[Helen Stohlman]

Helen Stohlman  asked the Naked Scientists:

Hi Chris,

I just finished listening to last week's show on mosquitoes and other biting insects, and it has made me wonder about something.  I am one of those lucky people that mosquitos don't seem to be very attracted to.  However, when I moved to Guatemala (from the Northeastern United States) several years ago, I discovered that they have a type of mosquito here, locally called a zancudo, that loves me.  

Fortunately they live in the warm coastal areas and not here in the chilly mountains where I live, but whenever I travel to a lower altitude I get eaten alive, and the bites are nasty:  the second day they show up as red bumps with a tiny scab in the middle, and they tich, form blisters, and seep a milky yellow liquid for about two weeks.  

Zancudos are smaller than other mosquitos, and more compact (shorter legs and smaller wings in proportion to the body size).  Here in Xela, where I live, I never get mosquito bites, although there are some here.  Do different breeds of mosquitos prefer different human emissions? Why am I delicious exotic meat to zancudos and totally boring to the regular mosquitoes?  

The locals say it's because I am white, but some white Guatemalans are much more immune to zancoudos than I am, and some dark-skinned Guatemalans get bitten by regular mosquitos and zancudos all the time.  (They tend to blame everything on me being white, so I am always skeptical!)


Thanks as always for a great show.

Helen Stohlman

What do you think?

[Doc James]

Hi Helen, I am the guy who was on the show talking about mosquitoes and your questions are really interesting.

I have checked with a colleague of ours that works in Guatemala and apparently the name “Zancudo” is a colloquialism that is used in Guatemala and other Central American countries to describe mosquitoes generally. As far as I am aware, it doesn’t refer to a specific species. However, the zancudos that you will encounter are quite likely to be Aedes species (for example Aedes aegypti) – these mosquitoes are the vectors of dengue fever and yellow fever.  The can often be smaller than the types of mosquitoes you would be used to in North USA. These are likely to be Culex species mosquitoes. If they are Aedes mosquitoes in Guatemala, you will be able to tell by looking at their legs –they are black with white stripes. I would advise that you use a good repellent and avoid being bitten as much as possible as there is no other preventative against dengue. The annoying thing about Aedes mosquitoes is that they will quite happily feed during the day, so you do have to be quite careful. Alternatively, it could be Anopheles mosquitoes that are biting you. Again, you would be more unlikely to find them in the US. They mainly feed between sunset and sunrise and can carry the pathogens that cause malaria, so watch out for them. If you know what time you are being bitten you will be able to take a guess at which type of mosquito is biting you! Both types will also readily enter houses.

The reason that you react badly to these mosquitoes is probably because you are not used to being bitten by these species. People often become desensitized to bites after a while, although some people do go the other way and become sensitized (like me unfortunately!!)

The differential responses of different species can be because some prefer different vertebrate host species. Many Culex species prefer to feed on birds, some Anopheles mosquitoes prefer to feed on cows and some prefer to feed on humans. What we don’t know is whether different species of mosquito would be attracted or repelled by the same individual – this test hasn’t been done to my knowledge. But if it were to happen, it would likely be due to them responding differently to the same odours. For example, Aedes mosquitoes are strongly attracted to lactic acid, whereas Culex mosquitoes aren’t. Two of the repellent chemicals that we identified from people who rarely get bitten by mosquitoes (and that we are now developing as a product) seem to work against Culex, Anopheles and Aedes mosquitoes. However, one of the other repellents that we have only works against Aedes and Anopheles. Culex mosquitoes actually find it attractive!

I doubt that it has much to do with the fact that you are white, although to be honest, we haven’t been able to compare the odours of people from different countries to see if it has an effect. Skin colour has been implicated in the past, but most of the scientific literature is quite contradictory and I would say that most evidence suggests that skin colour makes no difference. What we need to do is an experiment to test the attractiveness of a large and random selection of the world’s population - only then will we be able to tease out the factors that underlie differential attractiveness. This would be a massive study and unfortunately we do not have the funding to do it, but I believe it should be done and would provide crucial information about mosquito-human interactions which will aid the development of targeted control efforts and feed into predictive modeling for the spread of diseases like malaria.

Hope that is of some help!

Cheers,

James

[chris]

Thanks James, what a brilliant answer.

Chris

[Helen Stohlman]

Hi!  I figured out how to get registered.  Thanks, James, for the very complete answer!  At least in my city, people do distinguish between mosquitos (the big ones that don't bite me) and zancudos (the nasty little guys that do).  I'll have to try and get a look at their legs next time I'm in the lowlands, but they do seem to bite during the day since a day trip to the beach is enough for me to wake up speckled the next day.  Thanks also for the warning about dengue; I guess I've been a little cavalier about that, especially since the hemmoragic type is still pretty uncommon here. 

If I react to mosquitos that are new to my organism, does that mean that different breeds of mosquitos inject different substances into their "dinners"?

[Doc James]

Hi Helen,

Sorry for the delayed response - I have just returned from a trip to Ghana. The sensitivity to bites is quite an interesting thing. When the mosquito bites it injects saliva into you which contains a cocktail of substances. The saliva contains anticlotting factors and vasodilators which stops the blood from clotting and probably speeds up the feeding process (so less chance of them being splatted!), substances that are involved in transmitting parasites (like the malaria parasite), enzymes associated with sugar feeding (mosquitoes also feed on sugar/nectar from plants for energy - blood is only used for the production of eggs), immunomodulators (including immunosuppressants which decrease the activity of the immune system during feeding) and lysozyme which is a substance that may help control bacterial growth in the mosquito crop (where the mosquito stores its sugar meal). The types of substances (including proteins) are known to be allergens and so this is what people react to when they are bitten. Interestingly, different species of mosquito do have different proteins in their saliva and so it is a different cocktail of substances being injected into you. That is why the reaction to different species can occur.

If you have never been bitten by a certain species before, apparently what normally happens is that you develop very little reaction at all to the bites. However, subsequent bites do cause a reaction although the itchy lumps are normally delayed. If you continue to be bitten the delay disappears and you usually react straight away. After a while, if you have lots of exposure to the bites, most people eventually lose the reaction and therefore are considered to be desensitised. This can take years to occur and in fact may never happen for some people (especially since people who do react tend to avoid being bitten and so are never exposed enough to become desensitised!)

Anyway, I hope that is of some interest to you!

Cheers,

J

[Helen Stohlman]

Thanks for taking the time to reply and I hope you had a good trip!  Your information is very interesting and would seem to confirm my guess!  I still seem to get the delayed reaction but the last time I got bitten the bites seemed to go away a bit more quickly.  I guess I would have to travel to the low areas a lot to become desensitized!  If I can ever catch a zancudo and look at its legs I'll let you know if they are black and white!

[Doc James]

No worries - any time!

James

[Helen Stohlman]

So, James, last week I spent about 48 hours in Antigua, and NO zancudo bites!!!  How do we explain that?  Does weather affect them (it was pretty cloudy)?  Could I possibly have become desensitized?  I do remember the last time I was there my bites were fewer and took less time to heal (this was about six months ago).  Needless to say, I wasn't able to examine any zancudo legs either.

[Helen Stohlman]

Just spent the weekend on Lake Atitlán and still no zancudo bites, although my American goddaughter who was with me got eaten alive.  I did see and swat one mosquito, but it wasn't what I know as a zancudo and it was at night.  I felt the sting but got no reaction whatsoever.  I'm totally confused now, although not unhappy to be bite-free!  Maybe mind control can prevent mosquito bites... []

[RD]

Related TNS topic...
http://www.thenakedscientists.com/forum/index.php?topic=15150.0

[Doc James]

Hi Helen,

Sorry for the delay in replying - just returned from South Africa. Weather does definately affect mosquitoes - they have optimum temperatures and humidity, etc. Although heavy cloud cover is usually quite good for them. Am I right in thinking it is the dry season over there just now? If so, there will be fewer mosquitoes around. They need water to breed in and their populations usually boom in the rainy season (which i think starts around September?). I'm not sure whether you have become desensitised or not to be honest - the only way to tell would be to actually see a mosquito landing on you and feeding and then observing that you dont have a reaction afterwards. Although i wouldnt advise anyone doing that of course!

It is quite common that some people are bitten more than others so your goddaughter is probably more attractive than you. It is interesting that you felt them bite you as you dont normally feel a mosquito bite until afterwards. Perhaps it was another type of blood-feeding insect?

Confusion as a control method doesn't work I can tell you. I am confused on a daily basis and the mosquitoes still love me!

 []

[Helen Stohlman]

Hey Dr. James,

Don't worry about the delay in responding, and I appreciate your getting back to me! 

I am mystified about this; we are in the rainy season, and it has been especially rainy this year, so I would expect to encounter more mosquitos.  I went away again the last week of July (this is REALLY an unusual amount of traveling for me!), this time three days in Cobán, which is cloud forest and much warmer than Xela, where I live.  I got a couple of bites, but they were the kind that just itch and disappeared in a day or two.  The other time I mentioned in Lake Atitlán, I felt the bite (and yes, this is unusual for me), and swatted the mosquito.  It was what we call a mosquito, not a zancudo, and looked like the ones I know in the US.  And yes, I had no reaction whatsoever to the bite later.  I mentioned my goddaughter as evidence that there were zancudos around (she got the scab in the middle kind of bite I associate with zancudos). 

By the way, several of my dark-skinned coworkers got bitten a lot in Cobán, and offered the explanation that the zancudos like dark skinned people better than whites!  So I'm quite sure we are inconsistent, if nothing more.

Thanks!

[Doc James]

Cheers, that's all interesting stuff! We are about to do a survey in Scotland to try and work out whether there are any characteristics such as those associated with attractiveness to biting midges (a huge pest on the west coast!).

J

[RD]

Examples of the commercial trap Mosquito Magnet® Pro™ (MMP emitting attractant 1-octen-3-ol in carbon dioxide 500 mL/min generated from propane fuel), were run 24 h/day on the Isle of Skye, Scotland, during June–August 2001 and evaluated for catching Culicoides biting midges (Diptera: Ceratopogonidae). From 30 days trapping, the catch averaged 2626 ± 1358 Culicoides females/trap/day (mean ± SE, range 558 ± 139 to 6088 ± 3597, for five sets of six consecutive nights), predominantly the pest Culicoides impunctatus Goetghebuer (68% overall), plus C. vexans (Stæger) > C. delta Edwards > C. pulicaris (L.) > C. lupicaris Downs & Kettle > C. albicans (Winnertz) > other Culicoides spp.

Attempts were made to enhance the odour baiting system by adding hexane-extracts (2.1 mg/day) of hair samples from large host animals, resulting in the following effects on Culicoides collections: sheep - 53 %, red deer - 26 %, calf + 20%, pony + 40%, water buffalo + 262%, with greatest increases for C. impunctatus and C. pulicaris.

http://www.ingentaconnect.com/content/bsc/mve/2004/00000018/00000004/art00004

If you're going to Skye leave your water buffalo at home.  []

[Helen Stohlman]

Good luck with the study, Dr. James!  Hope to hear about it on the Naked Scientists when it's done.

[Doc James]

Thanks H!