Fact Check

Zika Virus Caused by GMO Mosquitoes?

A number of articles inspired by a single speculative comment in Reddit's conspiracy section suggested the Zika virus was transmitted by genetically modified mosquitoes.

Published Jan. 29, 2016

A recent uptick in Zika virus infections is linked to the introduction of a genetically modified mosquito farm in Brazil in 2012.

In January 2016, a number of news outlets reported an outbreak of the Zika virus across a number of Latin American and Caribbean countries, appearing to originate from Brazil. On 23 January 2016, BBC published an article that confirmed the virus as a suspected factor in a sharp increase in the incidence of microcephaly:

Colombia, Ecuador, El Salvador and Jamaica recommended to delay pregnancies until more was known about the mosquito-borne Zika virus ... Brazil said the number of babies born with suspected microcephaly - or abnormally small heads - had reached nearly 4,000 since October.

The link between microcephaly and Zika has not been confirmed - but a small number of babies who died had the virus in their brain and no other explanation for the surge in microcephaly has been suggested.

Although the outbreak caused alarm, Zika virus was itself not new. According to a New York Times item, the virus was isolated more than half a century earlier, in April 1947:

Until recently, health officials paid little attention to Zika virus. It circulated in the same regions as dengue and chikungunya, and compared to those two painful infections – nicknamed “break-bone fever” and “bending-up fever” – Zika was usually mild.

The virus is thought to have reached Asia from Africa at least 50 years ago. While it may have caused spikes in microcephaly as it first spread, there was no testing to pin down which of many possible causes was to blame ... In 2007, a Southeast Asian strain of the Zika virus began leap-frogging the South Pacific, sparking rapid outbreaks on islands where no one had immunity to it.

However, the 2015-2016 outbreaks apepared to be different. The Times piece surmised that introduction to a new population was a potential factor in the microcephaly spikes:

Zika was first confirmed in Brazil — a country of 200 million — last May, and it spread like wildfire. The first alarms about microcephaly were raised only in October, when doctors in the northeastern state of Pernambuco reported a surge in babies born with it. Pernambuco has nine million people and 129,000 annual births. In a typical year, nine are microcephalic infants.

On 29 January 2016, the unreliable alternative medicine web site Health Nut News published an article titled "Breaking: Zika Outbreak Epicenter in Same Area Where GM Mosquitoes Were Released in 2015." It speculated that the Zika outbreak was caused by the introduction of mosquitoes, which were genetically modified to decrease their ability to be vectors of disease, in Brazil:

Zika seemingly exploded out of nowhere. Though it was first discovered in 1947, cases only sporadically occurred throughout Africa and southern Asia. In 2007, the first case was reported in the Pacific. In 2013, a smattering of small outbreaks and individual cases were officially documented in Africa and the western Pacific. They also began showing up in the Americas. In May 2015, Brazil reported its first case of Zika virus — and the situation changed dramatically.

Oxitec first unveiled its large-scale, genetically-modified mosquito farm in Brazil in July 2012, with the goal of reducing “the incidence of dengue fever,” as The Disease Daily reported. Dengue fever is spread by the same Aedes mosquitoes which spread the Zika virus — and though they “cannot fly more than 400 meters,” WHO stated, “it may inadvertently be transported by humans from one place to another.” By July 2015, shortly after the GM mosquitoes were first released into the wild in Juazeiro, Brazil, Oxitec proudly announced they had “successfully controlled the Aedes aegypti mosquito that spreads dengue fever, chikungunya and zika virus, by reducing the target population by more than 90%.”

The article pointed to a Reddit thread published to the Conspiracy subreddit. Its original poster described the confluence of genetically modified mosquitoes appearing in 2012 and a Zika outbreak in 2015:

Zika was first confirmed in Brazil in may of 2015, but had been seen in other nations before. Question: Why didn't it cause an epidemic of birth defects in any other countries? How exactly would you miss a tenfold increase in children born with most of their brain missing? Zika in Brazil does not seem to behave like the Zika we were familiar with before.

How could the Zika catastrophe be linked to genetically modified mosquitoes? ... What is the effect on these mosquitoes that grow up with a mutilated genome? It is thought that this should introduce a fitness cost, that is, they should have greater difficulty surviving. What do we know about these mosquitoes? Has adequate research ever been done on how a genetically mutilated mosquito copes with viral infections? Could the mosquito be more susceptible to certain pathogens, that it then passes on to humans? If a pathogen like the Zika virus can thrive in the mosquito without restraint, it 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.

This echoed similar fears expressed in the Florida Keys in 2012, when residents feared that genetically modified mosquitoes posed unpredictable threats:

Hadyn Parry, Oxitec's chief executive, said that that the company had been approached by officials in Florida after dengue fever was reported in the Keys in 2009 and 2010. "The decision to go ahead is entirely a local Floridal decision — it's not up to us," he said.

The petition calls on the federal regulator the US Food and Drug Administration to withhold permission for Oxitec's mosquito to be released. De Mier said there are too many questions left hanging, such as whether the gene introduced into the insect could itself mutate, or whether the reduction of the A. aegypti mosquito could have adverse connotations for the local ecosystem.

Parry dismissed those fears. Only male mosquitoes would be released, he said, that do not bite and do not carry dengue fever.

The potential implications of fear about the Zika outbreak caught the attention of the World Health Organization, which quickly released a briefing reporting that the "level of alarm [was] extremely high" and the virus was "spreading explosively" in 23 countries. It continued:

Arrival of the virus in some places has been associated with a steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome.

A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but is strongly suspected.

The possible links, only recently suspected, have rapidly changed the risk profile of Zika, from a mild threat to one of alarming proportions. The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities.

However, the release didn't mention genetically modified mosquitoes as an area of epidemiology focus. Noting that having more questions than answers was exacerbating the described alarm over the zika virus, the WHO's press release stated:

Moreover, conditions associated with this year's El Nino weather pattern are expected to increase mosquito populations greatly in many areas.

The level of concern is high, as is the level of uncertainty. Questions abound. We need to get some answers quickly.

A 14 January 2016 U.S. News and World Report article titled "Zika Disease in Pregnant Women Can Be Devastating" reported that no fetal abnormalities were initially observed during a 2007 outbreak in French Polynesia. After the 2015 outbreak in Brazil researchers discovered that an unspecified number of microcephaly cases emerged concurrent with the 2007 outbreak, but were either unreported or not initially attributed to Zika:

In a 2007 outbreak on the island of Yap, a diving mecca in Micronesia, roughly 75 percent of the island's 5,000 inhabitants were infected, says Dr. Lyle Petersen, director of the CDC's division of vector-borne diseases.

There were no birth defects reported, but that may be because the population of Yap is so small, Petersen says. A much larger outbreak was reported in French Polynesia in 2013 and 2014, affecting 30,000 people. At the time of the outbreak, no fetal abnormalities were identified, Petersen says. "When they saw what was happening in Brazil, they went back and found microcephaly cases."

While the scope of those fetal abnormalities was indeterminate, it was clear public health agencies worldwide took the 2015-2016 Zika outbreak seriously. But CNN noted that the aggregate threat posed was minimal, and the spread was likely a function of countries ill-equipped to fight unusual mosquito-borne viruses:

For those who aren't pregnant, most will either have no symptoms or mild ones, and it's not serious at all. Eighty percent of those affected never know they have the disease. In fact, what typically happens is that once you get it, you become immune ... However, there has also been an association with the Zika virus and Guillain-Barre syndrome, an inflammatory syndrome of the central nervous system. It occurs with bad viral infections, sometimes the flu. But again, most people really have no problems at all.

... But it's spreading in many different countries where people are in close quarters, lots of mosquitoes are more present, and where there is a lack of screens on windows, air conditioning in buildings and insect repellent.

South Africa's Sunday Times addressed credible epidemiological theories as to how the virus became a problem in previously unaffected regions:

Genetic analysis of the virus revealed that the strain in Brazil was most similar to one that had been circulating in the Pacific ... Brazil had been on alert for an introduction of a new virus following the 2014 FIFA World Cup, because the event concentrated people from all over the world. However, no Pacific island nation with Zika transmission had competed at this event, making it less likely to be the source.

There is another theory that Zika virus may have been introduced following an international canoe event held in Rio de Janeiro in August of 2014, which hosted competitors from various Pacific islands.

Another possible route of introduction was overland from Chile, since that country had detected a case of Zika disease in a returning traveler from Easter Island.

Dr. Amy Y. Vittor, an assistant professor with University of Florida's Division of Infectious Diseases and Global Medicine, concluded by explaining how researchers worked to untangle the causative factors to eventually slow or stop Zika's transmission:

My colleagues and I are studying the role of these [myriad factors] as they relate to the outbreak of yet another mosquito-borne virus ... There, we are examining the association between deforestation, mosquito vector factors, and the susceptibility of migrants compared to indigenous people in the affected area.

In our highly interconnected world which is being subjected to massive ecological change, we can expect ongoing outbreaks of viruses originating in far-flung regions with names we can barely pronounce – yet.

In short, epidemiologists worldwide were hard at work to identify the cause or causes of Zika's spread in previously unaffected regions, determine the specifics of the possible link to microcephaly births, and generally resolve the many lingering questions about what was an unexpected and severe impact on pregnant women and developing fetuses.

By all accounts, the Zika outbreak was both mosquito-borne and potentially severe. Epidemiologists developed some theories as to how the virus shifted course, but none have definitively been pinpointed as an absolute, supported-by-evidence factor. Not present, however, among those developing threads was any suspicion that genetic modification of mosquitoes led to the outbreak. The only factor cited in the WHO's report was potential disruption to ecosystems by El Niño.

Finally, no aspect of the rumor made any clear or distinct connection between Zika and genetically modified mosquitoes. The claims simply pointed to an outbreak after the introduction of the mosquitoes in 2012, without any additional elaboration on how one might have affected the other. If genetically modified mosquitoes were a possible factor in the spread of Zika or increase in microcephaly births, it remained extremely unlikely such a link would escape the world's epidemiology experts and be discovered only by speculation by internet commenters in Reddit's conspiracy sub.


BBC. "Zika Virus Triggers Pregnancy Delay Calls." 23 January 2016.

McNeil, Donald G., Saint Louis, Catherine, and St. Fleur, Nicholas. "Short Answers to Hard Questions About Zika Virus."   The New York Times. 29 January 2016.

The World Health Organization, "WHO Director-General Briefs Executive Board on Zika Situation." 28 January 2016.

Gupta, Sanjay. "Sanjay Gupta Answers Your questions About the Zika Virus."   CNN. 29 January 2016.

Vittor, Amy Y.  "Explainer: Where Did Zika Virus Come From and Why Is It a Problem in Brazil?"   Sunday Times. 29 January 2016.

Kim LaCapria is a former writer for Snopes.