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

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.

Perhaps 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.

 

Talking wetlands, wildlife and mosquitoes at the 2017 Australian Entomological Society Meeting

I’ll be in Terrigal, on the NSW Central Coast, for the 2017 Australian Entomological Society conference and taking the opportunity to present a summary of a number of collaborative projects undertaken in recent years, from working out how surrounding landuse influences the mosquito populations in urban mangroves to how important mosquitoes are to the diet of local bats.

Together with a range of colleagues, I’ve been undertaking research into the factors driving mosquito and mosquito-borne disease risk in urban wetlands. It is a complex puzzle to solve with more than just mosquitoes determining local pest and public health risks. However, with outbreaks of mosquito-borne Ross River virus on the rise in recent years, including urban areas of Australia, there is a need to better understand the factors at play.

There is a range of factors that may increase the risk of Ross River virus, they include suitable wetlands, wildlife reservoirs of the pathogen and mosquitoes. Understanding the mosquitoes associated with urban estuarine and freshwater wetlands is critical.

Investigating the role of surrounding landuse in determining the mosquito communities of urban mangroves, we found that industrial and residential areas tended to increase abundance of mosquitoes, perhaps due to direct or indirect impacts on the health of those mangroves. We’ve found previously that mosquitoes problems are often associated with estuarine wetlands suffering poor health, perhaps this is determining the increased mosquito risk we identified? You can read more in our publication here.

Expanding the investigation to look at urban freshwater wetlands, it was found that there was a high degree of variability in local mosquito populations and that each wetland needed to be assessed with consideration to be given to site-specific characteristics. You can read more about our work investigating mosquito assemblages associated with urban water bodies in our publication here.

More research is underway in this field and my PhD student, Jayne Hanford, has already started collecting some fascinating data on wetland biodiversity and local mosquito populations.

While the focus of our studies is often prompted by concern about Ross River virus, interestingly, in recent years we’ve found considerable activity of Stratford virus. This is not currently considered a major human health concern but given how widespread it is, it raises concerns about the suitability of local wildlife, even in Western Sydney, to represent important reservoirs of mosquito-borne pathogens. You can read more about Stratford virus in our publication here.

The final piece of the puzzle is to understand the ecological role of mosquitoes. Where their potential health threats are deemed significant, how could management of mosquito populations have unintended consequences for other wildlife. What about the animals that eat mosquitoes? A number of years ago we did some research to determine the importance of mosquitoes in the diet of coastal bats. While there was no indication that mosquitoes are a critical component of their diet, they are still being snacked on and mosquito control programs need to consider any local ecological impacts.

Now, how am I going to squeeze all this into 15 minutes….

The presentation abstract is below:

What drives mosquito-borne disease risk in urban wetlands?

Webb, C. (1, 2), J. Hanford (3), S. Claflin (4), W. Crocker (5), K. Maute (5), K. French (5), L. Gonsalves (6) & D. Hochuli (3)

(1) Department of Medical Entomology, NSW Health Pathology, Westmead Hospital, NSW 2145; (2) Marie Bashir Institute of Infectious Diseases and Biosecurity, University of Sydney, Camperdown, NSW 2006; (3) School of Life and Environmental Sciences, Faculty of Science, The University of Sydney, Camperdown, NSW, 2006; (4) Menzies Institute for Medical Research, University of Tasmania, Hobart, TAS, 7000; (5) Centre for Sustainable Ecosystem Solutions, Biological Sciences, Faculty of Science, Medicine & Health, University of Wollongong NSW, 2522; (6) School of Arts and Sciences, Australian Catholic University, North Sydney, NSW, 2060.

Managing pest and public health risks associated with constructed and rehabilitated urban wetlands is of increasing concern for local authorities. While strategic conservation of wetlands and wildlife is required to mitigate the impacts of urbanisation and climate change, concomitant increases in mosquitoes and mosquito-borne disease outbreak risk must be addressed. However, with gaps in our understanding of the ecological role of mosquitoes, could control strategies have unintended adverse impacts on vertebrate and invertebrate communities? A series of studies were undertaken in urban wetlands of greater Sydney to investigate the role of land use, wetland type and wetland aquatic biodiversity in driving the abundance and diversity of mosquito populations. A diverse range of mosquitoes, including key pest an vector species, were found in urban environments and mosquito-borne pathogens were detected in local populations, implicating local wildlife (e.g. water birds and macropods) as potential public health risk factors. Estuarine wetlands are locally important with the percentage of residential land and bushland surrounding wetlands having a negative effect on mosquito abundance and species richness while the amount of industrial land had a significant positive effect on species richness. Mosquito control in these habitats is required but insectivorous bats were identified as mosquito predators and the indirect implications of mosquito control should be considered. The aquatic biodiversity of urban freshwater wetlands influenced the species richness of local mosquito populations indicating vegetation plays an important role in determining local pest species. However, the matrix of wetland types also influences the abundance of mosquitoes in the local area. These results demonstrate the need for site-specific investigations of mosquito communities to assist local authorities develop policies for urban development and wetland rehabilitation that balance the need for conservation with reduced public health risks.

To keep up to date on what’s happening at the conference, check out the program online or follow the conversation on Twitter.

 

What do dogs and bird drawings have to do with conservation?

Ask a dog owner why they let their pets chase shorebirds across the local mudflats and the response will usually be “Don’t worry, they never catch ‘em”. But dogs don’t have to catch the birds to have a serious impact on them, especially when they’re chasing them off wetlands full of mud dwelling invertebrates that the birds love munching on.

Perhaps if the community was more aware of how vulnerable the shorebirds are,  maybe they’d be more likely to keep their dogs on a leash, off the local mudflats and the birds could feed in peace. But how can we get the message across to local residents?

A global traveller and annual visitor to local wetlands

The Bar-tailed Godwit, Limosa lapponica, is a migratory bird that flies all the way from Alaska to feed in Australia’s coastal wetlands each summer. A medium sized brown and white bird with a distinctively long upturned bill, the Bar-tailed Godwits are commonly seen feeding on aquatic insects and molluscs along the muddy foreshore of Sydney’s Parramatta River. They’ve become an iconic bird of the region, even featuring as one of the “mascots” of the “Our Living River”, an initiative to improve the health of the  Parramatta River.

Each spring they make the trip from the Arctic to Australia (non-stop flight around 11,000km has been recorded!) but to make the long flight back to Alaska, the Bar-tailed Godwits rely on finding enough food among the wetlands during their time along the river. The birds are commonly seen from spring to early autumn, often in flocks of a dozen or more, foraging in local mudflats. But their habitats are under constant threat from pollution and the encroachment of mangroves.

One location where the birds are commonly seen is Hen and Chicken Bay in the City of Canada Bay Council local government area. The suburbs of Abbotsford, Wareemba and Five Dock are nearby and the foreshore pathways and parklands are popular locations for exercise and recreation by both residents and visitors (please check out my favourite cafe, The Cove Dining Co, when you’re next visiting).

Unfortunately, the ever increasing human population, together with the easy access to the foreshore, brings threats to the Bar-tailed Godwits and their habitats.

People, pets and wildlife conservation

The Bar-tailed Godwits are quite tolerant of people. They’re happy to keep feeding while people and pets walk past. It provides a wonderful opportunity for the community to see these unique global travellers up close.

They have a tough time though when constantly chased by dogs or disturbed by people. Many times I’ve seen dogs running along the low tide shoreline, no doubt it is fun for both the pet and their owner. I have no doubt there is complete ignorance of the unintended consequences and that this may be a problem for the birds.

Awareness of shorebird conservation issues among those sharing habitats is a critical issue to address.

Reducing the impact of dogs on shorebirds is an issue faced by authorities around Australia as well as overseas. Beyond wetland habitats, studies have also shown dog walkers can have a negative impact on bushland birds too.

Putting a fence up around the wetlands isn’t a solution. The use of buffers is impractical and nobody wants to start setting traps or baits to stop the dogs. What else could be tried?

Illustration and wildlife conservation

Inspired by a project in Ku-Ring-Gai Council where school students drew signs encouraging dog owners to pick up after their pets, a similar approach was proposed for Hen and Chicken Bay. Could signs featuring the artwork of local students help get the message across?

Jointly funded by the Abbotsford Public School Parents and Citizens Association, and City of Canada Bay Council (Council’s Bushcare Department contributed funds in additional to a Canada Bay Community Grant), a series of educational signs were installed along the foreshore pathway of Hen and Chicken Bay as well as nearby Henry Lawson Park and Halliday Park.

When trying to decide on the design of the signs, it was important that there was some community ownership of the messages, they were aesthetically pleasing and lacked a serious authoritarian tone.

A competition was launched among students at Abbotsford Public School to draw Bar-tailed Godwits, one illustration was selected from each grade to be showcased on the signs. While the student’s drawing may not have been taxonomically accurate, they certainly reflected the great enthusiasm for the birds and this conservation process.

The school itself was wonderful and a component of the curriculum involved the students learning about the Bar-tailed Godwits and the importance of local wetlands. A local author, Jeannie Baker, had recently published the wonderful children’s book “Circle”, and a copy was purchased for almost every classroom and became a reference point for learning about the birds and their amazing global migratory journal to the local wetlands each year. Directly involving the students not only yielded some wonderful illustrations to personalise the signs but it also engaged the broader school community and assisted raising the profile of the project.

Along with an unique illustration on each sign, the key messages promoted were that the Bar-tailed Godwits visits the local mudflats from the Northern Hemisphere each year between August and April and to please don’t allow your dog onto the mudflats and please don’t allow off-leash dogs to chase birds. Dog owners were also directed towards to local off leash area in a nearby park.

An attitudinal study of dog owners in Victoria, Australia found that “Dog owners were more likely to feel obliged to leash their dog when they believed other people expected dogs to be leashed, and when they believed their dog was a threat to wildlife or people.” While this study was associated with beach-nesting birds, perhaps a similar attitude exists among those dog owners prone to allowing their dogs to run across bird-filled mudflat?

The signs were unveiled at an official launch in December 2016 by the Mayor of Canada Bay Council, Helen McCaffrey, and in attendance were representatives of Birdlife Australia, Our Living River and dozens of Abbotsford Public School students. Judy Harrington, Sydney Olympic Park Authority, kindly gave a talk to students on the day about the Bar-tailed Godwits and in the shadow of an approaching summer storm, a class alongside the local wetlands was a wonderful way to launch a novel and engaging project of environmental conservation.

Got any other ideas about how we can protect our local wetlands wildlife? Join the conversation on Twitter!