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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Is eventual eradication of malaria possible?

A new technique for sterilising certain mosquitoes looks promising

After a break of two weeks, here is my latest Mind and Matter column in the Wall Street Journal:

April 25 is World Malaria Day, designed to draw attention to the planet's biggest infectious killer. The news is generally good. Never has malaria, which is carried by the Anopheles mosquito, been in more rapid retreat. Deaths are down by a third in Africa over the past decade alone, and malaria has vanished from much of the world, including the U.S.

As so often happens in the battle against disease, however, evolution aids the enemy. The selection pressure on pathogens to develop resistance to new drugs is huge. In recent weeks, the emergence on the Thai-Myanmar border of malaria strains resistant to artemisin, a plant-derived drug, have led to pessimistic headlines and reminders of the setback caused by resistance to the drug chloroquine, which began in the 1950s.

For this reason, prevention generally works better than cure in eradicating infectious diseases: Vaccination beat smallpox, clean water beats cholera, less crowded living beats tuberculosis and protection from mosquitoes beats malaria. Good prevention keeps bad evolution from getting started. Yet two can play at evolution. The newest weapon in the fight against mosquitoes ingeniously turns the evolutionary tables on the pests.

The mosquito Aedes aegypti lives almost exclusively in human settlements, breeding in small pools inside discarded objects like car tires and coconut husks. Dengue fever, which it carries, now infects at least 50 million people a year and rising. Both Aedes and dengue have evolved to exploit dense urban settlements.

Yet this also makes the mosquito vulnerable to a new control technique developed by a former Oxford University scientist named Luke Alphey. He genetically modified mosquitoes so that they would produce no viable progeny unless supplied with a dietary supplement. His idea was to release the modified males (only the females bite humans) and let them mate with wild females, whose offspring would then die. He can also make females genetically flightless or doomed to die young.

A similar technique eradicated the screw worm (a blowfly maggot that infects livestock) from North and Central America. Gamma rays made male flies sterile but didn't affect their ability to mate, and since females only mate once, this led to an epidemic of infertility in wild flies. The beauty of this technique is that it generates the opposite of diminishing returns: The rarer the pest gets, the more likely it is that the released males will mate with the few remaining females.

But gamma rays damage mosquitoes too much, so a subtler, gene-based form of sterilization was needed. Enter Dr. Alphey and his company Oxitec, which last year announced the results of a trial of his technique in the Cayman Islands, showing that released male Aedes mosquitoes did indeed succeed in mating with wild females. Further tests are planned for Key West, Fla., and other areas before moving into larger cities.

Predictably, perhaps, the genetic modifications have led to objections from some Western pressure groups, showing their now customary tendency to elevate theoretical principles above the battle against human suffering. Yet the great advantage of Dr. Alphey's approach, in contrast to the fogging of dengue-affected areas with insecticide, is that it is pest-specific. No other insect is hurt.

It will work best for a mosquito like Aedes that breeds in modest numbers and only in urban refuse. Common Culex mosquitoes, which also breed in ditches, sewage systems and rural habitats, may be too widespread to be controlled this way. But there is another fastidious mosquito genus that prefers small pools close to human habitation and that could also be vulnerable to a campaign of genetic sterilization: Anopheles, the carrier of malaria. With such a technique, the eventual eradication of human malaria from the planet is far from being an impossible dream.