Why do mosquitoes seem to bite some people more?

Back in 2015, I had an article published at The Conversation on why some people are more likely to be bitten by mosquitoes than others. It is one of the most commonly asked questions I get whenever I give public talks (or friends and family are quizzing me at summer BBQs).

This article was incredibly successful and has currently been read by approximately 1.4 million people. That is a lot of people. Hopefully the science of mosquito bites has got out there and actually helped a few people stop themselves or their family being bitten by mosquitoes!

The warm weather is starting to arrive here in Australia so I am sharing this once more for those wondering why they’re always the “mosquito magnet” among their friends…

Health Check: why mosquitoes seem to bite some people more

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There are up to 400 chemical compounds on human skin that could play a role in attracting mosquitoes.  sookie/Flickr, CC BY-SA

There’s always one in a crowd, a sort of harbinger of the oncoming mosquito onslaught: a person mosquitoes seem to target more than others. What is it about these unlucky chosen few that makes them mosquito magnets?

There are hundreds of mosquito species and they all have slightly different preferences when it comes to what or who they bite. But only females bite; they need a nutritional hit to develop eggs.

Finding someone to bite

Mosquitoes are stimulated by a number of factors when seeking out a blood meal. Initially, they’re attracted by the carbon dioxide we exhale. Body heat is probably important too, but once the mosquito gets closer, she will respond to the smell of a potential blood source’s skin.

Studies have suggested blood type (particularly type O), pregnancy and beer drinking all make you marginally more attractive to mosquitoes. But most of this research uses only one mosquito species. Switch to another species and the results are likely to be different.

There are up to 400 chemical compounds on human skin that could play a role in attracting (and perhaps repulsing) mosquitoes. This smelly mix, produced by bacteria living on our skin and exuded in sweat, varies from person to person and is likely to explain why there is substantial variation in how many mozzies we attract. Genetics probably plays the biggest role in this, but a little of it may be down to diet or physiology.

One of the best studied substances contained in sweat is lactic acid. Research shows it’s a key mosquito attractant, particularly for human-biting species such as Aedes aegypti. This should act as fair warning against exercising close to wetlands; a hot and sweaty body is probably the “pick of the bunch” for a hungry mosquito!

Probably the most famous study about their biting habits demonstrated that the mosquitoes that spread malaria (Anopheles gambiae) are attracted to Limburger cheese. The bacteria that gives this cheese its distinctive aroma is closely related to germs living between our toes. That explains why these mosquitoes are attracted to smelly feet.

But when another mosquito (such as Aedes aegypti) is exposed to the same cheese, the phenomenon is not repeated. This difference between mosquitoes highlights the difficulty of studying their biting behaviours. Even pathogens such as malaria may make us more attractive to mosquitoes once we’re infected.

Only females bite because they need a nutritional hit to develop eggs.
Sean McCann/Flickr, CC BY-NC-SA

Researchers are trying to unscramble the irresistible smelly cocktails on the skins of “mosquito magnets”. But the bad news is that if you’re one of these people, there isn’t much you can do about it other than wearing insect repellents.

The good news is that you may one day help isolate a substance, or mixes of substances, that will help them find the perfect lure to use in mosquito traps. We could all then possibly say goodbye to topical insect repellents altogether.

Attraction or reaction?

Sometimes, it’s not the bite as much as the reaction that raises concerns. Think of the last time the mosquito magnets in your circle of friends started complaining about being bitten after the event where the purported mosquito feast took place. At least, they appear to have attracted more than the “bite free” people who were also at the picnic, or concert or whatever.

But just because some people didn’t react to mosquito bites, doesn’t mean they weren’t bitten. Just as we do with a range of environmental, chemical or food allergens, we all differ in our reaction to the saliva mosquitoes spit while feeding.

People who don’t react badly to mosquito bites may think they haven’t been bitten when they’ve actually been bitten as much as their itchy friends. In fact, while some people attract more mosquito bites than others, there’s unlikely to be anyone who never, ever, gets bitten.

The problem is that people who don’t react to mosquito bites may all too easily become complacent. If you’re one of them, remember that it only takes one bite to contract a mosquito-borne disease.

Finally, there is no evidence from anywhere in the world that there is something you can eat or drink that will stop you being bitten by mosquitoes. No, not even eating garlic, or swallowing vitamin B supplements.

The ConversationPerhaps if we spent as much time thinking about how to choose and use mosquito repellents as we do about why mosquitoes bite our friends and family less than us, there’d be fewer bites all around.

Cameron Webb, Clinical Lecturer and Principal Hospital Scientist, University of Sydney

This article was originally published on The Conversation. Read the original article.

 

Do mosquito repellent wrist bands work?

wristbandIf you don’t know if commercial mosquito repellents work, how can authorities provide useful advice to the community? What about non-topical mosquito repellents? How do you help the community make informed decisions on how to choose and use mosquito repellents effectively?

I’ve written about mosquito repellents on a number of occasions. The most popular post on my blog has been one describing how we test new repellents. That post was promoted by the enormous support received by a crowdfunding campaign by the developers of the Kite “mosquito repellent” patch that raised over $550,000 back in August 2013. There is a steady stream of new repellents in the news, either newly published research on active ingredients or new approaches to formulations and delivery systems. What was most appealing with the Kite patch was that is wasn’t a topical formulation but a “spatial repellent”. All you needed to do was put a sticker on your shirt you’d be protected from biting mosquitoes. An effective non-topical mosquito repellent would be a great asset in our battle against mosquito-borne disease.

Much of the research I do with mosquito repellents is directed towards better informing local health authorities on what works and how it should be used. My development of guidelines on mosquito repellent use came from the paucity of information provided by local health authorities. Health authorities generally provide the right advice, just not with enough detail that allows the public to make informed choices on mosquito repellents and how they should be used.

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Topical mosquito repellents are the most commonly used and widely distributed in Australia. These products, particularly those containing DEET or picaridin are most commonly recommended by health authorities

One of the most common questions I’m asked at public events is “do those mosquito repellent wrist bands and patches work?”

In 2010, I tested a product currently registered for use in Australia by the APVMA for use against mosquitoes. This was a plastic wrist band impregnated with peppermint oil. The results of the study were published in the 2011 volume of General and Applied Entomology. All papers in that volume are now freely available for download.

We tested the bands in laboratory conditions against the mosquito Aedes aegypti. This mosquito is one of the major vectors of dengue and yellow fever viruses internationally. It is also the most common species used in laboratory assessments of mosquito repellents due to its persistent biting behaviour and preference for humans.

The bands were tested to determine if any protection or repellency was provided against the mosquitoes compared to a DEET-based topical repellent. Although fewer mosquitoes landed on arms with the wrist bands compared to arms without wrist bands, there was no complete protection provided (as observed with DEET-based topical repellents). In fact, even on arms with wrist bands, beyond a small area around the band, the reduction in landing mosquitoes further up the arm was only marginally better than on arms without the wrist bands.

In short, while there was a reduction in total bites in close proximity to the bands, the bands we tested won’t prevent all bites. They won’t completely prevent bites on the arms wearing the bands and there is certainly no evidence that other parts of the body will be protected. They won’t create a “halo” of protection against mosquito bites around you.

There aren’t a lot of published studies investigating these “non-topical” repellents. There are a few “wrist band” and “patch” type devices available but all generally contain a botanical based active ingredient. In Australia, there are no DEET-based spatial repellents registered (to my knowledge). Studies from overseas have yielded mixed results. The level of protection (if any) provided is generally dose-related. For the most part, these devices have been demonstrated to assist in reducing the number of bites but not protecting you from all bites. Similarly, studies investigating the effectiveness of burning “mosquito sticks” demonstrate that while fewer bites are received, there is no complete protection provided.

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A summer approaches, the local hardware stores boost their stocks of Christmas lights and mosquito repellents

The results prompt an interesting question for health authorities (as well as regulators of mosquito repellents). Is it good enough to simply reduce the number bites, do you really need to prevent all bites?

My personal feeling is that you need to prevent as many as possible. Transmission of mosquito-borne pathogens such as Ross River virus, dengue virus or West Nile virus isn’t dependent on the number of bites. A single mosquito bite is all it takes. It may be true that the more bites you receive, the more likely it is that one of those mozzies will be infected but what if it is the first bite of the day that infects you?

So, do these devices have any use? My advice is to go with a DEET-based topical repellent. However, in some cases there may be some benefit in using a repellent wrist band to protect the hands if you’re undertaking an activity where the topical application of repellent may be considered inappropriate (perhaps fishing?). You could even wear these devices around your ankles to prevent bites but if you’re in a region where dengue or Chikungunya viruses are active (see here and here), I’d always recommend a topical product.

So, in answer to that commonly asked question, “Do mosquito repellent wrist bands work?”, I’d say they may offer some very limited protection but they are an ineffective way to prevent mosquito bites compared to a DEET or picaridin based topical repellent.

Here is the abstract of our paper:

A wide range of insect repellent formulations, as well as active ingredients, are currently registered for use in Australia. While topical repellents are most common, there are also commercial products in the form of wristbands impregnated with botanical extracts that purport to repel mosquitoes. In laboratory tests, wristbands impregnated with peppermint oil were tested against the mosquito Aedes aegypti to determine their efficacy in repelling mosquitoes from the forearms of human volunteers compared with a commercial DEET-based topical repellent. The wristbands failed to stop landing by the mosquitoes, although the mean landing rate of mosquitoes was significantly lower on forearms in the presence of the wristband compared with untreated controls. The mean landing rate of mosquitoes on forearms treated with DEET was significantly lower than those of forearms in the presences of the wristband. The results indicated that while wristbands impregnated with botanical products may assist in repelling mosquitoes, their inability to completely protect individuals from mosquito bites suggests that they should not be recommended for use in areas of endemic or epidemic mosquito-borne disease.

The full paper [PDF] can be downloaded for free here:

Webb CE and Russell RC. (2011) Do wrist bands impregnated with botanical extracts assist in repelling mosquitoes? General and Applied Entomology 40:1-5. PDF

[UPDATE 25 February 2015] It is interesting to note that in the U.S., the Federal Trade Commission has charged a manufacturer of mosquito repellent wrist bands with “making deceptive, unsubstantiated claims” regarding the effectiveness of their product. It will be interesting to see how this decision impacts the availability of these products in North America.

Want to learn more about the amazing world of Australian mosquitoes? Check out “A Field Guide to Mosquitoes of Australia” out now through CSIRO Publishing. Over 200 pages containing a pictorial guide to almost 100 different mosquitoes along with tips on beating their bite and protecting your family from the health risks of mosquitoes. You can order online or through your favourite local bookstore or online retailer.


Read more at The Conversation: Are mosquito coils good or bad for our health?


Read more at The Conversation: What can I eat to stop mosquitoes biting me?


Read more at The Conversation: The best (and worst) ways to beat mosquito bites