An experiment to sabotage Brazil’s mosquito population by releasing 450,000 gene-modified insects may have backfired, says a new study. The insect-breeding company is fighting back against the claims.

Editor’s note: The article has been amended to better reflect the position of Oxitec after the company raised concerns over the study’s credibility.

British biotech firm Oxitec conducted a 27-month-long experiment in 2013 in Jacobina, Brazil, aimed at reducing the local mosquito population by 90 percent while preserving the genetic integrity of the local insect population.

The overall goal was to curb the spread of mosquito-borne diseases, such as yellow fever, dengue, chikungunya, and Zika, by releasing half a million OX513A mosquitoes. The insects are a genetically-modified version of the Aedes aegypti mosquito, which combined a breed from Mexico with a breed from Cuba.

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The mosquitoes were supposed to contain a dominant lethal gene that would render the first generation of future offspring infertile (while also marking them with a fluorescent green protein gene). But, according to a recent study published in Scientific Reports, the experiment backfired and life found a way.

An Oxitec spokesperson claims the research contains “numerous false, speculative and unsubstantiated claims and statements” about its mosquito technology, claiming instead, in a three-page document, that the paper did not identify any “negative, deleterious or unanticipated effect to people or the environment from the release of OX513A mosquitoes.”

Indeed, that is notably different from what was published in the research journal.

“The claim was that genes from the release strain would not get into the general population because offspring would die,” said ecologist and evolutionary biologist Jeffrey Powell, who is one of the authors of the study into the efficacy and success of the GM mosquito trial. “That obviously was not what happened.”

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The original Oxitec research suggested that just three to four percent of the “infertile” offspring would survive into adulthood and would be too weak to reproduce anyway.

Anywhere from 10 to 60 percent of the mosquitoes analyzed by Powell and his team featured genomes tainted by OX513A. While the scheme apparently worked initially, with a dramatic reduction in the population, it would later seemingly backfire around the 18-month mark, returning the number of mosquitoes in the area to pre-release levels.

According to the study, the female population, opted not to mate with the weaker, genetically-modified mosquitoes anyway, in a phenomenon known as “mating discrimination.” But Oxitec disputes this claim, saying that selective mating “has never been observed” in any releases of close to 1 billion males worldwide.

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Powell and his co-authors warned that some genetically-modified mosquitoes even showed signs of “hybrid vigor” in which the artificially-introduced genetic diversity actually made the mosquitoes stronger and more resilient. Oxitec, however, claims the data “does not support this hypothesis.”

As for the study’s claim that the new mosquitoes showed some insecticide resistance, Oxitec said it had demonstrated that OX513A “is not resistant to commonly used insecticides.”

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