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EVERY 10 seconds a child dies from malaria. But there might be a way to
control the Anopheles mosquito, which spreads the disease. Scientists
say they can genetically modify whole mosquito populations so that the flies are
either susceptible to pesticides, or can’t transmit the malaria parasite. And
they’d only need to release relatively few GM mosquitoes to kick-start the
process.

According to the WHO, malaria claims more lives than any other communicable
disease bar TB, infecting around 500 million people each year. Yet it’s proved
impossible to eradicate the mosquitoes that carry the disease, because the
insects are so numerous.

One trick that has worked against the tsetse fly in Zanzibar and the screw
worm fly in North America is to release billions of sterilised males to swamp
the wild population. Almost all the wild females mate with the sterilised males
and the population crashes.

But it’s extremely difficult to rear huge numbers of sterile male
Anopheles mosquitoes in captivity as females require constant blood meals
to lay eggs, says Chris Curtis at the London School of Hygiene and Tropical
Medicine. And anyway, it’s only a short-term solution—even if you could
wipe out all the mosquitoes in one area, nearby populations would soon take
over.

Another idea is to modify a population so that it no longer carries the
parasite. Two potential methods involve introducing a GM bacterium or harnessing
so-called “jumping genes” that occur naturally in fruit flies. But there are
fears that the bacteria could spread to other species. And the fruit fly genes
won’t spread particularly well through populations of Anopheles.

Now Stephen Davis’s team at the Commonwealth Science and Industry Research
Organisation in Canberra, Australia, think they have found a way to safely
“infect” whole populations of mosquitoes with detrimental genes. Davis’s idea is
to release engineered males that have two copies of a “type A” gene, and two
copies of “type B”. Subsequently, individuals who inherit A and B together will
be fine. But those who inherit either A without B or vice versa will die.

Computer models show that modifying just 3 per cent of the population is
enough to spread the genes. “Such a low threshold was a bit of a shock to us,”
explains Davis. Hybrid offspring from matings between engineered males and wild
females are fine because they have one A and one B. But things get interesting
when the hybrids start mating. While all the offspring from hybrid/engineered
crosses are fine, some of the offspring from the hybrid/wild crosses die because
they have an A or a B in isolation. This creates a selection pressure which
drives the genes through the population, as the offspring of wild flies die more
often than those of engineered or hybrid flies.

You could exploit this “drive mechanism” to tackle malaria. Other genes, that
either kill the malaria parasite or make the flies susceptible to insecticides,
can be tagged onto the modified genes, and will also spread through the
population. While no one has tried this out on real flies, Davis thinks it could
be the first practical way to modify a whole population.