To Fight Malaria, Scientists Target Old Mosquitoes
An insecticide aimed only at old mosquitoes could solve a major problem in malaria eradication, say Penn State researchers.
About every 30 seconds, malaria kills a child somewhere in the world, according to the World Health Organization. It kills about one mother every hour.
Insecticides can kill the mosquitoes that spread malaria, but spraying a population of mosquitoes kills only the ones that are vulnerable to the poison. A handful of super-mosquitoes that are immune to insecticides survive and reproduce. So health programs use insecticides sparingly.
Earlier this month, Andrew Read of Penn State University and his colleagues proposed a solution: Kill only the older mosquitoes that actually transmit malaria. They describe their theory in Public Library of Science Biology.
The ability of mosquitoes to withstand insecticides was first noticed after World War II and continues to be a significant problem.
"Insecticides are such a good way of controlling malaria, we need them to continue to work," says Read. "The problem is if you get resistant mosquitoes spreading in a population, then most of the mosquitoes are no longer controlled by the insecticide, and malaria comes back."
Read and his colleagues are looking to exploit a key feature of the malaria life cycle. When a mosquito feeds on a person with malaria and picks up malaria parasites, it can't transmit those parasites right away. The parasites circulate and develop in a mosquito's body for 10 to 14 days before getting into the mosquito's saliva. From there it can be transmitted to the next human that the mosquito bites. Mosquitoes live a harsh life, Read says, and most don't live long enough to transmit malaria.
"From the point of view of malaria control, you need to take out only the old mosquitoes," he says. And killing off only the old mosquitoes means resistance to insecticides is less likely to develop; there are plenty of young mosquitoes around competing for resources, and keeping the resistant bugs from dominating.
It's an interesting idea, says Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, which funds research on malaria treatment, prevention and eradication. "If mosquitoes did not develop resistance to insecticide, one limb of the malaria control program would be much more robust and effective."
Read and his colleagues are working on a fungus that kills mosquitoes after 10 to 12 days, targeting the older mosquitoes most likely to carry malaria. He says he'll be ready to field-test the fungus in a year or so.
An additional strategy Read suggests is to use lower doses of existing insecticides, which would take longer to kill and thereby select the older mosquitoes for death.
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ROBERT SIEGEL, host:
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. Transcript provided by NPR, Copyright National Public Radio.
