’. Thus, the daily rhythm of OBP expression is more deeply ingrained than simply following the position of the sun.

Flying and feeding behavior also peaked at about the same time as the OBPs, as determined by human volunteers sticking their arms into mosquito cages. Aren’t you glad you’re not a graduate student in that lab? 

In summary, it’s not that mosquitoes are avoiding daytime predators (though they are also doing that) but that their prey-detecting senses are most acute at night. This makes bed nets for people in malaria hot spots all the more critical.

Rund SS, Bonar NA, Champion MM, Ghazi JP, Houk CM, Leming MT, Syed Z, & Duffield GE (2013). Daily rhythms in antennal protein and olfactory sensitivity in the malaria mosquito Anopheles gambiae. Scientific reports, 3 PMID: 23986098.