The anopheles mosquito, Anopheles gambiae, is the primary vector for human malaria. Mosquitoes in general, the A. gambiae included, find their prey by tracking body odor exuded from the breath and skin. Apparently, the composition of body odor determines A. gambiae‘s preference for one individual over another. It has been known for some time now that A. gambiae preferentially seek out and draw blood from pregnant women (Linsay et al 2000; Ansell et al 2002; Himeidan, Elbashir and Adam 2004), preferring pregnant over none pregnant women at about a 2:1 ratio.
Researchers working in Burkina Faso recently decided to test the hypothesis that beer drinking would have an effect on mosquito prey preference, and they found that it did. Twenty-five human volunteers drank beer and were explosed to 2,500 A. gambiae mosquitoes, and 18 volunteers drank only water and were exposed to 1,800 A. gambiae mosquitoes (100 mosquitoes per volunteer). The behavior of the mosquitoes was observed, and it was determined that they go after the beer-drinkers preferentially.
The volunteers, healthy adult male Burkinabe not using medicine of any kind were instructed to avoid deodorants and beer on the day of the trial. The beer was a local dolo made of sorghum, the commonly consumed beer for the region. The amount of beer or water consumed for each person was one liter.
There were two trials separated by 15 minutes, with the drinking of the beer or water in between. A gadget called the Y-olfactometer was used to measure the human odors, and was set up like so:
(A) The two tents set up outdoors and connected to the two traps of the Y-olfactometer by lay-flat tubing, and the olfactometer room located between the two tents. (B) Fan drawing air from a tent to the olfactometer via lay-flat tubing. (C) The Y tube-olfactometer.
Batches of 50 mosquitoes were released into the downwind box of the Y-olfactometer (figure 1C) and given a choice between outdoor air and human odour. They were allowed to respond for 30 min. During this time frame, mosquitoes that responded to the stimuli left the downwind box and flew upwind into the traps from which they were retrieved (figure 1C). At the end of each test, the mosquitoes inside the two traps were removed with an aspirator and counted.
Various statistical analyses were done, but the results are best indicated in the following graph:
(A) Effects of beer (n = 25 volunteers) or water (n = 18 volunteers) consumption on mosquito activation, expressed as the proportion of mosquitoes caught in both traps out of the total number released in the downwind box of the Y-olfactometer. In parentheses are the total numbers of mosquitoes entering both traps. (B) Effects of beer (n = 25 volunteers) or water (n = 18 volunteers) consumption on the mosquito orientation, expressed as the proportion of mosquitoes caught in the odour-baited trap out of the total number retrieved from both traps. In parentheses are indicated the numbers of mosquitoes entering the volunteer odour-baited trap. Error bars show 95% confidence interval of the mean proportion. Asterisks indicate significant effect of treatments on the response variables (GLMM); ns = not significant; *** = P<0.001.
So, drinking beer = more likely to get bit by potentially malaria-carrying A. gambiae mosquitoes. But why? It is not clear. Pregnant women seem to be more often targeted by A. gambiae because of increased body temperatures and exhaled breath. In this study, however, no increase in body temperature was observed; Temperatures of beer-drinkers actually went down, no up. There was no increase of exhaled breath either.
It is possible that mosquitoes track beer drinkers adaptively. It is probably easier to bite an intoxicated person and get away alive. There is no direct evidence for this in the current study, but it is an interesting idea.
We postulate that the metabolism of alcohol following beer consumption induces changes in breath and odour markers (i.e. increases the production of kairomones such as 1-octen-3-ol) that increases attractiveness to An. gambiae. Beyond this coincidental side effect of beer consumption, mosquitoes may have evolved preferences for people who recently consumed beer – possibly due to reduced host defensive behaviours or highly nutritious blood-meals. This hypothesis is appealing but requires further investigations.
The paper, published in PLoS ONE, is available here. Full citation:
Lefèvre, T., Gouagna, L., Dabiré, K., Elguero, E., Fontenille, D., Renaud, F., Costantini, C., & Thomas, F. (2010). Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009546
Lindsay S, Ansell J, Selman C, Cox V, Hamilton K, et al. (2000) Effect of pregnancy on exposure to malaria mosquitoes. Lancet 355: 1972.
Ansell J, Hamilton KA, Pinder M, Walraven GEL, Lindsay SW (2002) Short-range attractiveness of pregnant women to Anopheles gambiae mosquitoes. Trans R Soc Trop Med Hyg 96: 113-116.
Himeidan YE, Elbashir MI, Adam I (2004) Attractiveness of pregnant women to the malaria vector, Anopheles arabiensis, in Sudan. Ann Trop Med Parasitol 98: 631-3.