A Genetically Engineered Swat

Oliver Judson's articles in the New York Times are without fail informative and entertaining. Here she discusses a promising technique to combat mosquito borne diseases...

Last week, I discussed rewriting the genes of viruses in order to make better vaccines. This week, I’d like to discuss the genetic engineering of mosquitoes as a way to stop the spread of dengue fever.

Dengue is caused by any of four related viruses. The disease can take a number of forms, from a mild sense of feeling below par, to dengue hemorrhagic fever, which can be lethal. Compared to diseases like malaria, dengue is a minor problem. Each year, more than 500 million people are infected with malaria, compared to “just” 50 million people for dengue. As diseases go, it’s not terribly dangerous either: the death rate from dengue hemorrhagic fever is around 2.5 percent.
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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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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.