08.06.13 8:45 AM ET
Mosquitoes Love Some People More and Science Wants to Know Why
“It’s like stepping out of the airport in Florida,” says Dr. Leslie Vosshall, as we move from her air-conditioned laboratory at New York City’s Rockefeller University into the mosquito chamber. The air is hot and damp, and lining the walls are clear-plastic-labeled “bug dorms.” Each box contains a swarm of buzzing mosquitoes. They cling to the walls, hang off the ceiling, bounce persistently against the mesh opening, trying to get at us. “I should have asked if you have a phobia,” says Vosshall.
I don’t have a phobia, exactly, but like a lot of people I feel uniquely hunted by mosquitoes. At barbecues and on hikes, mosquitoes always seem to seek me out. I’ve tried DEET spray, citronella torches, permethrin butane repellers of the sort used by hunters in the South. I once bought a mosquito net on Amazon at 3 a.m., after accidentally swatting myself in the ear while half asleep. But nothing seemed to change the fact that the best repellent, for everyone else, was to have me around. I’m at Vosshall’s lab because she’s just begun a six-month study of this phenomenon. I’ve come here for answers.
And I am not alone. Lately there are a lot of people trying to learn more about how mosquitoes choose their prey, people funded by the Bill and Melinda Gates Foundation, the National Institutes of Health, or, as Vosshall is, the Howard Hughes Medical Institute. Mosquitoes, some researchers say, are the deadliest creatures in the world, killing over a million people a year through diseases like malaria, dengue, and West Nile. They’ve proven remarkably good at adapting to new regions, traveling the world in water inside used tires, ornamental bamboo plants, or shipping containers, and quickly making themselves at home. Yet until recently many of our strategies for fighting them have remained ad hoc, limited by how little we know about how mosquitoes hunt.
Vosshall reassures me that it’s true, mosquitoes really do find some people more attractive than others. A lot of people doubted that this was the case, she says, despite the plethora of anecdotal evidence. Skeptics thought that everyone gets bitten about the same, but people whose immune systems react more strongly to the mosquito’s anticoagulant simply notice bites more. Vosshall designed her pilot study to weed out this variable. Volunteers exposed a patch of their forearm and a machine blew air across it, enticing mosquitoes into a trap. She found that mosquitoes found some people’s body odor four times as appetizing as others.
She also found that those people don’t really know whether they’re mosquito super attractors or not. In an attempt to get a diverse sample for her study, she designed a questionnaire asking people whether mosquitoes sought them out or ignored them. “We found zero correlation,” she says. What she concluded is that mosquito attractiveness is all relative. You could be a 40, she says, on an attractiveness scale of 1 to 100, but everyone else in your family is a 10, so mosquitoes go to you. But if you start hanging out with a bunch of 80s, you’ll be safe. “It could be a Craigslist thing,” she says. “Hey, we’re having a picnic, we’ll pay you $100 to come attract all the mosquitoes.” Since she started researching mosquitoes four years ago, she’s spoken to plenty of people who do this already, though less transactionally. “I hear people say, ‘I like bringing my husband to outdoor concerts because he gets all the bites, or I sit next to Uncle Ted at barbecues, because then I don’t get bit.”
For the next six months, paid volunteers will come to the lab and have their mosquito attractiveness tested. Some will put their arms in the scent trap, like the one in the pilot study. Others will stick their arms in containers and let mosquitoes feed upon them. “We give them an iPad with HBO GO, so they don’t have to watch,” says Vosshall.
She’s looking for anything that correlates with attractiveness. “There’s a whole folklore about it,” she says, “sweet blood, blood type, diet, gender, skin temperature, body size, garlic, spicy foods, certain vitamins.” She’s spoken with people who say that before they became pregnant, mosquitoes loved them, but after, they were ignored, and visa versa. She spoke to one person convinced that his appendectomy made him less attractive. Another who thinks his blood pressure medication lessened his appeal. This summer, she’s been approached by vegans who claim mosquitoes leave them alone. That mosquitoes are more attracted to people who drink beer is one of the few things we know with some degree of certainty, she says, though we don't know why. Everyone has a mosquito story they want to tell her—not like when she was studying drosophila, the fruit fly. “I’ll listen to anyone who has a hypothesis,” she says. “I keep a file—somewhere in there is an answer.”
With the feeding mosquitoes, she’s looking to see whether attractiveness correlates with breeding success. Only female mosquitoes drink blood, and only for the extra protein they need to lay eggs. (The males eat nectar.) Have mosquitoes evolved to seek out people whose blood lets them lay more eggs, or does something else determine attractiveness? She says body odor is clearly key, but it could be that humans have a “bouquet,” with some elements of odor attracting mosquitoes and others repelling them. Attractive people could just lack the odorants that mosquitoes don’t like.
The reason it’s been so hard to pin down what makes some people more attractive than others, despite the copious anecdotal evidence, is that mosquitoes rely on many different variables when seeking blood. “They use some of the same strategies that humans use when hunting,” says Vosshall. “They take everything into account.” First they smell us, detecting molecules from bacteria on our skin and carbon dioxide from our breath. As they get closer, they start to take heat into account. And when they land, they taste us through receptors on their legs. Each element plays a role. Mosquitoes seem to like the way Vosshall smells, for example, but not how she tastes. “When I was hiking in New Jersey, mosquitoes flew up to me, landed, and flew off without biting.” She compares the phenomenon to vanilla: it smells delicious but tastes awful. “It’s almost like I have DEET.”
The more we know about how mosquitoes hunt, the better we can defend ourselves against them. DEET was developed back in 1952 by USDA chemists at the request of the military, but for decades no one knew how it worked—just that it did. Vosshall recently published a study in Nature in which removing a gene from mosquitoes robbed them of their ability to distinguish between humans and other animals—mosquitoes like the dengue-carrying Aedes aegypti have evolved to hunt humans in particular—and no longer smelled DEET. Yet they still disliked the taste of DEET once they landed on it, suggesting that DEET works both by blocking mosquitoes’ smell receptors and by tasting repulsive.
This is why basic research is important, says Vosshall. People keep saying we need something better than DEET, but no one has come up with something better in 60 years, she says, because we still don’t fully understand how mosquitoes operate. Vosshall compares mosquito research, and research on malaria and dengue fever that they carry, to research on HIV/AIDS. “It’s a disease, something is attacking you. You can’t just say we need to cure AIDS and start pulling stuff off the shelf. You need to understand how the virus works.”
“The mosquito is really wily, but the pathogen is even more wily,” she says. “Malaria is a shape-shifter, like HIV, always reshuffling the deck to make a new pathogen.” It evolves to get around vaccines. Mosquitoes adapt to bed nets, hunting in the daytime. They evolve resistance to pesticides. They hunt by carbon dioxide and heat when odor is blocked by repellents. Vosshall thinks the fight against mosquitoes will settle on an integrated approach—as happened with HIV and combination therapies—directed by a better understanding of what they’re fighting: a mix of nets, insecticides, repellents, and vaccines to fight mosquitoes and the diseases they carry to a draw.