The multimillion-dollar effort to eradicate one of the world's deadliest diseases received a significant but controversial boost on Monday when scientists announced the creation of genetically modified (GM) mosquitoes that cannot pass on malaria.
Trials revealed that the GM mosquitoes could quickly establish themselves in the wild and drive out natural malaria-carrying insects, thereby breaking the route through which humans are infected.
The strategy is likely to prove contentious, as it would require the unprecedented release of tens of thousands of GM organisms into the wild. But it has raised hopes among scientists, some of whom believe it may be powerful enough to finally bring under control a disease which strikes 300 million people a year and causes more than 1 million deaths, mostly of children in sub-Saharan Africa.
Plans to combat malaria with disease-resistant mosquitoes have been hampered in the past by fears that adding crucial resistance genes weakens the insects, making them too feeble to survive in the wild. For the idea to work, the malaria-resistant insects must breed and become dominant, so that the parasite is not picked up from infected animals and passed on to humans through insect bites.
The Malaria Research Institute, at Johns Hopkins University, led by Marcelo Jacobs-Lorena, created GM mosquitoes by giving them a gene that made it impossible for them to pass on the plasmodium parasite that causes malaria. Around 1,200 GM mosquitoes were then released into a cage holding malaria-infected mice and the same number of wild mosquitoes.
Over time, the researchers found that the GM mosquitoes slowly became the majority, reaching 70 percent in nine generations. The scientists believe that although malaria-resistance weakened the mosquitoes by making them immune to the parasite, they fared better in the long term than insects infected with it because they lived longer and laid more eggs.
"This fitness advantage has important implications for devising malaria control strategies," the team write in the journal Proceedings of the National Academy of Sciences.
The finding was hailed as proof that GM mosquitoes, made with cheap laboratory techniques, could ultimately have a greater impact on malaria than chemical sprays and other treatments.
Andrea Crisanti, head of a team working on GM mosquitoes at Imperial College, London, said the latest work raised hopes by showing the insects would be unlikely to die out if released into the wild.
"This has tremendous potential. You can start with a few mosquitoes and in future generations, all of them are resistant to the disease. It should be much cheaper than the controls used now, because you have a strategy where the mosquito spreads the gene which confers resistance. You don't need insecticides any more," he said.
In wild populations, only a small fraction of females carry the malaria parasite, so disease-resistant strains must become well established to affect the spread of disease. Scientists are focusing on ways to perfect how resistance genes are inherited, ensuring they are passed on in every mosquito egg. Normally, offspring have a 50 percent chance of inheriting a specific gene from the mother.
Trials in sub-Saharan Africa, where malaria claims the life of a child every 30 seconds, could be conducted within five years, but scientists will first have to prove as far as possible that the resistance genes will not trigger a more aggressive form of malaria, or spread to other insects.
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