The pesky Aedes aegypti mosquito has been buzzing around infecting millions of people in the Western Hemisphere with the Zika virus. Now some scientists think we should retaliate — and infect the mosquito with a strange little bug of our own.
It sounds zany, but it might actually work. Scientists have long known that if they infect just a few A. aegypti mosquitoes with a type of bacterium known as Wolbachia, the microbe will rapidly spread and infect much of the population. Once that happens, the infected mosquitoes lose their ability to transmit dengue fever among humans.
Now it appears this technique can help block Zika, too. A new paper published by Brazilian researchers in Cell Host & Microbe finds that when A. aegypti mosquitoes are infected with a specific strain of Wolbachia, they become far, far less effective at carrying the Zika virus currently wreaking havoc in Brazil and beyond.
Other scientists were impressed by the new finding. "Overall, I think this is a sound and very important approach," says Marcelo Jacobs-Lorena, a professor at the Malaria Research Institute at Johns Hopkins who wasn't involved in this newest study. Scientists have already been releasing Wolbachia-infected mosquitoes in parts of Australia and Vietnam to see how well they reduced dengue fever. This new study suggests countries fighting Zika should give Wolbachia a try, too.
How to fight Zika mosquitoes with Wolbachia
Wolbachia is a very bizarre and very common type of bacterium that already infects millions of insects and spiders in the wild. The bacterium often hijacks its hosts' reproductive abilities, allowing it to spread quickly within a population. (For example, some strains of Wolbachia prevent infected males from mating with uninfected females — thereby ensuring that infected females have a competitive advantage.)
This bacterium doesn't, however, naturally infect the Aedes aegypti mosquito — the species that is currently spreading diseases like dengue fever, yellow fever, chikungunya, and Zika.
So what scientists can do is capture a few wild A. aegypti mosquitoes, inject their eggs with Wolbachia, and then release the mosquitoes back into the wild. Once that happens, the infection quickly spreads throughout the broader population.
Now here's the neat part: What this new study found is that if you infect A. aegypti mosquitoes with a very particular strain of Wolbachia — known as wMel Wolbachia — then something fascinating happens. The mosquitoes don't die off. They survive just fine. But they become less able to carry the Zika virus in their saliva. That means if they bite one infected human and then go bite someone else … nothing would happen!
This all points to a promising Zika control strategy. Capture some wild A. aegypti mosquitoes and inject their eggs with Wolbachia. Release the infected into the wild at strategic points. Monitor the mosquitoes to make sure the infection spreads throughout the entire population. Pretty soon, Zika infections should go down. "That takes some effort, and you do need funding to do it," says Jacobs-Lorena. But modeling studies have suggested that this approach could decrease transmission significantly.
For those worried about the risks of releasing a bunch of infected mosquitoes into the wild … well, the dangers are minimal. Remember, Wolbachia already exists in nature — plenty of common household mosquitoes have it. (In any case, scientists have found that the bacteria are too big to fit through the mosquitoes' salivary ducts, which means they can't be passed to humans through bites.) Researchers have also been releasing A. aegypti mosquitoes infected with Wolbachia into the wild in various countries since 2011 to reduce dengue fever, and they've found no ill environmental effects.
Arguably the biggest pitfall is that the Zika or dengue viruses might one day develop resistance to Wolbachia, rendering this strategy ineffective. Some scientists have suggested infecting the mosquitoes with multiple strains of Wolbachia to reduce this risk, but it remains a concern.
How Wolbachia compares with GMO mosquitoes
Now, this isn't the only clever scheme out there for fighting Zika. Insecticides and other control methods can help kill off mosquitoes altogether, although A. aegypti are notoriously hard to eradicate and have developed pesticide resistance over time. Plus, as soon as you stop spraying, the mosquitoes bounce right back.
Or here's another creative idea: A British company called Oxitec has genetically modified A. aegypti mosquitoes so that they contain a self-destructive gene — these mosquitoes will die unless they receive antibiotics in a lab. So the company releases genetically modified males into the wild. The GM males mate with wild females and pass on the self-destructive gene to their offspring. The offspring, in turn, die before reaching adulthood. Early results suggest this technique can kill 80 percent of local mosquitoes — way more than conventional treatments.
Jacobs-Lorena says the GMO mosquito strategy could also prove useful, but it does have a few drawbacks. For one, the second you stop releasing those GM mosquitoes into the wild, the population is likely to rebound. That means the company may have to continually manufacture millions and millions of mosquitoes — carefully separating out the males for release — in order to keep suppressing the population. That's doable, but it does increase the cost.
By contrast, once Wolbachia wiggles its way into A. aegypti populations, it tends to establish itself fairly permanently. In theory, that should prove lower maintenance. You don't have to kill off the mosquitoes. Just … make them less harmful to humans.
The other advantage is regulatory: Oxitec's genetically modified mosquitoes still have to jump through all sorts of hoops with Brazil's regulators (and America's). But because Wolbachia already exists in nature, there are far fewer regulations around its use.
Granted, there are plenty of further tests that will be required to figure out how best to use Wolbachia to stop Zika transmission — and how well it actually reduces the spread of disease. And Jacobs-Lorena warns that even if this does become an accepted strategy, it will likely just be part of a broader arsenal against mosquitoes — complementing public education, conventional control campaigns such as removing standing water where A. aegypti breeds, and possibly a vaccine.
It's too early to rule anything out. In the future we may learn that combining GM mosquitoes and Wolbachia works better than any one method alone, for instance.
Still, every little bit helps. We've been losing the war on mosquitoes for decades. It's always nice to have another weapon.
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