As countries across the world rush to understand and control the spread of the Zika virus, are we missing the bigger picture?
That’s what Dr. Lok Pokhrel wonders. Dr. Pokhrel, an assistant professor of environmental health in the Department of Epidemiology and Biostatistics at Temple’s College of Public Health, says that as we fight the mosquitoes that transmit Zika, we should also consider the side effects of the battle.
Dr. Pokhrel argues that there are some big problems with the chemicals we currently rely on to control mosquitoes. Insects are gradually becoming resistant to the most common treatments, meaning that harsher chemicals and higher doses are required in order to be effective. Using more chemicals is costly—both in price and in their impact on the environment, where they accumulate and affect other organisms besides the pests they’re meant to target.
“There are no sustainable pesticides available,” says Dr. Pokhrel. “They’re all high-volume and high-dose, and their environmental impact is very high. Once they’re applied, they tend to linger in the environment for a long time.”
So Dr. Pokhrel is turning to a decidedly different substance—silver—to control disease-bearing mosquitoes without the heavy environmental footprint. Silver is naturally antimicrobial and has been used for millennia to kill bacteria. And it turns out to be lethal to mosquitoes, too.
Dr. Pokhrel is especially interested in a modified nano version of silver, which he says is both more effective and better for the environment than existing pesticides.
When silver particles are reduced in size to the nano level, they become more toxic for pests like Aedes aegypti, the mosquito species that carries Zika. In Dr. Pokhrel’s lab studies, silver nanoparticles kill a majority of Aedes eggs before they ever hatch, and the mosquitoes that do make it out end up dying before they can bite.
Dr. Pokhrel has found that a miniscule dose of modified silver nanoparticles will achieve the same effects as much larger amounts of conventional pesticides. That makes his technique much less expensive—and means fewer chemicals end up in the environment. Another potential benefit: while Aedes mosquitoes are increasingly resistant to existing pesticides, Pokhrel suspects that they might not develop resistance to silver.
So what’s the nanosilver actually doing? Dr. Pokhrel is still working to understand what he’s observing in his lab. “A normal mosquito egg will hatch very quickly when you add water,” says Dr. Pokhrel. “But when it is also exposed to nanosilver, the eggshell takes on abnormalities and breaks open before the mosquito hatches. Something is happening inside the egg which makes it rupture.”
That question highlights a bigger issue: nanoparticles are so new that we don’t yet know how they behave in the environment. But Dr. Pokhrel says research suggests that silver nanoparticles may naturally aggregate with each other in the environment, creating larger—and therefore less toxic—silver particles. And the vastly smaller amounts of the substance needed in the first place would also minimize risks to other organisms.
Dr. Pokhrel is now working to combine nanosilver with a set of chemicals called insect growth regulators (IGRs) that prevent newly-hatched mosquitoes from developing into adults, and that are relatively harmless to non-target organisms. By attaching them to silver nanoparticles, Pokhrel hopes he can create a hybrid particle that’s even more effective than either chemical would be on its own, but that also remains safe for the environment.
There’s a long way to go before something like this could be used to control mosquitoes in the real world. But despite the questions that remain, Dr. Pokhrel argues that nanoparticles deserve far more attention for their potential ability to stem the tide of epidemics like Zika in a more environmentally-neutral way.
And from everything Dr. Pokhrel has found so far, the benefits seem to clearly outweigh the risks. “We should be worrying about infants with Guillain-Barre syndrome or microcephaly,” the debilitating illnesses linked to Zika infection, he says. “If we eradicate the Aedes aegypti mosquito for now, those illnesses will be gone.”
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