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Get the females and beat the disease

Photo: Bloomberg

Genetic engineering can help reduce population of disease-carrying mosquitoes

Someone I know is in the Florida Keys this week, planning to board a catamaran and wander the surrounding seas for several days. “Our final destination is the Dry Tortugas," she wrote, “look it up, quite something!" Which I did, and they are. Naturally I’m envious.

Someone I know is in the Florida Keys this week, planning to board a catamaran and wander the surrounding seas for several days. “Our final destination is the Dry Tortugas," she wrote, “look it up, quite something!" Which I did, and they are. Naturally I’m envious.

Now I have never been to Florida. But the state is known for its mosquitoes. The humorist Dave Barry lives there and has often mentioned the insects in his columns: “... as the Sun set, we experienced a sensation that I will never forget: The sensation of being landed on by every mosquito in the Western Hemisphere. There were so many of them that they needed Air Traffic Control mosquitoes to give directions."

Long story short: Florida has swarms of mosquitoes. They are constantly biting residents of and visitors to the state, so much so that I feel for the person I know who is going there. Still, get this: in an effort to fight the mosquito menace last April, a biotech firm went to the Keys to release ... more mosquitoes. Hundreds of thousands of mosquitoes, brought to the Keys as eggs actually, allowed to hatch there and live out their lives.

What hare-brained scheme is this, you may wonder. Many people have so wondered, and in the Keys, there has been plenty of opposition—so it is a controversial programme. Yet, it at least deserves some thought, especially given that swarms of mosquitoes are a feature of life in much of India too.

The mosquitoes introduced into the Keys were genetically engineered.

A little background, first. There are plenty of mosquitoes in Florida, certainly, and it can’t be pleasant to suffer their bites. But only the species Aedes aegypti actually carries diseases—chikungunya, dengue and more—and they make up only 4% of the mosquito population in Florida. What’s more, only female mosquitoes actually bite humans. Males feed on nectar and their sole purpose in life is to mate with females and produce more mosquitoes. None of this is meant to say that we should ignore these pests. But it does suggest a possible way to fight them that’s more efficient than blanket applications of insecticide: target the females.

It’s true, the male and the female of the species do look different, but that’s if you get a chance to peer closely at them. So, it’s in no way practical to visually identify only the female mosquitoes in a given area and whack them dead. But what if there’s a way to ensure that when a mosquito pair reproduces, the female, and only the female, offspring die? What if such death comes early in their lives, even before they attack humans for the first time? Carnage like this means that the offspring left alive will mostly be males. They will mate with the remaining females, with the same morbid results for the resulting female offspring. Over time, you’d expect the mosquito population to become more and more male. With less and less females to mate with, the Aedes aegypti population will naturally decline.

Genetic engineering (or genetic modification) offers a way to accomplish more or less this. Though with various plant species especially, plenty of controversy surrounds the process. Consider:

Proponents point out that humans have been doing such engineering indirectly for many millennia: breeding plants and animals selectively for certain desirable characteristics. For example, modern corn looks nothing like the grass-like Mexican plant with rudimentary ears, teosinte, that it is descended from. That’s because we humans have for uncounted generations selected plants with juicier, bigger and more succulent ears and kernels —and used only those plants to generate their next crop. Much the same applies to plenty of other crops and domesticated animals.

Critics, though, say that today’s techniques of actually modifying genes are entirely different from selective breeding, and there’s definite danger there. For example, the wind can carry pollen from genetically modified (GM) crops to fields of non-modified crops, causing unpredictable and undesirable problems. Besides, the GM crop industry is dominated by a few large biotech firms. So, the prospect of widespread use of such crops raises serious concerns about monopolies, especially for small farmers like in India.

The fear that genetic engineering can have unpredictable consequences is why many residents of the Keys opposed the new genetically-engineered male mosquitoes.

Still, let’s look at how they were engineered and then released. These Aedes aegypti males have had their DNA altered: scientists have “edited" two particular genes into particular locations in the mosquito’s genome:

* a fluorescent “marker" gene that glows in red light, which will later be used to identify engineered mosquitoes.

* a “self-limiting" gene.

When the insects reproduce, both genes are passed on to their offspring. The “self-limiting" gene has no effect on males. But in larval females, it inhibits the storage of a specific protein that would otherwise build up as the insect grows. The result is that the female dies before it can mature.

This is the theory, of course. But these engineered mosquitoes have been released in Brazil, Panama and even India—in the last two years, over a billion of them. The British biotech company that produced them, Oxitec, reports that in those areas, the populations of Aedes aegypti shrank by over 90%. You’d think that would certainly have an effect on the incidence of mosquito-borne diseases.

What of unpredictable consequences? The Brazil trial suggested that the self-limiting gene did not kill all the female offspring before they could mate, because other genes from the engineered mosquitoes appeared among other local mosquitoes. What effect this will have on the local ecosystem is not yet clear. But this is the kind of fallout of genetic engineering that worries many people.

Still, in April, Oxitec placed boxes containing eggs of the engineered mosquitoes in six different locations in the Florida Keys. Each week between May and August, about 12,000 of the mosquitoes will hatch from their eggs and emerge into the Florida air, ready to find willing females to mate with. Every now and then, Oxitec’s researchers will collect mosquitoes and use red light to identify the engineered ones. They want to know such details as their life spans, the distance they have travelled from their boxes, and how many of the females who inherit the self-limiting gene have actually died. All this will shape a second and larger trial later this year, when Oxitec plans to release 20 million engineered mosquitoes. Data from these trials will help decide whether it is worth releasing mosquitoes more widely across the US.

Clearly, there’s still plenty to learn about genetically engineered mosquitoes. But till now, insecticides have been our weapons of choice against mosquitoes. They kill the insects, sure, but also other insects we would rather save, like honeybees.

Consider this parallel to cancer. Our weapon of choice there —one that’s just as blunt as insecticides—remains chemotherapy. It kills cancer cells, sure, but also plenty of other cells in our bodies. What if we instead found a way to introduce a particular kind of cancer cell into the body, one that would single out and kill the malignant cells?

We don’t know of such a cell (yet, anyway), but that’s how to think of genetically engineered mosquitoes. And if you think about it some more, there’s also a parallel of sorts to vaccines for a certain virus that we are all a little too familiar with these days.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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