A mosquito flaps its wingsGeoff Howe
The latest offence in the global fight against malaria: Build a better mosquito.
Scientists have developed the first genetically modified mosquito (GMM) that is completely immune to the disease the insects so efficiently spread. An estimated 250 million people worldwide contract the deadly blood-borne disease a year; one million of them die.
The GMM mosquitoes will still bite; they just won't leave behind the malaria-causing parasite, called Plasmodium.
"Hopefully, down the road this will play a part in controlling malaria," says lead researcher, entomologist Michael Riehle. The research was published Thursday afternoon in the journal Public Library of Science Pathogens.
Although releasing the new mosquito into the wild is a long way off, Dr. Riehle says that given the drawbacks of other malaria-fighters, especially the mosquito's growing resistance to various insecticides and vaccines, it is a method worth investigating.
"Malaria is a very smart parasite and our best efforts haven't done much to reduce the impact of it. That's why we have to keep trying these different approaches, to find something that works."
While previous research has been able to create mosquitoes that are 97 per cent resistant to malaria, that's just not enough, Dr. Riehle says.
"If even one parasite makes it through, it will produce thousands of parasites, ultimately," he says. "And that mosquito can transmit malaria."
What's more, he says, if any parasites evade your killing mechanism, they're going to develop resistance to that mechanism quickly. So in a few generations, malaria will be back in full force.
To understand the making of the new mosquito, it's important to understand just how the insect acts as a so-called "vector" for the disease.
It all starts when a female mosquito bites a human whose blood is infected with malaria. From the mosquito's "midgut" the parasites squeeze through the wall of the digestive tract and try to attach to its outer wall.
Only a tiny fraction make it, but those who do develop into cells called oocysts. After 10-12 days, thousands of new parasite cells, called sporozoites, have divided. These burst out of that cyst and make their way into the mosquito's salivary glands, ready to find a new host.
The mosquito can then transmit these youthful parasites to humans via saliva. The mosquito transmits about 40 of these sporozoites in one bite - but it only takes one to infect a human and work its way into their bloodstream. And the cycle continues.
Each time the parasites multiply in the human bloodstream, their host cells burst and release themselves into the blood, causing the painful bouts of fever for which malaria is known.
So, how to throw a wrench into that very involved process? Dr. Riehle, an associate professor at the University of Arizona, designed a piece of genetic code that could insert itself into the mosquito's genome.
Once injected into mosquito eggs, the piece of code acts as a switch that is always set to "on" and thus causes a signalling molecule called Akt to go into overdrive. Akt affects many functions, including larval development, immune response and lifespan.
Dr. Riehle says he thought the Akt burst would mostly affect the lifespan of the mosquito, which is also an effective malaria curb if the mosquito dies before the parasite has matured fully.
That did happen, but the more interesting development was that the parasites were completely killed off in the mosquitoes.
"Lo and behold, we had a completely resistant mosquito," he says. "That was a pleasant surprise. It's always the unexpected stuff that's the most exciting."
Why this works remains a mystery, which is the next step of his investigation, Dr. Riehle says.
"It might be affecting immunity. Or it might just be making it a more hostile environment for the parasite to pass through. We just don't know at this point."
Because it's a genetic change, future generations of mosquitoes inherit the immunity.
The experiment was conducted on one particular type of mosquito, the Anopheles stephensi. It is common to Asia and India.
But Dr. Riehle says the technique could also work with the other species, including the Anopheles gambiae, which spread malaria in Africa.