Research bites

by Seema Jolly on March 23, 2012

Like many of my fellow Michiganders, due to the unseasonably warm weather*, I got to dust off my sandals and let my feet peak into the life of sunshine and warmth . . . and mosquito bites!  Ever since I was little, I was prone to getting bit (my sibling used to tell me it was because I smelled, my parents said it was because I had “sweet blood.”  Either way, I was always putting on aloe or dotting myself with calamine lotion to soothe those bites).  Thankfully, due to our temperate climate, I don’t have to be concerned about malaria, or Dengue Fever, (although maybe I should pay attention to West Nile) but as I’ve been holding back on itching my bites, mosquitoes have certainly been on my mind.

When I think about why a mosquito decides to bite me (or rather, suck my blood) over the next person (maybe I shouldn’t take it so personally but I can’t help it), there are a number of factors that come into play.  Heat, moisture, odors (fragrant perfumes, lotions), dark colored clothing, and carbon dioxide can all attract a female mosquito (the males don’t bite) to a human host.  But what are some of the factors that impact where a mosquito decides to bite you?

I stumbled upon this not-so-recent study conducted in 1998 that essentially took 5 white adult male volunteers and exposed them to different types of mosquitoes to see if the type of mosquito and whether the person was sitting or laying down impacted where the mosquito would bite.  The participants were not allowed to bathe for 9 hours prior to the study, only wore underwear, and sat underneath a mosquito bed net. Three types of mosquitoes were used in the study.  Mosquitoes of one type were released through a small slit in the bed net, and the participant had to sit or lay in a particular position for 5 minutes while being bit by the mosquitoes (this sounds awful!).  After 5 minutes of attack, researchers counted the number of mosquito bites and location on the body.

All three mosquito types bit the feet and legs more often than other parts of the body.  The An. Gambiae s.s. has a preference for biting feet, so one trial included washing the subject’s foot with an unscented soap prior to the test period.  Even after washing, this type of mosquito still preferred the foot, which may indicate that it’s not smelly feet that they are attracted to (interestingly, this particular finding is counter to what previous studies found).

When participants were lying on the ground with their legs straight up in the air, the mosquitoes bit the parts of the body that were closest to the ground (head, arms or trunk) rather than the legs in the air.  The authors’ explanation for this is because mosquitoes may be following “convection currents” resulting from the person’s body heat and the mosquito will bite on the lowest body part that it encounters (if the mosquitoes were attracted to a smell or other property of the legs/feet, it would be expected that the mosquitoes would have still been biting there).

There are a number of limitations with this study.  There were only 5 participants, all male, they were the same race, there was no indication of how the subjects were selected (or where they lived, life styles), and their ages varied.  Given that individual variation (carbon dioxide emitted from breathing, moisture of the skin, chemicals released through the skin) was not accounted for, it’s hard to ascertain whether these findings would be replicable or are representative of any particular group.

So, no one really knows the secret as to why a mosquito decides to bite me over my friend or why certain mosquitoes like my feet.  I suppose if I’d like to avoid getting bit on my feet, I could lay on the ground with my feet sticking straight up, or could cover up my feet.  But what I’ll probably do is make sure I always have a steady supply of aloe on hand and let my feet enjoy being free.

*As much as I love the warm weather, I can’t help but worry about our great Michigan fruit this summer!

Source: Dekker T., Takken W, Knowls B.G.J., Bouman E., van de Laak S, de Bever A., Huisman P.W.T. (1998). “Selection of biting sites on a human host by Anopeheles gambiae s.s., An arabiensis and An. quadriannulatus.  Entomologia Experimentallis et Applicata. 87:295-300.

Image: “James D. Gathany.” Anopheles gambiae mosquito. 12 August 2002. From http://en.wikipedia.org/wiki/File:AnophelesGambiaemosquito.jpg  Original: http://phil.cdc.gov/Phil/home.asp (Image ID#444)