When Materialists Try To Fix Things
“Dengue, in other words, is on the rise. It is an up-and-coming virus.
But there is no vaccine and no cure. The only way to reduce the incidence of the disease is to control the mosquitoes that spread it. Which is not working.
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The mosquito chiefly responsible for spreading dengue (as well as several other nasties including yellow fever and chikungunya fever (but not malaria) is a small tropical species called Aedes aegypti. Females transmit the viruses that cause disease when they bite an infected person, and then bite someone else. As usual with mosquitoes, males don’t bite. (Dengue is also spread, though less efficiently, by other Aedes mosquitoes, including the Asian tiger mosquito, Aedes albopictus.)
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Because of its lifestyle, A. aegypti is extremely difficult to control: eliminating places for it to breed is almost impossible, and because it lives inside houses, controlling it requires a big, ongoing effort on the part of everybody.
So far, nothing has proved sustainable. For instance, A. aegypti was successfully eradicated from much of central and south America in the 1950s and 1960s (mostly by a paramilitary campaign that involved the intensive spraying of DDT), but the insects have since staged a magnificent comeback, and are now more widespread than they were before the eradication program began. Even Singapore, which has managed to reduce the numbers of breeding sites considerably (the premises index — the proportion of inspected premises found to contain mosquito larvae — is around 2 percent), is experiencing a resurgence of dengue. (There are probably several reasons for this; but one of them is thought to have to do with a shift in surveillance.) One group of scientists, writing earlier this year in the medical journal The Lancet, complained that existing methods of A. aegypti control are “ineffective, expensive, and environmentally aggressive.”
Hence the interest in genetic engineering.
There are several ways that A. aegypti could be engineered so as to interrupt the transmission of dengue. One possibility is to make mosquitoes that are unable to transmit the viruses. The idea would be to release them into the wild in the hopes that they would mate with normal mosquitoes, and resistance would spread.
But I prefer a simpler approach. Here, mosquitoes are engineered to have a built-in flaw: a gene that is lethal when the insect becomes a pupa. Males carrying this gene would then be released. Wild females who mated with one of these males would lay eggs as usual, the larvae would develop as usual — but when they got to the pupa stage, the insects would die. (From the point of view of control, death at this late stage is an advantage, because the animals still occupy the pool as larvae. This is useful because larvae compete with each other for food, so their presence in a pool helps, in and of itself, to keep the population down.)
But if mosquitoes carrying this gene die at the pupal stage, how do you ever manage to rear any males to release? This is the clever part. The mosquitoes are engineered so that whether the gene is lethal depends on what the larvae eat. If their diet contains a certain crucial ingredient, the killer gene does not get turned on. But in the absence of the crucial ingredient, the gene is turned on, and the animals die.
Such a mosquito has been made. In this case, the crucial ingredient is tetracycline, an antibiotic. This is added to the food the larvae eat in the laboratory, but it’s absent from pools of water in the bottoms of tires or old cans. (Tetracycline does sometimes occur in nature. Some bacteria produce it in small quantities; also, since it is used in agriculture, water near farm runoff sometimes contains it. However, polluted water of this sort is avoided by Aedes aegypti females, which are rather fussy about where they put their eggs.)
This system has several attractive features. First, since it is only males who carry these genes, no one will be bitten by genetically engineered mosquitoes. Second, because the gene construct is lethal, it shouldn’t spread into the wild mosquito population — instead, it should eliminate it. Third, the fact that the gene is lethal at the pupal stage means that fewer engineered mosquitoes need to be released. Finally, in a traditional control program, the hardest part is finding the last of the animals you seek to eradicate; in programs of this kind, the males will do it for you. Males, after all, have evolved to be good at detecting females.
Above all, the technology is clean and green: you don’t need to use pesticides. Assuming that field trials show that it works as planned, the benefits of using it could be enormous.”
