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Scientists at the University of Arizona have successfully bred genetically modified mosquitoes that are 100 percent resistant to the malaria parasite, rendering the mosquito incapable of infecting humans with malaria.

For years, researchers have tried to engineer mosquitoes so that they’re immune to the parasite that carries malaria — a single-celled organism called Plasmodium. But previous attempts only succeeded in destroying about 97 percent of malaria parasites in mosquitoes’ bodies. The difference between 97 and 100 percent might seem negligible, but Michael Riehle, who led the new study, says that 3 percent means the difference between success and failure. “If you want to effectively stop the spreading of the malaria parasite, you need mosquitoes that are no less than 100 percent resistant to it,” he said.

In the new study, Riehle’s team designed a piece of genetic information that inserts itself into a mosquito’s genome. When the researchers fed malaria-infested blood to the modified mosquitoes, the Plasmodium parasites did not infect a single animal in the study. And once the anti-malaria molecule is injected into the mosquito’s eggs, the next generation then carries the altered genes and passes it on to future generations.

The team also found that the anti-malaria molecule shortened the mosquito’s lifespan, which minimizes chances that the malaria parasite will develop. Wild mosquitoes typically live for 2-3 weeks, but the malaria parasite needs 12-16 days to develop within the mosquito before it can be transmitted back to people. “The oldest mosquitoes are responsible for most malaria transmission, so reducing the lifespan of the mosquito can reduce or eliminate the number of people the mosquito can infect,” said Riehle.

The malaria parasite is carried by the female Anopheles mosquito. When transmitted to a human, the parasite travels first to the liver and then on to the bloodstream, where it reproduces and destroys red blood cells. An estimated 250 million people contract malaria each year, and about 1 million die — many of them children. There are currently no effective or approved malaria vaccines, although a few have been tested. Riehle says that even if a vaccine were developed, distribution would be a major challenge.

According to Riehle, completely eradicating the malaria parasite carried by mosquitoes requires three things: the ability to engineer the mosquito, finding genes or molecules that can kill the malaria parasite, and giving the modified mosquitoes a competitive advantage so they can replace the wild population. The first two components have been accomplished, but Riehle says the third represents a bigger hurdle. “A lot of research is being done now to give the mosquitoes fitness advantages so that they can replace the wild populations,” he said. “But it’s probably at least a decade away, and if this is ever used for malaria control it will take several years for population replacement to actually occur.”

Riehle stressed that complete blockage of the malaria parasite is essential to any future control strategy — if some of the parasites slip through the mechanism, then the next generation will likely be resistant to it. “If you release the mosquito and within two or three generations, it’s no longer resistant to the malaria parasite, then you’re back where you started,” he said.

The genetically altered mosquitoes from the new study are being held in a secure lab environment that ensures they won’t escape. Once researchers find a way to replace wild mosquito populations with lab-bred ones, the altered genes will hopefully spread through the natural population. If the approach ultimately succeeds, malaria could be a disease of the past.

The results of the new study were published today in the journal Public Library of Science Pathogens.

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