Mosquitoes Bred to Die

Genetically Modified Bugs
May Be Released to Stem Disease

By GAUTAM NAIK
June 12, 2008

OXFORD, England -- In a hot and humid laboratory, an army of genetically altered mosquitoes zip around translucent plastic boxes, feasting on horse blood. Bred by a start-up called Oxitec Ltd., the bugs are on a mission to kill off their brethren with the most potent of weapons: sex.

Thanks to genetic tweaking, all of the male mosquitoes are born sterile. When they mate, the progeny inherit the defect and die at the larval stage. If enough are released into the wild, they could overtake regular males in the battle for females and eventually cause the population to collapse.

A plan, still in its early stages, has been hatched to unleash the newly armed insects 6,500 miles away in Malaysia. Authorities have been desperate to tame the growing scourge of dengue fever, a largely urban disease transmitted via a striped mosquito known as Aedes aegypti. In what would be the first release of an insect genetically programmed to kill other insects, scientists may release millions of genetically modified mosquitoes to crash the wild population of aegypti, and, they hope, stop dengue in its tracks.

But that's a big hope. Dengue, whose incidence has soared in recent decades, has been nicknamed "break-bone fever"; the flu-like illness causes severe joint and muscle pains. A rarer form kills. The World Health Organization estimates that there are 50 million dengue infections each year and as many as 2.5 billion people are at risk.

The project is causing a buzz of worry. "What's the consequence on the ecosystem if you wipe out an entire species?" says Gurmit Singh, chairman of the nonprofit Centre for Environment, Technology and Development in Malaysia. "You may solve one problem but create another."

In April, local newspapers reported that the Malaysian government would soon test-release the bugs on an island near Kuala Lumpur, the capital. After a public outcry, the government insisted the move wasn't imminent.

Oxitec

The World Health Organization estimates there are 50 million dengue infections a year.

As a form of pest control, sterile male insects have been unleashed plenty of times before. The U.S. used the method to rid itself of a flesh-eating fly called the screw worm by 1966. In Zanzibar, sterile males vanquished the tsetse fly in the mid-1990s. In both cases, the bugs were sterilized with old-fashioned radiation. The technique has never worked too well on mosquitoes.

Fiddling with an insect's genetic makeup is a more radical idea. Researchers in India and France have created silkworms that were genetically modified to defend against a deadly virus that usually kills them. There are similar early-stage efforts to make GM mosquitoes that don't transmit malaria.

In the U.S., scientists irradiate and release millions of pink bollworms in order to control populations of the cotton pest. Last summer, the U.S. Department of Agriculture went one step further. Of the 2.4 million creatures released in southern Arizona, half had a genetic modification: an extra piece of DNA that gave them red fluorescent markings on their body, making them easier to identify under a microscope.

Created by Britain's Oxitec, the transgenic bollworms -- they're really caterpillars that become moths -- "behave quite like the untransformed sterile strain," says Greg Simmons, a USDA entomologist.

Oxitec was spun off from Oxford University in 2002 and is still partly owned by that institution. At its labs on a recent afternoon, a researcher used a thin glass needle to inject liquid DNA into a mosquito embryo. The hope was that the foreign DNA would lodge in a suitable part of the embryo genome.

It's a laborious process. "We need to do a few thousand injections to get just one success," says Luke Alphey, co-founder and chief scientist of Oxitec. When it works, the inserted gene produces fluorescent proteins in the larvae that hatch.

The transgenic critters have a more important property: Their offspring are programmed to die in the larval stage if they aren't fed tetracycline. The antibiotic is part of their feed in the lab. Without access to tetracycline in the wild, any larvae born there can't survive.

Only female mosquitoes bite humans and animals on whom they feed. (Males don't bite; they sup on plant juices and sugary saps.) Upon mating, they require proteins from a blood meal to develop their eggs. Consequently, when female mosquitoes smell a person nearby, they are more inclined to mate.

"Humans emit more than 300 odors -- that's how mosquitoes track you down," says Bart Knols, entomologist at Wageningen University & Research Center in the Netherlands, who is familiar with Oxitec's research. (Dr. Knols is big on bad smells: In 1996, he published a paper showing that the female malaria mosquito was as attracted to the stink of Limburger cheese as to the odor of human feet.)

Malaysian scientists recently tested Oxitec's bugs in a specially built four-room house. Research volunteers took turns sitting behind a protective mesh screen, acting as olfactory stimuli for 10 female mosquitoes that had been released into the rooms. Also buzzing around were 10 males from the wild and 10 of Oxitec's special male bugs. The big question: Would the females go for the genetically engineered sterile males or the wild ones?

After eight hours, the females were separated and allowed to dine on the blood of rats. They were then placed in special containers where they could lay their eggs. If a particular larva that hatched had the fluorescent marker on its body, it was clear that female had mated with an Oxitec mosquito.

After a few dozen such experiments, the results indicated that the females showed no preference for the regular males over their genetically modified rivals. The next possible step is a controlled experiment to measure the technique's effectiveness in the wild.

There are safety concerns. Geneticists, for example, fret about horizontal gene transfer, whereby the alien DNA from the Oxitec mosquitoes would somehow jump into microbes or other organisms, with unforeseen consequences, such as harming another species. And while the chances are low that random mutations will restore fertility to sterile males, the probability goes up when millions of genetically modified mosquitoes mate in the wild.

Scientists "presume these accidents won't happen, but there's no evidence they won't," says Joe Cummins, professor emeritus of genetics at Canada's University of Western Ontario, who recently cowrote an article that criticized the safety aspects of Malaysia's mosquito project.

Oxitec's Mr. Alphey is less concerned. "You have to look at this on a case-by-case basis to understand what DNA was inserted and how it was inserted" into a transgenic insect" he says. "We don't think our strain poses a significant risk."