Wolbachia keep deadly viruses from replicating inside the mosquito, stopping disease transmission
While they live in 60% of insects, Wolbachia must be manually placed inside mosquitoes
Editor’s Note: This is part one in a four-part series about the efforts to stop or slow the spread of Zika virus though the use of modified and engineered mosquitoes. See all four pieces here.
Remember the movie “Alien,” where the creature grows in the crewman’s belly until it finally rips through his stomach to the outside world? Now imagine a kinder, gentler creature living inside the cells of up to 60% of the insects around us. This creature, a strain of bacteria called Wolbachia, manages to live off its hosts and transform them, but doesn’t have to kill them to survive.
In the case of the Aedes aegypti mosquito, the transformation is quite helpful: Wolbachia doesn’t allow the viruses that cause Zika, dengue, yellow fever and chikungunya to replicate inside the mosquito. That discovery belongs to Scott O’Neill, a professor at Australia’s Monash University, and his Eliminate Dengue project.
“We think that Wolbachia is competing for essential resources inside the insect cell,” explained University of Melbourne’s Cam Simmons, one of many scientists working on the nonprofit endeavor. “In that competition, the Wolbachia wins. It stops the virus from duplicating itself to a level that allows transmission by the mosquito through its saliva to a new human.”
Even though Wolbachia is often mentioned along with other genetically engineered options to end Zika, scientists working on the project don’t consider their mosquito a “GMO” product because Wolbachia is a naturally occurring bacterium.
“We did a horizontal infection of the mosquito, and that’s not what the field would describe as genetic engineering, because we are not manipulating the insect genome,” explains Simmons. “It’s not genetic engineering, but at the same time it’s not a natural event.”
Because the Aedes wasn’t one of the insect species that carry Wolbachia, the bacterium had to be taken out of a fruit fly, conditioned so it would accept its new host, and introduced manually.
“If you can imagine very, very fine injection needles injecting a tiny volume of liquid that contains the Wolbachia bacterium into a mosquito egg,” said Simmons, “you can imagine the challenge. To actually get successful infection of the mosquito in this process required hundreds of thousands of injections.”
And over a decade of effort.
Once the researchers had a viable pair of Wolbachia-carrying mosquitoes, they had to breed them and test their offspring for safety. The Eliminate Dengue group says lab tests showed the Wolbachia bacteria was too big to be passed through the mosquito’s salivary gland to humans, and three years of allowing Wolbachia-carrying mosquitoes to feed on human volunteers produced no signs of the bacterium in those volunteers.
The group looked at the environment as well, testing animals, including other insects, that ate the Wolbachia-carrying Aedes aegypti as part of the food chain. The researchers say they have found no signs of harm from the modified meal. Eliminate Dengue also commissioned a study by the Commonwealth Scientific and Industrial Research Organisation, Australia’s national science agency, which stamped the mosquito’s risk “negligible,” the lowest possible rating.
The first Wolbachia-carrying mosquitoes were released into the suburbs of Cairns, Australia, in 2011 and quickly spread into the wild, replacing the disease-carrying population with the new, disease-free one, and replicating quickly into subsequent generations. According to Eliminate Dengue, testing so far shows the helpful bacteria remain.
“Once Wolbachia is established in a mosquito population it stays there. It’s a one-and-done application,” said Simmons. “It’s not something you have to go back and do every year or every two or three weeks like some of the GMO options.”
That’s good news, he said, because it means that one release should do the trick, keeping costs to a minimum. To cut costs even more, the team is now releasing mosquito eggs into the wild, instead of the more expensive route of raising adults.
Besides Australia, Eliminate Dengue now has field trials in Indonesia, Vietnam, and some Zika-plagued areas of Colombia and Brazil.
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So why haven’t you heard more about this endeavor?
“It’s about resources,” said Simmons. “This is a nonprofit effort and there’s only so much we can do right now. Obviously with the Zika emergency, there’s more attention on mosquito-borne diseases, and hopefully that will change.”
The next step needed before Wolbachia can be considered a true contender in the battle against Zika is proof of disease reduction. While there have been no reported cases of locally acquired dengue in the Cairns suburb where the first mosquitoes were released five years ago, that’s not a controlled trial. Those begin this year, starting in Indonesia.