First U.S. Open-Air Test of Genetically Modified Mosquitoes Deemed a Success

Biotech firm Oxitec engineered the bugs in an effort to curb their numbers and help stop the spread of disease

Aedes aegypti mosquito on skin
Biotech firm Oxitec is genetically modifying mosquitoes in the hopes of curbing the overall population. The company completed its first open-air release of the bugs in Florida. Centers for Disease Control and Prevention

A private company that released genetically modified mosquitoes into the open air of the Florida Keys last spring says the experiment was a success, reports Emily Waltz for Nature. Oxitec, a United Kingdom-based biotech firm initially founded at Oxford University, released the bugs in an effort to curb wild mosquito populations.

The company has not yet published data from the experiment, but representatives said during a webinar earlier this month that the results were promising. Oxitec had already tested the genetically modified mosquitoes in the field in Brazil, Panama, the Cayman Islands and Malaysia, according to Nature, but this was the first open-air trial of genetically modified mosquitoes in the United States.

“We had quite a number of key performance outcomes that we were hoping to hit and we were able to hit all of those in this trial,” Nathan Rose, Oxitec’s head of regulatory affairs, said during the webinar.

FKMCD - #Oxitec Public Educational Webinar #16

Oxitec scientists genetically modified Aedes aegypti mosquitoes, an invasive species that can carry an array of harmful diseases, including yellow fever, dengue fever and Zika. They engineered the mosquitoes to be non-biting and male and injected them with a gene that is lethal to female offspring, thus killing off about half of all new mosquitoes and helping to reduce the population overall.

When scientists placed the modified mosquito eggs on various private properties around the Florida Keys last year, their hope was that the engineered bugs would mate with wild mosquitoes and confirm the efficacy of the company’s genetic tweaks.

To study the insects, researchers installed capture devices to collect both adult mosquitoes and eggs, according to Nature. They found that when the genetically modified mosquitoes matured into adults, their behaviors and fly ranges were similar to their wild counterparts. The modified male bugs indeed also mated with wild female mosquitoes, which, in turn, laid eggs that the company brought into a lab for observation as they hatched. Their analysis of more than 22,000 eggs found that only male mosquitoes made it to adulthood; the lethal gene worked as they had hoped.

The modified gene that killed off female offspring only lasted for two to three months — about three generations of mosquitoes — and mosquitoes carrying the lethal gene were only found within 400 meters (437 yards) of the launch sites, per Nature.

“This is pretty much what we expected,” Rose said during the webinar. “This really confirms the self-limiting nature of the genes, they’re not going to persist long-term in the environment.”

Pending state approval, Oxitec hopes to do further testing in Florida to study the effects of the genetic modifications on local mosquito numbers. The company also plans to release genetically modified mosquitoes at a testing site in California, according to Nature. In March, the Environmental Protection Agency gave its blessing for Oxitec to release 2.4 million genetically modified mosquitoes in the two states.

The overall project has been met with opposition from local residents since it was first announced in 2011, but, ultimately, federal regulators gave it the go-ahead.

Biotech Oxitec released genetically modified mosquitoes in the Florida Keys last spring in an effort to decrease the overall mosquito population. Now, the company says the trial was a success. Centers for Disease Control and Prevention

Though the overarching end goal of the company’s experiment is to slow the transmission of mosquito-borne diseases by reducing the overall population of the bugs, that outcome will likely prove difficult to study, Thomas Scott, an entomologist at the University of California, Davis, tells Nature. For starters, there aren’t enough mosquito-borne disease outbreaks in the continental U.S. to make up a robust field study and, beyond that, outbreaks can occur even when mosquito numbers are low, Scott says.

“They’re not going to be able to do a trial to show that it actually has a public-health impact,” Scott tells Nature.

Even so, local pest control groups are supportive of Oxitec’s efforts. Before 2009, there had been no recorded cases of dengue in Florida since 1934, according to the Florida Department of Health. But in 2009 and 2010, 88 people in Key West came down with the disease, which can cause vomiting, rashes, nausea and body aches and pains. Then, in 2020, 72 people contracted the virus, according to Andrea Leal, executive director of the Florida Keys Mosquito Control District, who also spoke during the webinar.

Leal said the Aedes aegypti species makes up just 4 percent of the local mosquito population but causes 100 percent of the area’s mosquito-borne illnesses. She added that the bugs are difficult to control and may be becoming resistant to pesticides.

“We’re looking for new tools to put in our toolbox to help us control this particular mosquito,” she said during the webinar. “There is no silver bullet, and we’re just really hopeful that we are finding something that we can integrate with the rest of our control methods.”

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