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A Project of The Annenberg Public Policy Center

GMOs Didn’t Cause Zika Outbreak

Q: Did genetically modified mosquitoes cause the Zika outbreak?

A: No. In fact, GM mosquitoes may be able to help control the virus’ spread.


Any truth to Zika virus linked to genetically modified mosquitoes released through the Gates Foundation?


The rumor that GM mosquitoes could be behind the Zika outbreak in Brazil began on Jan. 25 with a Reddit thread titled: “Genetically modified mosquitoes released in Brazil in 2015 linked to the current Zika epidemic?” Some media outlets, including Fox News, The Ecologist and The Daily Mail went on to spread the rumor. Some websites, such as Natural News, cited the involvement of Bill Gates. (The Bill & Melinda Gates Foundation provided $19.7 million for a project to develop and test GM mosquitoes, according to Science.)

SciCHECKinsertThe initial Reddit post was a reaction to the Zika virus’ spread through the Americas, including Brazil, and a rise in suspected cases of microcephaly, which can signify abnormal brain development. The Zika virus likely spread to Brazil around August 2014, as reported by the New York Times. As of Feb. 5 this year, the World Health Organization reported that Brazilian authorities estimate between 497,593 and 1,482,701 cases of Zika infection have occurred since the outbreak began. Zika is primarily spread by the Aedes mosquito.

The Aedes mosquito also spreads the dengue and chikungunya viruses. To reduce Aedes mosquito populations and combat these widespread viruses, a British biotech company called Oxitec genetically engineered male Aedes mosquitoes to produce offspring that die before they reach adulthood. This technique has been shown to reduce mosquito populations by 95 percent in some areas where Oxitec released the GM mosquitoes. And fewer mosquitoes mean less disease transmission.

Why do some claim GM mosquitoes caused the Zika outbreak? The initial Reddit contributor, who posted to a subreddit geared toward conspiracies, included three reasons: (1) The mosquitoes were released in the same area as Zika’s epicenter; (2) some offspring will still survive and pass on their genes; and (3) GM mosquito and Zika genes will then mix, resulting in a super virus that causes microcephaly.

This is all false: (1) Oxitec released its mosquitoes 400 miles away from Zika’s epicenter; (2) only a small percentage of offspring survive; and (3) there are no known ways for mosquito genes to infiltrate those of Zika.

Nevertheless, a newly released survey indicated that more than a third of Americans believe the rumor. The Annenberg Science Knowledge survey, which was conducted between Feb. 12 and Feb. 16, asked respondents which statement came closer to their view: “Genetically modified mosquitoes have caused the spread of the Zika virus OR Genetically modified mosquitoes could minimize the spread of the Zika virus?” Thirty-five percent said GM mosquitoes caused the spread, 43 percent said minimize the spread, and 19 percent said they didn’t know. 

Fifty-one percent of those polled also said that they are concerned that the Zika virus will spread to where they live. The Aedes mosquito distribution does extend to parts of the southern and eastern United States and Hawaii, but, as of Feb. 17, the Centers for Disease Control and Prevention reported no locally transmitted cases of Zika to the United States (beyond some local cases in U.S. territories). The CDC did, however, report 82 cases associated with travel to Zika-infected regions, such as Brazil.

What, Why and How on GM Mosquitoes

Oxitec first engineered its Aedes mosquitoes to combat the spread of the dengue and chikungunya viruses, but the same mosquitoes also transmit Zika. As a result, the company recently began discussing its technology’s potential to combat Zika.

So how are GM mosquitoes made? Oxitec engineered its male mosquitoes by disrupting their DNA, such that they produce offspring that are inviable in the absence of tetracycline, an antibiotic. Researchers took advantage of a mechanism that normally occurs within the cell to do this.

Within the cell, an enzyme called the transposase has the job of recognizing specific sequences of DNA, binding to them and then catalyzing the movement of that section of DNA to another part of the genome. In other words, the transposase cuts and pastes DNA from one section of the genome to another. That’s why scientists like to call these sequences of DNA “jumping genes.”

Taking advantage of this mechanism, Oxitec researchers inserted a gene that leads to developmental problems in the GM mosquitoes’ offspring. But if mosquito larva are reared on tetracycline, this gene is deactivated and normal development ensues.

Once released, the GM males go on to mate with wild females, who only mate once in their lifetime. With each release of the male GM mosquitoes, more pairs produce inviable offspring, and the overall mosquito population decreases within a given area. And when there are fewer mosquitoes to pass on dengue and Zika, there’s less opportunity for people to catch the viruses.

Still, some people believe GM mosquitoes are causing the Zika outbreak. We’ll explain why the claims made in the initial Reddit post were wrong.

Wrong Location

The Reddit contributor starts by implying that the area where Oxitec released the GM mosquitoes is the same area where “all the deformed babies are being born.” That’s false.

Oxitec first released GM mosquitoes in Brazil near Juazeiro, Bahia, which is roughly 400 miles away from the epicenter of the Zika outbreak in Recife, Pernambuco. That’s like claiming Washington, D.C., is the same area as Boston.

Moreover, the Reddit contributor inaccurately stated that GM mosquitoes were released in Juazeiro do Norte – a different city than Juazeiro, Bahia, that’s still 300 miles away from Recife.

Mosquitoes can’t travel very far. According to the World Health Organization, the Aedes mosquito, which transmits Zika, has a lifetime flight range of around 400 meters – that’s 0.25 miles.

Also, Oxitec released GM Aedes mosquitoes near Juazeiro, Bahia, between 2011 and 2013, not 2015, as the thread title claimed. Moscamed, one of Oxitec’s partners, did release more GM mosquitoes between 2013 and 2015 in Jacobina, Bahia. But this is roughly 440 miles away from Recife by air.

Since April 2015, Oxitec also has been releasing GM mosquitoes in Piracicaba, São Paulo. This is even further from Recife, at roughly 1,320 miles away. Not to mention the Zika outbreak had already started by then.

Matthew Warren, a press officer at Oxitec, told us that the locations we cite above include all of the releases of GM mosquitoes in Brazil.

As we explained previously, scientists have wondered whether Zika causes microcephaly because rises in reported cases of the virus matched increases in reported cases of the birth defect in time and place. But for GM mosquitoes, a similar correlation has not been found. As a result, there is no epidemiological evidence to support the claim that GM mosquitoes caused the Zika outbreak or an increase in reported cases of microcephaly.

Slim Survival

The Reddit contributor does correctly point out that researchers found roughly 4 percent of the GM mosquito offspring “survive to adulthood in the absence of the tetracycline antibiotic.” But that’s only in a laboratory setting.

Glen Slade, director of Oxitec’s Brazilian branch, told Discover that it is “unlikely that the survival rate is anywhere near that high in the harsher field conditions since offspring reaching adulthood will have been weakened by the self-limiting gene.”

Even in the lab, that number should be chopped roughly in half — only female mosquitoes can transmit Zika because only females bite people. And it should be doubly noted that Oxitec only releases male GM mosquitoes.

However, traces of tetracycline, the antibiotic the GM mosquitoes would need to survive, can be found in the environment globally, including in Brazil, due to runoff from agricultural practices, among other causes.

In 2015 researchers at Oxitec and elsewhere published a collection of studies that investigated the likelihood that leached tetracycline would increase the survival rates of GM mosquitoes in Brazil. However, their data showed “that potential routes of exposure of [GM mosquito] individuals to tetracycline and its analogues in the environment are not expected to increase the survivorship of [GM mosquitoes].”

Slippery Slope Debunked

Even if a female offspring of a GM mosquito were to survive (and fly significantly outside its normal range), there’s no mechanism to explain how the Zika virus and the GM mosquito genes would interact to bring about microcephaly or change the virus’ physiology. In other words, some evidence suggests that Zika alone could cause microcephaly, but this has nothing to do with the mosquito — let alone GM mosquitoes. The mosquito is only a carrier of the virus.

Still, the Reddit contributor uses a slippery slope argument commonly employed against GMOs: By interacting with the GM mosquito, the contributor wrote, Zika “could evolve into something far more dangerous than its original incarnation, pulling the lever on the slot machine with every replication until it hits the genetic jackpot.”

As we explained above, genes do “jump” from one place in the same genome to another. Researchers, including those at Oxitec, can also use this mechanism to insert genes artificially into an organism’s genome, thereby making it a genetically modified organism.

Viruses have been known to insert their genes by the same process into their host organism’s DNA — but this only occurs with specific kinds of viruses. Some viruses, such as Herpes, have double-stranded DNA as their genetic material. Others only have single-stranded DNA. Others still, including Zika, have only RNA. By comparison, humans — and all plants and animals for that matter — have double-stranded DNA.

But do host genes ever make it into virus genomes? In other words, could GM mosquito DNA make it into the Zika genome? No, virologist Kenneth Stedman told Discover: “There are a quite small, but not insignificant, number of examples of partial genomes of RNA and single-stranded DNA viruses that have been incorporated into [host] DNA genomes. … However, there are NO examples of the inverse, that is to say purely RNA viruses or ssDNA viruses picking up genes from host genomes.”

In short, there is no known way Zika could “evolve into something far more dangerous than its original incarnation,” as the original Reddit contributor claimed — at least not at the hand of GM mosquitoes.

Overall, the rumor that GM mosquitoes are behind the recent Zika outbreak in Brazil is completely ill-founded. The mosquitoes weren’t released in the same area; very few offspring survive; and even if they do, there’s no mechanism to explain how the mosquitoes’ genes could make their way into Zika’s genome.

Correction, Feb. 23: This article was updated to accurately reflect the Annenberg Science Knowledge survey question on whether GM mosquitoes have caused the spread of the Zika virus or minimized the spread of the virus. 

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.


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