|Why do mosquitoes target some of us while others remain unscathed? Roger Dobson reports.|
It's a warm, balmy evening, the music's playing, the barbecue's smoking, and the food is ready . . . and then they arrive. First one, then more, and finally the whole squadron zooms in, picking out victims and attacking them with all the zeal and single-mindedness of kamikaze pilots.
No amount of shouting, arm-waving or handslapping can deter the invaders as they swoop, intent on blood. But the mosquitoes appear to be picking out some people and ignoring others. The blonde by the barbecue, a white-suited man by the trees, the woman who has been sweating all evening and the next-door neighbour with halitosis and smelly feet seem to be the most popular targets.
As the arm-flaying victims flee indoors, one of them looks backwards and upwards and lets out a forlorn cry that will be repeated by tens of thousands of mossie victims this summer: "Why me?"
Despite years of research, and centuries of human suffering, the answer remains unclear: much about how and why, and even if, the biting mosquito chooses her targets — in most cases only female mossies bite — remains a mystery.
Any clues about the hunting habits of the mosquito and how they can be tamed are, of course, important because the insect carries more diseases and kills more people than any other creature. Malaria, just one of the diseases spread by the bug, kills more than one million people a year (that's one victim every 13 seconds).
But malaria is not the only weapon of mass destruction in the mosquito armoury: there's also West Nile virus, several strains of encephalitis, dengue fever, Rift Valley fever, yellow fever and elephantiasis.
An added problem is that the different types of mosquito — and there are thought to be more than 3000 of them — have particular behaviours, and carry different diseases. Some of the more dangerous diseases are, for instance, spread by Anopheles mosquitoes that work at night, while the Aedes mosquito that bites during the day carries dengue and yellow fever.
The mosquito is not only found in exotic climes, either. There is some evidence that numbers are increasing again in Britain, and the risk that they may carry malaria is growing, too. Weather changes brought on by global warming mean that large parts of England and Wales are already at risk of malaria.
Certainly Australia is vulnerable, especially in the north where mossies bite all year round. Malaria was largely eradicated in Australia more than 15 years ago. However, both global warming and the worsening problems with the disease in neighbouring countries where Australians travel increase the risks.
In warmer months, the likelihood of getting nipped extends south to NSW, Victoria, South Australia and even Tasmania. Since 1997, small outbreaks of Ross River virus appeared for the first time on the fringes of the metropolitan sprawls in Sydney and Melbourne. Ross River and Barmah Forest are the two most common mosquito-borne viruses in Victoria.
But how do you know if you have a higher risk of being a target, and just what kind of people the mossie is most attracted to?
"That's a hard one to answer," says Professor Andrew Spielman of the Harvard School of Public Health, and one of the world's leading experts on the mosquito.
"The US military tried to come up with the solution and completely failed. What we do know is that some people are more attractive to them, but who and why is still unclear.
"There are visual elements involved — although the mosquito does not detect form — and movement is important. Someone in light-coloured clothing standing against dark green trees, for example, might attract their attention. Carbon dioxide in the breath is also an attractant, and once you get in close there are different amino acids and other things that seem to excite them."
Research shows that mosquitoes can detect carbon dioxide in exhaled breath at a distance of 15 metres or more. It is also thought that there are volatile compounds in the breath, as yet unidentified, that the insect uses as a homing device.
Lactic acid, secreted by sweat glands, is another favourite, and is one of the reasons why people who work up a sweat waving their arms about to defend themselves become more of a target. A theory with less grounding in research is that fragrances in some shampoos, soaps and powders are used as homing beacons by the insects.
Mosquitoes are also attracted to contrasts, a fact that lies behind one theory that they are attracted more to platinum blondes and redheads who are, it’s proposed, more likely to stand out in a crowd. A variety of other research has suggested that ovulating women, people with smelly feet and those who have garlic on their breath are also preferred targets.
The number of theories about how mosquitoes choose their targets is almost as great as the list of devices that are sold to combat them. Chemicals, sprays, nets, buzzers, ultrasound devices, electronic zappers, smoking coils, flashers, blue lights, natural oils, homeopathy remedies, lavender, cloves, drinks and scented candles are just some of the products in the anti-mosquito armoury.
"Natural repellents work for a shorter time, and lemon eucalyptus is particularly effective," says Dr Jo Lines, senior lecturer at the London School of Hygiene and Tropical Medicine. "But there are also many that don't work as well. What could be used more and which does work very well is an electric vapourising device that emits safe but effective repellents.
"The buzzing devices really don't work. We carried out some research a while ago, and we found that mosquitoes fed happily off a hand that was actually holding a buzzing device."
Nets treated with insecticide and draped over the bed are effective, and so, too, are citronella candles. But eating various substances, such as garlic and thiamine, probably isn't so much use. Calamine lotion is a help for those people bothered by the itch left by the bite, as is a tiny drop of ammonia.
New research at Iowa University shows that naturally produced catnip oil deters mosquitoes 10 times more effectively than DEET, the active ingredient in most insect repellents. Research on mosquito coils has also found them to work well.
The aromatherapy solution is to apply one drop of neat lavender oil per bite, while herbalists advise using cloves as a deterrent. Homeopathy also has a number of suggestions, from using ointments made from Apis Mell (a natural compound extracted from honey bees) to treat mosquito bites, to applying catnip oil three times a day as a repellent.
"All homeopathic remedies are best dealt with by the individual, but there is one first aid remedy for mosquitoes and that is staphisagria, which is made from the delphinium flower," says homeopath Annette Middleton. "If the person takes one tablet daily they should avoid being bitten. I also suggest they take vitamin B6, which helps people to avoid mosquitoes too."
With such a wide range of protectors and treatments on sale, it's not surprising that surveys show most people are confused about what to do.
Spielman, however, has kept mosquitoes in check and malaria at bay for about 70 years by following this formula: "I would wear clothing that had been sprayed with permethrin, a synthetic material which occurs in some flowers. Then, I would use DEET oil on the exposed parts of my skin if I were very concerned. I would cover up as much as I could, but most of all, if it was a malarious area I would avoid being in the village after dusk."
One line of defence he certainly wouldn't use is that of sound-based devices: "I have never seen any experiment that in any way shows that mosquitoes are repelled by any sonic thing," he says.
A new device on the block is hoping to change all that. A South Korean company is offering its mobile phone users a service that it claims will repel mosquitoes. Phones will emit a special sound wave — inaudible to human ears — to annoy any mosquitoes within a one metre radius.
Just what mossies do when they are annoyed by mobile phones, however, is as yet unresearched.