Evolution October 2011

Building a Better Bug

Can scientists beat malaria by reengineering the mosquito?
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Edwin Remsberg/Alamy

Ridding a region of malaria is, in theory at least, fairly simple. Female mosquitoes transmit the disease when they make a meal of infected blood, gestate the malaria-causing parasites (called sporozoites), and then inject them into the bloodstream of another victim at a later feeding. Break any part of that cycle, and the parasite can’t reproduce; keep up that effort, and you can halt any malaria epidemic. In practice, however, breaking the malaria cycle takes a lot of work. Italy and the American South have all but eradicated the disease by draining swamps, spraying insecticides, improving medication, and introducing air conditioners, screen doors, and mosquito nets. But for many places—including most of Africa—the resources for a broad, sustained effort are out of reach.

Which is why I found myself in a basement laboratory in central Italy last spring, peering into a cage of mosquitoes. The scientist in charge of the lab, Andrea Crisanti, a parasitologist and microbiologist at Imperial College London, has developed a technique that can spread a genetic modification through generations of the insects. “In one or two seasons, you can thoroughly attack an entire wild population at a chosen site,” he says. This breakthrough, heralded in Nature last spring, opens the possibility of replacing a region’s malarial mosquitoes with genetically modified competitors. Give them a trait that cuts the transmission of malaria, and the epidemic will likely be beaten. “We want to ask the mosquitoes to do what humans have not been able to,” says Crisanti, who is considering three promising approaches. One modification would render the insect unable to recognize the human scent. Another would significantly reduce the mosquito’s lifespan, giving the parasite less time to reproduce before dying along with its host. The one that Crisanti thinks is likeliest to succeed would kill female mosquitoes in the embryonic stage, leaving the males to spread the modification until the resulting sex imbalance caused the population to crash.

On the day I visited, most of the mosquitoes being bred in Crisanti’s lab were unmodified, but a few had been engineered to carry a jellyfish gene that rendered their sperm fluorescent so that scientists could study their reproduction. A graduate student was busy transferring eggs from a shallow dish into Tupperware tubs. I watched as a single mosquito slipped its netted cage, took to the air, evaded the student’s halfhearted swipes, and vanished into the shelves that lined the walls. A few minutes later, as I leaned forward for a better look at the swarming in the cages, I rubbed at an itch on my arm and looked down. A mix of blood and insect was smeared across my skin. Oh great, I thought. I’ve been bitten by a genetically modified mosquito.

A broad public consensus holds that one messes with nature at one’s peril; that Darwinian selection will outmaneuver the cleverest constraints; that, in words made famous by Jurassic Park (the layman’s primer on genetic engineering), “Life, uh, finds a way.” Crisanti is all too aware of that consensus, but he’s convinced that the dangers of his work are minimal, and pale in comparison with the problems posed by existing technology (cost, toxicity, mosquito resistance). Still, to measure the possibility that genetic change will drift down the DNA strand or migrate to similar species or predators, Crisanti is building a bigger lab, large enough to fill with soil, plants, puddles, and upwards of 100,000 mosquitoes, and rigged to simulate the temperature, light, and humidity of an equatorial climate. “You want to convince the local and international authorities about the efficacy and safety,” he says. These large-scale laboratory tests will be followed by further experiments in Italy, first with the mosquitoes contained but exposed to the elements, and then in an open-air, but geographically constrained, pilot trial—on an island in Africa’s Lake Victoria, perhaps.

As Crisanti steps toward a pilot trial, he is betting that the prospect of liberating some of the world’s poorest from a disease that kills close to a million people every year will outweigh misgivings about the release of swarms of self-replicating, genetically modified flying organisms. But risks and benefits look different depending on where you’re sitting. If the trial succeeds, its benefits will be local. The ramifications of spreading the new mosquitoes, less so. One day soon, if all goes as planned, a country in Africa will weigh the factors and make that decision.

Stephan Faris is working on a book about global immigration.
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Stephan Faris is working on a book about global immigration.

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