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With World Malaria Day coming up Saturday, international health experts are focusing on the challenges of fighting an infection that kills up to one million children a year. Possibilities include a vaccine against the parasite that causes the disease, broader distribution of new and effective treatments, and this idea from researchers at Penn state: an insecticide that knocks off only old mosquitoes.

NPR's Joanne Silberner reports.

JOANNE SILBERNER: Here's what keeps biologist Andrew Read up at night.

Dr. ANDREW READ (Biologist, Penn State University): When you use insecticides there will always be a few mosquitoes that are hardy enough to survive the insecticide.

SILBERNER: Those few survivors will spread and reproduce and fill the ecological niche once pretty much filled by the mosquitoes that were sensitive to the insecticide.

Mr. READ: Eventually, the world fills up with resistant mosquitoes, and your insecticides no longer work.

SILBERNER: Insecticide-resistant mosquitoes thrived in Europe after the second World War and more recently in South African and Mexico. This prospect of insecticide resistance is a major reason why health planners don't blanket heavily infested areas with insecticides. Blame Darwin, says Read.

Mr. READ: A lot of our best public health tools like drugs, like insecticides are undermined by the evolution or our target organisms.

SILBERNER: Several years ago, Read and his colleagues at Penn State University began working on a new insecticide, a fungus actually, that infects and kills mosquitoes. Read knew his fungus was likely to take a few days to work. It needs time to multiply to lethal levels. He found that it took 10 to 12 days, and that's old for a mosquito, but mosquitoes don't transmit malaria until then anyway.

His lab experiments showed that killing just old mosquitoes was enough to stop malaria transmission. That got him to theorizing that generally killing only old mosquitoes would be a great strategy to counter resistance. A late kill would get the malaria transmitters without risking the development of widespread resistance.

Mr. READ: Most of the mosquitoes will have done their reproduction. So their offspring are filling the world, filling up the mosquitoes' niche, and so there won't be a vacuum into which resistant mosquitoes can pour.

SILBERNER: He and his colleagues worked out the mathematics of the process in the current of a journal called Public Library of Science Biology. It's an interesting approach, says Anthony Fauci. He's head of the National Institute of Allergy and Infectious Diseases, which finances a lot of malaria research. Resistance to insecticides is a huge challenge, he says.

Dr. ANTHONY FAUCI (Director, National Institute of Allergy and Infectious Diseases): If you had a product that there would be no resistance, and you will guarantee there was no resistance, and it was environmentally safe, you would definitely use it much more widely as opposed to saving it only for specific situations to avoid resistance.

SILBERNER: Penn State's Andrew Read hopes that his fungus will be such a product, though he says much more work needs to be done, and not just lab work.

Mr. READ: It's a shift in thinking. It's not the mosquito that's the enemy, it's the malaria, and the malaria is only dangerous when it's in old mosquitoes.

SILBERNER: But will people accept an insecticide that only kills old mosquitoes and spares the nuisance mosquitoes? Read says yes.

Mr. READ: Malaria is a really devastating disease. If you're losing children or siblings to the disease, you want to stop it.

SILBERNER: Joanne Silberner, NPR News.

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