Biotech firm announces results from first US trial of genetically modified mosquitoes

released into the environment, the engineered males should mate with wild females, and their female offspring will die before they can reproduce. Male offspring will carry the gene and pass it on to half of their progeny. As each generation mates, more females die, and the A. aegypti population should dwindle.

To make sure the mosquitoes follow this scheme, researchers placed boxes of Oxitec mosquito eggs on private properties in the Keys and surrounded them with traps, covering a radius of more than 400 metres. Some traps served as egg-laying sites, and others caught adult mosquitoes.

The researchers found that the males that hatched from the eggs typically travelled within a one-hectare area around the release box — the same range over which wild A. aegypti fly. The engineered mosquitoes, which don’t bite, mated with the wild population, and wild females laid eggs in Oxitec traps, as well as in sites such as flower pots, rubbish-bin lids and soft-drink cans.

Oxitec researchers collected more than 22,000 eggs from the traps and brought them back to their laboratory to hatch under observation. The firm reported that all females that inherited the lethal gene died before reaching adulthood.

The team found that the lethal gene persisted in the wild population for two to three months, or about three generations of mosquito offspring, and then disappeared. No mosquitoes carrying the lethal gene were found beyond 400 metres of the release points, even after several generations. Oxitec monitors the sites for ten weeks after the last lethal gene-carrying mosquito is found.

“I like the way they’re going about it,” says Thomas Scott, an entomologist at the University of California, Davis. “They’re doing it in a systematic, thoughtful way. But they have a lot of work ahead of them,” he says. The pilot study was not intended to determine how well the method suppresses the wild population. Oxitec plans to gather that data in an extension of the Florida Keys study. It first needs approval from state regulators, but hopes to begin soon. The company plans to release mosquitoes at a second study site in Visalia, California, where it is building a research and development facility.

But these expanded studies will not assess whether Oxitec’s method reduces transmission of dengue or other viruses carried by A. aegypti. “They’re not going to be able to do a trial to show that it actually has a public-health impact,” Scott says. “There’s not enough Aedes-transmitted viral infection in the Florida Keys,” or anywhere in the continental United States to do that kind of study, he says. To run such an experiment, the company would have to invest in a controlled trial elsewhere, and run the study like a clinical trial, which would be enormously expensive.