Why do mozzies love some people but not others?

Some people suffer greatly from mosquito bites, while others never get itchy—and so might believe they have nothing to fear. As Dr Karl Kruszelnicki discovers, mosquito preferences only explain half of this story. Why are some people mosquito magnets, while others seem to be blissfully bite-free? It’s a two-part answer. Number one—it’s chemistry! Mosquitoes love people who drink beer, and they prefer pregnant women to non-pregnant women. Number two—some people who actually do get bitten, don’t notice, or even react to, the bite. Surprisingly, this can be a major health concern. Let me explain by starting with the mosquito. It’s only the female mosquito that bites you. No hard feelings, it’s just business—she needs the protein in your blood for her babies. This is probably the time to mention mosquito repellents. Most of us have heard of DEET. DEET works to repel mosquitoes in two ways—by its odour in the air, and by direct contact when the mozzie (as we Australians call it) tries to land on your skin. By the way, the protection DEET provides lasts for hours, while that from citronella lasts only minutes. But let’s assume you have no mosquito repellent. If the female mosquito merely jabbed you, took a microscopic amount of blood and left no scars or mementos of her visit—she wouldn’t even be an annoyance. However, on occasion, your immune system reacts to her visit and you might be left with an itchy swollen lump on your skin. But it can get really nasty when the mosquito herself is infected—and can pass that infection on to you. Thanks to evolution, different mosquitoes can carry different germs. Aedes aegypti can carry the Dengue virus and the Zika virus. A different species of mosquito, Anopheles gambiae, can carry malaria, which is a parasite, not a virus. Malaria continues to be a major health problem. In 2015, malaria infected 214 million people, and killed 306,000 of them—and the vast majority were children younger than five. Aedes vigilax, the saltwater mosquito, carries the Ross River virus. When it comes to finding their next nibble, mosquitoes are first attracted by heat and carbon dioxide. Once the mozzies get really close, they can be either attracted—or repelled—by any of the several hundred chemicals you exude. These chemicals can come from your diet, from bacteria on your skin, and from your genetic inheritance. This is a complex and poorly understood field, so it’s still early days for learning about the several hundred chemicals you emit. Some of the chemicals you release are programmed by your DNA—your genes. One study looked at identical twins, whose DNA is virtually the same, and non-identical twins, whose DNA is partly the same and partly different. Mozzies were equally attracted to each identical twin in a pair, but this didn’t happen with the non-identical twins. So your genes affect how much mosquitoes ‘love’ you. At least one species of mosquito loves blood group O, but we don’t know how all the other species feel about blood group O—or, indeed, any other blood group. But it’s not just your DNA that affects your odour, it can be your diet. Sorry to say, but eating garlic or vitamin B1 does not repel mosquitoes. We know that the Aedes aegypti mosquito loves the smell of lactic acid. So if you happen to be in her wetlands, don’t do any sweaty exercise. Mosquitoes love people who drink beer, and they prefer pregnant women to non-pregnant women. We also know that men are more attractive to mozzies—because their greater body mass is linked to more heat and more carbon dioxide emission. Darker coloured clothing can absorb heat better than lighter colours, so making you more attractive to mozzies. Hairy arms or legs can be a physical deterrent to mozzies. So Miss Mozzie might be expected to make a move on a smooth-skinned beer-drinking bloke wearing dark clothing. Sure enough, the mozzies can be fussy. As an example, the malaria mosquito, Anopheles gambiae is attracted to Limburger cheese. By an unfortunate coincidence, the bacteria that make this cheesy smell are closely related to the bacteria on some people’s feet—which mozzies will happily jab with their pointy proboscis. Mosquito saliva is complicated. Some people react very strongly to certain chemicals in the saliva—and so complain bitterly about being bitten. But other people can get bitten and don’t react. However, not reacting is very different from not being bitten. These people could still get bitten, but would not know it. They might think that they are unattractive to mosquitoes, but they’d be wrong. In that case, they might not apply the mozzie repellent DEET, could get bitten and not know it, and then suffer the mosquito-borne infection (Dengue fever, malaria, Zika virus, etc.). So mozzies can make you sick—even without making you itch.

ABC Science News, 27 September 2016 ;http://www.abc.net.au/news/ ;