Not Exactly Rocket Science

i-cdf7f3f0d2f8317973efda1fd5346a0f-Mozzie_barney.jpgWe’ve all heard about “beer goggles”, the mythical, invisible eyewear that makes everyone else seem incredibly attractive after a few pints too many. If only beer had the reverse effect, making the drinker seem irresistibly attractive. Well, the good news is that beer does actually do this. The bad news is that the ones who are attracted are malarial mosquitoes.

Anopheles gambiae (the mosquito that transmits malaria) tracks its victims by their smells. By wafting the aromas of humans over thousands of mosquitoes, Thierry Lefevre found that they find the body odour of beer drinkers to be quite tantalising. The smell of tee-total water drinkers just can’t compare. The somewhat quirky conclusion from the study, albeit one with public health implications, is that drinking beer could increase the risk of contracting malaria.

Lefevre recruited 43 men from Burkina Faso and sent them individually into one of two sealed, outdoors tents. One tent was kept unoccupied. In the second, the volunteer had to drink either a litre of water (just shy of two pints) or a litre of dolo (a local 3% beer and the country’s most popular alcoholic drink). A fan pumped air from the tents, body odour and all, into the two forks of a Y-shaped apparatus. Both branches met in a third arm, which ended in a cup full of mosquitoes. The insects had to decide which branch of the Y to fly down and two pieces of gauze trapped them in their chosen path (and saved the volunteers from an infectious bite).

Lefevre showed that the smell of a beer drinker, 15 minutes after chugging his litre, increased the proportion of mosquitoes inclined to fly into the tubes, and the proportion (65%) who headed down the beer-scented fork.  The smell of water-drinkers had no effect, nor did the smell of the occupied tent before its inhabitant started drinking.

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What is it about a beer drinker that is so appealing? No one knows. Mosquitoes are drawn to the smell of carbon dioxide, but the beer drinkers weren’t exhaling any more of this gas after their drink. Something about the smell of beery body odours attracts mosquitoes. Mosquitoes also fancy body heat, but beer actually lowered the volunteers’ temperature by a fraction of a degree. Metabolising beer probably releases a slew of chemicals that mosquitoes are drawn to but the identity of these airborne attractants is a mystery. Nor do we know if the chemicals in question are specific to beer, or common to all alcoholic drinks.

This study is perfect tabloid fodder, but it has a very serious side. To mosquitoes, not all humans are equal. The bloodsuckers are quite picky about whom they suck from, and an important global study revealed that in problem areas, 20% of people receive 80% of all malaria infections. As such, it’s quite important to work out what makes one person a delectable feast and another person a bloody turn-off.

Beer can’t explain all of this variation by any means. After all, Lefevre found that some people were naturally attractive to mosquitoes without drinking anything; beer merely boosted these natural charms. So a pint of lager in the African sunset isn’t going to guarantee a raging malarial fever, but it might increase the risk of one.  

There are other reasons to think that the beer effect may be more serious than shown in this study. For a start, A.gambiae is a night biter. It’s most active after sunset, which probably coincides with the time its prey is most likely to smell of beer. Drinking moderate to heavy amounts of alcohol can also suppress the immune system  so regular beer drinkers are not only more likely to encounter malarial mosquitoes but they could be more vulnerable to the parasites they carry. And finally, at least one previous study showed that Aedes mosquitoes, carriers of dengue fever, are drawn to the aroma of beer drinkers, so the public health implications may go beyond just malaria.

Finally, Lefevre even puts forward a completely speculative, but cool, idea – perhaps the fact that mosquitoes wear beer goggles isn’t just a coincidence. It’s possible that they might have evolved a slight preference for the smell of beer-drinkers, either because their blood is full of nutrients or as he dryly notes, “possibly due to reduced host defensive behaviours”! As Lefevre says, “This hypothesis is appealing but requires further investigations.”

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Reference: 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

More on mosquitoes:           

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Comments

  1. #1 becca
    March 4, 2010

    Tabloid headline- “to avoid malaria, you should drink beer for breakfast, not dinner”.
    Or, “Gin and tonic, not beer: a drinkers guide to avoiding malaria”
    How do we know the mozzies don’t feel the alcohol?
    I wish they’d done it with plasmodium carrying mozzies and with plasmodium free mozzies.

  2. #2 Ed Yong
    March 4, 2010

    I think the reason why they didn’t separate out Plasmodium carriers vs. non-carriers is that they wanted to do the study with mosquitoes from populations that hadn’t been bred in the lab. So the mozzies in the study were hatched in a lab, but the eggs were laid by pregnant females caught in the wild. Why this is important isn’t clear to me, but they make a thing of it in the paper. Anyway, more work to do. It would also be interesting to test other alcoholic beverages

  3. #3 southlakesmom
    March 4, 2010

    Interesting article. We have terrible mosquito problems around here (Virginia) but I don’t know that the beer drinker in the house suffers worse. On the other hand, he smokes so maybe that is a deterrent.

    Oh Ed, there’s a typo in the first para…The bad news is that the ones who are attracted [at] malarial mosquitoes.

  4. #4 phantomreader42
    March 4, 2010

    Since you mention that mosquitos are attracted to carbon dioxide, was the beer in question carbonated? Have other carbonated beverages been tested?

  5. #5 Ed Yong
    March 4, 2010

    Typo fixed – thanks.

    @4 – doesn’t really matter because the beer didn’t increase the concentration of CO2 in the tent air.

  6. #6 not_hippy
    March 4, 2010

    I was immediately left wondering why the decision was made to test beer vs water in the first place. Did the authors already suspect a link between beer consumption and ‘attractiveness of odour’…? A quick read of the paper answered the question, it seems that a previous study had already made the link between beer consumption and ‘odour attractiveness’ to laboratory bred Aedes mosquitoes and the authors wished to test the link in malaria mosquitoes from natural populations. I can’t find a full copy of the previous study online though (its past my bedtime so I’ve only looked for a few minutes) so I’m still wondering why that particular link was suspected in the first case. I wonder if it was already part of some common folklore that drunk people are more prone to bites.

  7. #7 not_hippy
    March 4, 2010

    In any case, I’m not usually prone to being bitten. I think I might get a case of beer on the weekend and conduct some experiments of my own on my local population of mosquitoes.

  8. #8 Tyler Dukes
    March 4, 2010

    This is incredibly interesting. I’m always attacked relentlessly by the mosquitoes here in North Carolina, especially compared to some of my friends.

    But this study has some important implications for tailgating during football season!

  9. #9 Eve Emshwiller
    March 4, 2010

    Shouldn’t they have had them drink the beer outside of the tent first, and then enter the tent for the test? How do we know that the odor of the beer itself wasn’t still in those tents, mixed in with the odor of the beer drinkers? Does it explain this in the paper?

  10. #10 Jason Smith
    March 4, 2010

    @Ed: Couldn’t belching from the beer tent (esp. if chugged) produce a slight bump in CO2 emissions which the mosquitoes could pick up on?

  11. #11 Nathan Myers
    March 4, 2010

    The title should have been Beer makes humans more attractive. The rest is a detail.

  12. #12 Lilian Nattel
    March 4, 2010

    One of my children is a mosquito magnet. It will be interesting to find out more about mosquito preferences. (I will advise her that when she is of age to avoid beer while camping!)

  13. #13 Ed Yong
    March 4, 2010

    Nathan, did you see the title for Figure 2? ;-)

    Jason and Eve – sorry, no time tonight, but the paper’s open access and not too difficult a read.

  14. #14 Briana
    March 4, 2010

    So we have carbon dioxide, nonanal, and now beer that makes us delicious.

    Did I read the nonanal findings from Not Exactly Rocket Science? I don’t see it in the “more on mosquitoes”, but I don’t remember where I heard that one…

  15. #15 Chris M.
    March 5, 2010

    @Ed: One problem with lab mosquito populations is that they quickly become slow. If you’ve ever been in a lab that raises houseflies, it’s fun swatting the ones that escape, because their reaction time is absolutely terrible. Probably some strong behavioral changes as well, such as rapid selection for willingness to feed on the blood bag membrane.

    Anyway, a bunch of things you’d rather not have to account for in your experimental design! So they kept it as close to wild behavior as reasonably possible.

    I seem to recall something about the smell of feet being attractive to mosquitoes; if any sort of yeast is involved in that effect, it could follow that beer would cause greater attractiveness. Unfortunately, in that study they were culturing bacteria, although if generalized it could imply that mosquitoes are attracted to feet in other ways also. If so, there might be a whole host of chemical attractants active, and yeast or other fungus could be among of them.

    A little contrived, but testable!

  16. #16 A. Fleming
    March 5, 2010

    I assume the local beer used in the experiment is a live-yeast home-brew, not factory-bottled?
    Folklore in Australia claims that drinking home-brewed beer (non-pasteurized live yeast beer) acts as a mosquito repellent, providing you’ve been consuming the stuff for 6 weeks or so.
    But in southern Australia there are no malaria- carrying mosquitoes – yet. Perhaps this needs testing.

  17. #17 Ed Yong
    March 5, 2010

    Did I read the nonanal findings from Not Exactly Rocket Science?

    Probably not – I try to feature only the most anal of discoveries.

    ChrisM – Thanks for that.

    A.Fleming – The idea that Aussies might drink home-brewed beer for 6 weeks straight does not surprise me in the slightest ;-)

  18. #18 not_hippy
    March 5, 2010

    Maybe that explains why mosquitos tend to ignore me, being Australian and a homebrewer… but now I’m confused. Mosquitos are attracted to beer drinkers, or at least Burkinabe males aged between 23 and 40 who drink dolo prepared from the fermented dough of sorghum. But mosquitos are repelled by beer drinkers, or at least according to folklore, Australians who drink homebrew. To be safe, I’ll continue to avoid drinking dolo and being Burkinise and stick with homebrew…

  19. #19 Briana
    March 5, 2010

    @Ed Yes, I’ve learned to appreciate the wonders of my colon and all it contains thanks to your features.

    NSF is where I read that article, in case anyone else is interested: http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=116286&org=NSF

    Apparently nonanal gives off a fruity odor, as opposed to nutty.

  20. #20 Happy
    March 9, 2010

    [blockquote]
    But in southern Australia there are no malaria- carrying mosquitoes – yet.
    [/blockquote]

    between Dengue Fever, Ross River Virus, and the tradition of going “fishing” with half a litre of bait and four hundred litres of beer, a local follow up on this is probably going happen any second now…

  21. #21 Happy
    March 9, 2010

    But in southern Australia there are no malaria- carrying mosquitoes – yet.

    between Dengue Fever, Ross River Virus, and the tradition of going “fishing” with half a litre of bait and four hundred litres of beer, a local follow up on this is probably going happen any second now…

  22. #22 John
    March 9, 2010

    Vitamin B1 (Thiamin) deters mesquitoes as most backpackers know. Beer depletes this vitamin in the system. Mystery solved.

  23. #23 harold
    March 9, 2010

    The only control is water, rendering the study silly. To say that beer has a specific effect, you need to test it against other foods, taken in similar caloric amounts, at a minimum.

    Perhaps a non-specific increase in blood sugar is responsible, for example. It’s highly plausible that mosquitoes would be attracted to blood with a higher glucose content. But then almost any snack or meal would have the same effect.

  24. #24 harold
    March 11, 2010

    John –

    Hard liquor depletes vitamin B1 (except in Australia, where they fortify to prevent Wernicke-Korsikoff syndrome).

    Beer, at least high quality all-malt beer, is loaded with thiamine.

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