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.

 

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West Aussies versus the local mozzies

This is a special guest post from Dr Abbey Potter, Senior Scientific Officer, Environmental Health Hazards, WA Health. I’m currently mentoring Abbey as part of The Public Health Advocacy Institute of WA (PHAIWA) Mentoring Program. Its been a great experience as we navigate through some of the strategies to raise awareness of mosquito-borne disease and advocate for better approaches to addressing the public health risks associated with mosquitoes.

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Living in WA, we’re all too familiar with the pesky mosquito. We know they bite but what we often don’t consider is that they can transmit serious and sometimes deadly diseases. In fact, a recent survey of locals indicated that knowledge of mosquito-borne disease is pretty limited, particularly among younger adults aged 18-34 years and those living in the Perth Metro. It’s pretty important we’re aware of the risks posed by these pint-sized blood suckers and how you can avoid them… and here’s why!

The Facts

On average, more than 1,000 people will be infected with a mosquito-borne disease in WA every year. Our mossies can transmit Ross River virus, Barmah Forest virus, West Nile virus (Kunjin substrain) and Murray Valley encephalitis virus. All four cause diseases that are debilitating at best, causing weeks to months of symptoms. Murray Valley encephalitis is limited to the north of the State but is so serious it can result in seizures, coma, brain damage and even death.

Forget the bush, most people bitten in their own backyard. West Aussies are all very prone to getting eaten alive while socialising outdoors but if you’re up in the north of the State, you’ve also got a much higher likelihood of being bitten while boating, camping or fishing or working outside, compared to the rest of the state.

And don’t think you’re off the hook when you head off on holidays. A further 500 WA residents return from overseas travel with an exotic mosquito-borne disease every year. Heading to Bali? Beware of dengue, especially young adult males who return home with the illness more than others. There is limited mosquito management in many overseas countries where disease-transmitting mozzies can bite aggressively both indoors and throughout the day. This catches West Aussies off guard, as we are accustomed to mozzies biting outdoors, around dusk and dawn. When you’re in holiday mode it’s likely that you’ll be relaxing, having a couple of drinks and not thinking about applying repellent. Oddly enough, mosquitoes may actually be more attracted to people whose body temperature is higher. This happens naturally when you consume alcohol, so best pull out the repellent before you crack your first beer.

Despite our attractiveness to mosquitoes, we aren’t really aware of the most effective ways to avoid bites or how we can do our bit to reduce breeding in our own backyards. If you live by the mantra Cover Up. Repel. Clean Up you’ll have no problems!

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Western Australia has some amazingly beautiful wetlands but these saltmarshes around Mandurah can produce large populations of nuisance-biting mosquitoes!

Cover Up

If you know you are going to be outdoors when mosquitoes are active, wear loose, long-fitting clothing that is light in colour. Believe it or not, mosquitoes can bite through tight pants as tough as jeans – I’ve witnessed it!

If you’re staying in accommodation that isn’t mosquito-proof, consider bed netting.

Try to keep children indoors when mosquitoes are most active. If exposure can’t be avoided, dress them appropriately and cover their feet with socks and shoes. Pram netting can also be really useful.

Admittedly, it’s not always practical to wear long sleeves during our warm summer nights, so there are going to be times when you need to use repellent. Choose a product that actually works and apply it appropriately so it does the job. Despite our best intentions, this is where we often go wrong. There are a few basic things to cover here, so stick with it!

Ingredient: Science tells us that the best active ingredient for repelling mosquitoes is diethyltoluamide (DEET for short) or picaridin. You need to look for either one of these names on the repellent label under the ‘active constituents’ section.

Unfortunately, natural repellents and anything wearable (e.g. bands, bracelets or patches) have very limited efficacy. Experts don’t recommend you use them and I consider this very wise advice. It only takes a single mosquito bite to become infected and chances are you will receive at least one if you rely solely on a product of this nature. It just isn’t worth the risk.

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Percentage: The next thing to consider is the percentage of the active ingredient. This can range anywhere from 7% to 80% which can make choosing a repellent confusing. Just remember, the higher the percentage, the LONGER the product will remain active for. It doesn’t mean it will repel mosquitoes better.

A repellent containing 16-20% DEET will provide around 4-6 hours of protection, and is a good place to start. Repellents labelled ‘tropical strength’ usually contain greater than 20% DEET – they are useful when you spend longer periods exposed to mosquitoes or if you are heading to a region where dengue, malaria or Zika is problematic. Kids repellents usually contain picaridin or <10% DEET.

Sometimes it can be tricky to work out the percentage of the active ingredient. You can see the Bushmans example below states this clearly, but the other bottles list the ingredient in grams per litre (g/L). No need for complex maths – just divide by 10 and you have the magic number! For example, the RID label below reports the product contains 160g/L of DEET. This would convert to 16% DEET – easy!

You can see a few examples here of effective repellents:

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How to Apply: No doubt we would all prefer if repellents didn’t feel quite so gross on our skin or didn’t smell so bad. Even I have to admit that before I moved into this field, I was guilty of putting just a dab here and a dab there. Unfortunately, this is flawed logic that will only result in you being bitten!

Repellents must be applied correctly to be effective. That means reading the label and applying it evenly to all areas of exposed skin. Remember to reapply the product if you are exposed to mosquitoes for longer than the repellent protects you for. You’ll also have to reapply the repellent after sweaty activity or swimming.

For more information on repellent use in adults and children, click here.

Clean Up

Mosquitoes need water to breed, but only a very small amount. Water commonly collects in a range of things you may find in your backyard including pot plant drip trays, toys, old tyres, trailers and clogged up gutters. Mosquitoes also love breeding in pet water bowls, bird baths and pools if the water is not changed weekly or they are not well maintained. Rain water tanks can also be problematic so place some insect proof meshing over any outlets. When you’re holidaying, cover up or remove anything that may collect water.

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If you need more official info from WA Health about mosquito-borne disease or simple ways to prevent being bitten click here. And if you want to read more about how much West Aussies know (or don’t know) about mossies, check out Abbey’s excellent paper here! Joint the conversation too on Twitter by following Abbey and Cameron.

Safely avoiding mosquito bites when pregnant

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Hundreds of millions of people fall ill due to mosquito-borne pathogens every year but the recent rise in birth defects associated with Zika virus emerging in the Americas has health authorities on alert.

Zika virus is transmitted by mosquitoes, primarily by the Yellow Fever mosquito, Aedes aegypti. Since its discovery in Africa around 70 years ago it has avoided the public health spotlight due to the relatively mild illness it causes. Throughout Africa and Asia it is overshadowed in importance by the diseases caused by malaria parasites as well as dengue and chikungunya viruses.

For background on the rise of Zika virus, see my article for The Conversation.

Zika and the health risks to those pregnant and their unborn children

While Zika virus has yet to be fully confirmed as the causative agent in birth defects (such as microcephaly), there is clearly enough concern among health authorities in many parts of the world to issue warnings to those pregnant to avoid travel to countries experiencing an outbreak of Zika virus.

Authorities in Columbia and El Salvador have even gone so far as to advise residents to avoid falling pregnant for up to two years.

The Australian Government issued the following advice via their SmartTraveller website:

“Until more is known about Zika virus, and taking a very cautious approach, we advise women who are pregnant (in any trimester) or who plan to become pregnant to consider postponing travel to any area where Zika virus transmission is ongoing. If you do decide to travel, talk to your doctor first and strictly follow steps to prevent mosquito bites during your trip.”

For many, the option of not travelling will be easy. But what if you still want to travel? What if you’re doing business in some of these countries? What if you need to travel to visit family? Cancelling a trip isn’t always the easiest options.

Reducing risk of mosquito-borne disease while travelling

Irrespective of the current Zika outbreak, travelling while pregnant brings various health and safety risks.  Other mosquito-borne diseases, such as malaria and dengue, also pose a risk to pregnant women and their unborn children. These serious risks existed long before Zika virus grabbed the public health spotlight.

Unfortunately, there is no vaccine currently available for Zika virus. Vaccines are in development for dengue viruses and anti-malaria drugs are available so consult your local travel health clinic.

While travelling, staying indoors as much as possible, particularly air-conditioned accommodation, will greatly reduce exposure to mosquitoes. This may not be how you expected to spend your time during a South American holiday!

Many people associate mosquito-borne disease with wetland or jungle environments but as Zika virus is spread by mosquitoes found in urban habitats (e.g. water-filled containers), travellers should not be complacent if only visiting cities. Some of the biggest recent outbreaks of mosquito-borne disease have been in major metropolitan regions in the Americas and Asia.

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Mosquito control in Brazil, a striking image of the battle against container-inhabting mosquitoes (Image: Ernesto Benavides via International Business Times)

If you’re staying at a high end resort, chances are there will be a well established insect control program. This typically includes widespread spraying for insecticides to knock down any mosquito populations. This may not completely remove risk but it will substantially lower potential exposure to mosquitoes. Again, don’t be complacent and take special care to avoid mosquitoes if taking a day trip to local villages or other tourist attractions.

Sleeping under a bed net is usually recommended in regions where malaria is an annual problem but this may not offer that much protection against Zika virus as the mosquitoes that spread the virus primarily bite during the day. If you’re planning on taking some afternoon naps, make sure it is under a bed net. A range of insecticide treated bed nets are available from your local camping store.

There is also an ever increasing range of “pre-treated” insect repellent clothing but evidence is scarce on just how effective these are at preventing bites. Treating clothing with insecticide (e.g permethrin) yourself would be a better option but don’t expect that wearing treated clothing means you don’t have to put insect repellents on exposed skin.

Safe and effective use of mosquito repellents

There will be anxiety among many about using insect repellents while pregnant. Are they safe? Will they impact the baby?

Without doubt, the most commonly used, safe and effective mosquito repellents is DEET (I’ve written about these repellents extensively, see here and here but I’ll summarize below). This is found in lots of major commercial brands and is a mainstay in the recommendations issued by health authorities the world over. Problem is, it can be hard to find information on how to choose and use the repellent that’s right for you and your situation.

The first point to remember is that the the strength of the formulation determines how long you’re protected against mosquito bite, not how many mosquitoes are kept away. For example, a 10% DEET based repellent may provide 2h protection, a 20% formulation may provide 4h protection. When choosing a repellent, think about how long you need protection for and how frequently you’re happy to reapply.

Secondly, the repellent must be applied as an even coverage on all exposed skin. If there are “gaps” in the application, mosquitoes are sneaky enough to pick a spot to bite. In the case of the mosquitoes that spread Zika, dengue and chikungunya viruses, pay special attention to application around the lower legs and feet, that’s where they like to bite.

Be sure to reapply repellent after swimming or sweaty exercise too.

There is no need to apply mosquito repellents to skin under clothing.

Are repellents safe to use when pregnant?

Health authorities and regulatory agencies rarely provide specific warnings on the use of insect repellents by those pregnant. While there haven’t been many clinical trials, these papers (here and here) demonstrate a lack of documented significant health risk associated with the used of DEET-based repellents. Most notably, a study of almost 900 women using mosquito repellent in the second or third trimester and no adverse neurologic, gastrointestinal, or dermatologic effects were observed in women or their infants for a year after birth.

It is important to balance the distinct lack of evidence of major health risks associated with repellents to the rapid rise in microcephaly in Brazil. Repellents can stop mosquitoes bites, stop mosquito bites and remove the risk of infection. If you use registered mosquito repellents as directed on the label, they are effective and safe.

Common sense must prevail. Even if you’re concerned about the use of repellents, make some compromises while still protecting yourself for infection. Choose a lower dose DEET-based repellent and reply more often. This is a better approach than trying a repellent that hasn’t been proven effective.

I’m often asked what formulation works best. There are aerosol sprays, roll-ons, pump-pack sprays, creams, gels, lotions and even towelettes. There really haven’t been many scientific studies looking at which if these formulations work best, and for good reason. As the active ingredients in these formulations are the same, it doesn’t really matter. The critical issue is to choose a formulation that you’re most comfortable using to ensure you get a good coverage over exposed skin. I like creams and pump-pack sprays but I generally apply the product to my hands first and them spread across skin.

Always ensure you avoid getting repellent in your eyes or any cuts or abrasions.

I don’t like the smell or feel of mosquito repellents!

There is often a temptation for those who dislike DEET to use a “natural“, plant-based repellents. Notwithstanding that these products provide shorter periods of protection, tea-tree oil (particularly when used in home-made concoctions) also has the potential to cause skin irritation. While plant-based mosquito repellents may offer some protection against nuisance-biting mosquitoes, they shouldn’t be relied on to prevent mosquito bites in regions of mosquito-bore disease outbreaks.

Many health authorities recommend para-Menthane-3,8-diol (PMD), a product commonly known as “oil of lemon eucalyptus”. This is not an essential oil but rather the by product of the distillation process of Corymbia citriodora. The product does repel a range of biting insects and there is no evidence suggesting it should not be used in pregnancy. However, in Australia, this product is generally more difficult to find in grocery stores and pharmacies than DEET- or picaridin-based repellent formulations.

It would be brilliant if there was a non-topical options for stopping mosquito bites. Unfortunately, there is nothing that has been proven effective. Do not rely on mosquito repellent wrist bands as they do not provide adequate protection against mosquitoes. Also, remember that there is nothing you can eat or drink that will stop you being bitten by mosquitoes.

Rounding out the advice on mosquito repellents, make sure you pack some before you leave. You can never be sure of what products will be available at your destination or whether it has gone through the process of registration (e.g. APVMA in Australia or EPA-registered in the U.S.). It is not unheard of for mosquito repellent stock to sell out during outbreaks of disease.

Lastly, if you’re travelling to regions experiencing dengue, chikungunya and Zika virus outbreaks, don’t necessarily expect to be swarmed by mosquitoes in the same way you will around many of Australia’s coastal wetlands. Don’t be complacent if there are only a few about, remember, it only take one bite to transmit a pathogen. Don’t wait until you notice mosquitoes biting, wake up and put on that repellent.

There is a great set of questions with answers provided by the CDC for pregnant travellers on Zika risk and prevention and here is another reminder of the travel advice provided by the CDC and Australian Government for pregnant women.

If planning to travel while pregnant, consult your local doctor or travel health clinic for advice.

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.

 

 

Perfume won’t protect you from mosquito bites

VictorissecretThe headlines have been awash with claims that a popular perfume may repel as many mosquitoes as those regularly recommended by health authorities. Could it be true?

In short, no. There is little surprise that the results of this recently published study in the Journal of Insect Science has attracted so much attention. Everyone loves the idea that some unexpected substance could be used as a mosquito repellent. Even better if it performs as well, or even better, than those such as DEET or picaridin that are widely recommended by health authorities.

The scientists tested a range of commercial insect repellents. Three formulations of DEET based repellent, an oil of lemon eucalyptus (aka PMD) based repellent, three botanical-based repellents, a mosquito repellent patch (Vitamin B), a product not specifically designed as a repellent but often quoted as being effective (Avon skin so soft) and the perfume. Why include the perfume at all?

The logic behind including the perfume was a good one. It is often said that floral perfumes and other cosmetics attract mosquitoes. I’ve never thought this is actually the case. I mean, there is stronger evidence that mosquitoes re attracted to smelly foot bacteria than pleasant smelling cosmetics! I always suspected that the idea comes from the fact the mosquitoes (mostly the non-biting males) will feed on plant sugars. However, it was worth including in this study. Always good to gather some quantitative evidence on the response of blood-seeking mosquitoes. It could be a good opportunity to bust (or perhaps confirm) some urban myths.

I’ve written before about how you can test mosquito repellents. While the “arm-in-cage” methodology typically provides the best indication of how a mosquito repellent will perform, there are other methods commonly employed. In this case, the researchers used a “Y-tube” setup. This system basically allows mosquitoes to make a choice as to whether they preferentially fly towards one or the other ends of the tube. If you insert a hand treated with a substance into one end and another untreated hand as a control into the other, it is possible to measure the overall repellent effect by tracking the movement of mosquitoes.

Firstly, it is interested to note that the researchers found that some mosquitoes were attracted to the hand treated with DEET. If I was conducting an “arm in cage” test. I would be very surprised if I had any mosquitoes biting a DEET-treated arm within 2h of application. In one study, I found an approximately 7% DEET-based repellent stopped bites for a little under 2h. It makes me wonder how many mosquitoes may fly up to tube towards the treated hand but, given the chance, would actually bite the hand?

Fewer mosquitoes were attracted to hands treated with oil of lemon eucalyptus, not surprising either given this product is regularly recommended as an effective repellent by health authorities.

The testing of the perfume provided the headline grabbing results! For the first couple of hours, there wasn’t much difference in the proportion of mosquitoes repelled by the perfume compared to the other repellents. Why? It may be related to the strength of the odour overpowering the sensory organs of the mosquito. I think this is how some strongly smelling essential oils can provide some protection. It masks the normal chemical cocktails of smells on our skin that attracts mosquitoes.

We all know how overpowering the smell of some cosmetics can be. In this case of this experiment, a relatively high dose of the products as used. The authors make note of this too when they state “It must be noted that the concentration of perfume we used in this test was rather high and that lower concentrations of the same fragrance might have different effects.”

Could this perfume be used as a repellent?

Studies like this provide some fun headlines but they can be misleading to the public. What “works” for a relatively short period in a small laboratory based study does not necessarily stand up the practicalities of real life.

Notwithstanding the expense (the perfume is about AUS$80 for 100ml compared to less than AUS$10 for about the same amount of DEET-based repellent) I must admit that for some of these products, the smell can be so overpowering that applying them to large areas of skin would probably be more unpleasant than the bites of mosquitoes!

When trying to help the public choose and use mosquito repellents more effectively, it is critical that health authorities stick to products that are currently registered for use as a mosquito repellent and that have been demonstrated to provide suitable protection from biting mosquitoes over extended periods of time.

Perhaps the most important finding of the paper is not that the perfume repelled some mosquitoes but that patches infused with Vitamin B provide absolutely no protection from mosquitoes. This is one urban myth that never really seems to go away!

We also know that Victoria’s Secret perfume doesn’t stop bed bugs invading lingerie stores!

[The image used at the top of this piece is taken from here.]