Matt Ridley: We need to defeat one particular mosquito before it gets us

Cancelling the Rio Olympics would do little to slow the spread of the zika virus. That horse has already bolted: more than 60 countries and territories already have zika. It will soon be almost anywhere that its mosquito host lives. Now that the link with microcephaly is well established, becoming pregnant in any country with zika carries a small but real risk of birth defects for the baby.

In the 1970s, troubled by the risks of using pesticides, we took our eye off the fight against mosquitoes and the diseases they carry. Zika is just the latest evidence that we are paying a heavy price for that. 


Between 1947 and 1958 Brazil managed to eradicate the Aedes aegypti mosquito from the entire country, as part of a continent-wide campaign against yellow fever. Yet the effort was not maintained, so the mosquito returned and now flourishes in the favelas of urban Brazil as well as most of the warm parts of the world.

The biology of Aedes aegypti makes it almost obliged to bring new and virulent pathogens to our species. As its name suggests, it originated somewhere in Africa, where some time around 4,000 years ago, unlike most mosquitoes, it became a domesticated species. That is to say, like swallows and house sparrows, it became entirely dependent on breeding in and around human houses, but also came to feed specifically on human blood. It spread to west Africa and there it picked up the yellow fever virus, which quickly became adapted to infecting human hosts.

It also collected the chikungunya virus, probably in Africa, and the dengue virus, possibly in Asia. It took all three across the Atlantic with the slave trade. More recently the mosquito collected the zika virus, probably in central Africa (it was first detected in monkeys in the Zika forest in Uganda) in the 20th century.

Because Aedes aegypti feeds mainly on human beings, every virus it picks up will either die out or become specialised at thriving in human bodies. And the trouble with insect-borne diseases is that, unlike colds, they do not always evolve towards lower virulence. Indeed, in some cases the more they debilitate their victim with fever, the better the chance of him or her being bitten again. So Aedes aegypti will go on bringing us new and nasty diseases as fast as we can fight them.

Aedes albopictus, the Asian tiger mosquito, is a bit less dangerous, because it is less domesticated and is not host-specific, so many of its bites are wasted on non-susceptible hosts such as rodents, birds and even amphibians. However, it can also carry all three viruses and it has spread around the world largely in the global trade in used tyres: tiny pools of water inside the tyres suit the larvae well. Because it can live in cooler climates, it is spreading through Europe.

Yet malaria, by far the most lethal of all mosquito-borne diseases, is retreating at an extraordinary rate. Malaria incidence has fallen by 37 per cent and mortality by 60 per cent since 2000. Why can we not do for zika what we have done for malaria? The two best weapons against malaria are insecticide-treated bednets and the spraying of the inside walls of houses, where the Anopheles mosquito rests during the day before feeding at night. Neither of these works against Aedes, a day-active mosquito.

The eradication of Aedes aegypti from Brazil and other countries in the middle of the last century was achieved by a monumental effort to spray insecticide into every urban nook and cranny where the larvae might live. Because the mosquito was domesticated, there was no need to treat the entire countryside. Today, that task would be near impossible, for three reasons: the huge expansion of cities, where the mosquito lives mostly on private property, in and around the home; the resistance of mosquitoes to many insecticides; and the objections of environmentalists to spraying.

Those objections, while sometimes overdone, have a point. DDT remains an especially effective way of reducing mosquito populations, but is best used in very small doses inside homes against malarial mosquitoes. Sprayed outside, it gets into the wild food chain where it accumulates in the bodies of predatory birds and mammals and eventually reduces their fertility. Though some deny it, DDT did devastate birds of prey in the 1960s.

So we need a better solution. Fortunately, one exists. The British start-up Oxitec, now part of Intrexon, developed a technique for genetically altering male Aedes aegypti in 2002 so that their offspring cannot survive to adulthood. In five separate trials over the past five years, more than 150 million such male OX513A mosquitoes have been released in Brazil (three places), Panama and the Cayman Islands. Males don’t bite, live for only a week, and in this case their offspring die young, so the long-term ecological effect is nil. The trials resulted in 92 per cent, 92 per cent, 93 per cent, 96 per cent and 99 per cent reduction in the respective local populations of Aedes aegypti — far higher than can be achieved with pesticides.

The technique was approved by Brazil’s National Biosafety Technical Commission and the World Health Organisation. Oxitec is building a factory in Brazil and will be in a position to release half a billion modified mosquitoes this year and over three billion in 2017.

Yet so obsessed are some activists with their opposition to genetic modification of any kind that they have been spreading bonkers theories that the release of GM mosquitoes actually caused the zika outbreak in Brazil, even though the release was 400 miles from and four years before the zika outbreak. There were also cases of microcephaly following an earlier outbreak in Polynesia, where Oxitec has never set foot. The idea is, in any case, physically impossible for many reasons, not least that the genetic insert is made of DNA and the virus genome is made of RNA. Yet this conspiracy theory has been repeated credulously by the editor of The Ecologist magazine, Oliver Tickell, and echoed in mainstream media, although recently he appears to have accepted new evidence that casts doubt on the theory.

Of course, despite it being invented in Europe, there is no plan to use the Oxitec technology on this continent even if it becomes urgent to do so. The process of approving genetically modified insects in Europe is too long, complex, uncertain and expensive for it to be even worth applying. Madness.

Matt Ridley, a member of the British House of Lords, is an acclaimed author who blogs at www.rationaloptimist.com.