Elena Scotti/FUSION

When scientists proposed releasing thousands of genetically modified Zika-killing mosquitoes in the Florida Keys, the biggest hurdle was not government regulation. On Friday, the Food and Drug Administration gave the project the green light, finding that its potential Zika-busting benefits outweighed any slim potential for environmental impact.

Instead, scientists at the biotech firm Oxitec face a formidable opponent in local residents armed with a powerful tool: the internet.

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In the small town of Key Haven, Oxitec has proposed releasing male Aedes aegypti mosquitoes engineered to kill off the local mosquito population by passing on a gene lethal to any offspring they might have with wild females. By doing so, Oxitec also hopes it will kill any chance of a local Zika outbreak.

Local residents are not enthusiastic about the plan. Organizing via hashtags like #‎buzzoffOxitec and a Facebook group titled "No to GM Mosquitoes in Key Haven" with just over 700 likes, a small group of Keys residents have become a very loud opposition to Oxitec's field test in Florida.

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"We should not be forced to be part of human experiment," the group explains on its page, "and do not consent."

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Oxitec first set its sights on the Keys back in 2011, after a local outbreak of the mosquito-born virus dengue fever. But its plan has become all the more urgent as an outbreak of Zika virus has moved rapidly across the globe, spread by international travel and the Aedes aegypti. To date, at least 42 countries have confirmed local transmission of Zika. In the US, there have been more than 7,000 cases. And in Florida, where the first US transmission of Zika spread by mosquitoes occurred in late July, an outbreak has now spread to at least 21 people. On Tuesday, the first Zika case was confirmed in the Keys, though it was a travel-related case, indicating that Keys mosquitoes are not infected with Zika—at least not yet.

This would not be the first time that Oxitec has performed this experiment. It has conducted trials in Brazil, Panama and the Cayman Islands, where it claims the mutant mosquitoes successfully reduced the local mosquito population by 90%.

The opposition has many specific concerns about Oxitec's proposal, with varying degrees of plausibility. Oxitec, for example, releases male mosquitoes because only the females bite humans. But what happens, the naysayers wonder, if an engineered female mosquito somehow gets released and bites a pregnant person? Can the antibiotics given to the mosquitoes in the lab inadvertently create super bugs? Is it possible that the engineered mosquitoes could thrive in the wild, establishing colonies of man-made mutants? Are Keys residents really just guinea pigs to a sinister money-hungry company?

"As we peeled this onion, the answers become less and less clear," Barry Wray, one of the Keys residents organizing against Oxitec, told me. "There are people in our ranks that are just against anything GM, but I'm not like that. I just don't feel that this technology was vetted with rigor."

These are the kinds of anxieties that drive Derric Nimmo, the head of Oxitec's Florida Keys project, absolutely crazy.

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"There is a very vocal minority that have these wild ideas," he said. "Some of the rumors about our project are just totally fake."

The anti-Oxitec rumor mill is swift and efficient. In June, for example, a local physician spoke out at a community meeting, raising concerns that the antibiotics given to the engineered mosquitoes could breed superbugs harmful to both animals and people. It wasn't long before those concerns began circulating in the community, both by word of mouth and online. A talking point parroted by many anti-GMO groups, a Facebook post of the meeting was shared 59 times and racked up 1,500 views—a lot for a community of just a few thousand people.

Oxitec’s mosquitoes are engineered to include two copies of a gene that kills them before they reach adulthood, overriding natural selection that makes it almost certain offspring will receive their dad's killer abilities. Mosquitoes in the lab receive small doses of tetracycline to counteract this gene's effect until they can pass it along to their ill-fated offspring.

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But the levels of tetracycline used, Nimmo said, are much lower than the dosages frequently given to people as treatment for things like urinary tract infections and acne. This means the levels of the antibiotic in any wastewater from Oxitec's facility—where critics think a superbug might breed—are also low. Studies have also shown that even if Oxitec were pumping out gallons of tetracycline-infused waste, the drug almost entirely degrades in just one day.

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"It is highly unlikely that the use of tetracycline in the production of OX513A mosquitoes would have any adverse effects on the environment," the FDA's report concluded.

Still, in a small community, rumors to travel far and fast.

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For this reason, Nimmo has spent much of his time in the Keys trying to win overs residents with "the facts." After the FDA's approval, Oxitec still needs approval from the board of the local Mosquito Control District. In the fall, the board plans to survey the community on their feelings about the proposal, and make a decision with those results in mind.

Past surveys have been favorable. A survey in 2013, found that more than 60% of local residents supported using genetic engineering technology in Key West to control mosquitoes that spread dengue. Only 18% said they were opposed; another 21% were neutral.

But as a trial of such technology has come closer to becoming a reality, opposition has grown, at least in volume if not number. A small, more recent survey of 88 of the community's 456 households by Johns Hopkins found that 58% of residents now either “oppose” or “strongly oppose” genetically modified mosquito use. All over Key Haven, protestors abhor the release of genetically modified mosquitoes, swarming community meetings wielding “No Consent” signs. (More than 160,000 people also signed a petition opposing Oxitec’s trial in the Florida Keys, though most did so before the Zika crisis.)

There are few places where the opinions of the public are so divorced from that of science as genetic engineering. According to a Pew Research Center survey last year, while 88% of scientists believe that it is safe to eat genetically modified foods, only 37% of the public does. That gulf is even wider than the one between the public and scientists on the subject of climate change.

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"People care at least as much about the process and motives of the project backers as they do about what is done," Kevin Esvelt, a scientist who runs MIT's Sculpting Evolution lab, told me. "That's especially true for a new technology, when most people start out neutral, and is of course less true for GMOs."

Esvelt, who has been working on his own project to combat Lyme disease in the Northeast using genetically engineered mice, says that any solution which relies on genetic engineering is bound to face more intense criticism. An approach to combatting Zika that Oxitec's dissenters endorse, for example, relies on infecting males with a virus-killing bacteria called Wolbachia instead, an approach that at least on the surface seems more 'natural.' (The Wolbachia approach is also promising, but tests have so far only been successful with a different species of mosquito, the Aedes albopictus. Tests using Aedes aegypti have only just begun.)

"People are apprehensive about the release of these mosquitoes simply because they are genetically modified," molecular geneticist Nina Fedoroff and former secretary of agriculture John Block wrote recently in a New York Times op-ed. "This is unfortunate, because biological insect control can eradicate pests over large areas."

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In the Keys, residents are suspicious at least in part because the project is being undertaken by a private corporation, in a way that at least to them feels opaque. Additionally, a 2010 outbreak of dengue fever, was conquered in the Keys without having to rely on genetic engineering. To residents of Key Haven, Oxitec's mosquito project seems like a lot of risk to solve a problem they don't actually have.

"When I've gone around to people and explained it to them most people are fine with it," Nimmo told me. "Other people say, 'You’re from a company, how can I believe you?' In some ways, I can understand that."

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At community meetings in the Keys, Nimmo has fielded questions from apprehensive residents on whether bites from Oxitec's mosquitoes could sterilize children. 

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Nimmo said that for most people, the hardest thing to swallow is that with such an experiment, there is no 100% guarantee of how anything will turn out. There are still lots of unanswered questions about what might happen if we release genetically engineered mosquitoes into the wilds of the Florida Keys. Nimmo can't promise for certain that it will prevent Zika, or that releasing thousands of mutant mosquitoes won't wind up having unforeseen consequences. In its assessment, the FDA determined that adverse effects are "highly unlikely," but there are no guarantees.

"I’m a scientist. We deal with the facts as much as we can," he told me. "We can never say anything is 100% certain. And some people are dead set to say that if it's not 100%, then it's not worth the risk. That's a general starting point I find quite challenging."

But Nimmo insists that the risk is minimal—and worth it. Discovering a way to stop mosquitoes from spreading multiple diseases would save tens of thousands of lives every year. Perhaps most importantly, while many other Zika interventions are in the works, Oxitec's is tested, FDA-approved and ready to go.

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“Everywhere else where we’ve done this there’s been 90% or better control of the population,” he told me. “If we can show that it’s the same in the Key Haven, it has a really good chance of being able to prevent Zika in Miami or wherever in the U.S.”