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Why This Reddit Conspiracy Theory About Zika Is Wrong

Conspiracy theorists have suggested that genetically modified mosquitoes released in Brazil are behind the recent outbreak. Here's why that's not true.

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The Zika virus outbreak has been declared a global health emergency by the World Health Organisation (WHO).

Mario Tama / Getty Images

It's a disease carried by mosquitoes, and has spread rapidly across South and Central America in the last few weeks. Cases have also been detected in the USA, Australia and Europe.

Most people who get the disease only get mild, flu-like symptoms or nothing at all. But the virus has been linked with a sudden rise in the number of babies born with microcephaly – a condition in which the baby's head is abnormally small, as in the picture above. These children often have learning difficulties.

Recently, a bizarre conspiracy theory has cropped up about it. People have claimed that genetically modified mosquitoes are behind the disease.

The Daily Mail is probably the most high-profile media outlet to have covered the conspiracy theory, asking: "Are scientists to blame for Zika virus? Researchers released genetically modified mosquitos into Brazil three years ago."

It has also been covered in the Ecologist and the Kremlin-controlled channel RT.

As is often the case with conspiracy theories, there is a kernel of truth behind it. A British company really did release millions of genetically modified mosquitoes in Brazil in 2012.


It's part of the fight against another mosquito-borne disease, dengue, a horrible and painful viral infection also known as "breakbone fever".

The mosquitoes, which were all male, were engineered to be unable to have healthy offspring. The idea was that they would breed with females in the wild. The resulting juveniles would die as larvae, and the populations would crash.

It was a trial by a British company, called Oxitec (short for Oxford Insect Technologies), which began in 2010. It was extremely successful. In the areas it was tried, the populations of Aedes aegypti – the species of mosquito which carries dengue, as well as Zika – fell by more than 90%.

The GM mosquitoes are a new idea, but the concept of releasing sterile mosquitoes is more than 60 years old. In the past, mosquitoes would be bred and then irradiated to make them sterile. However, this made them less healthy, and therefore less likely to find mates.

The Oxitec mosquitoes are perfectly healthy as long as they are given a drug called tetracycline. But in their wild-born offspring don't get that drug, so they die.


The conspiracy theory appears to have begun on Reddit, where someone pointed out that the centre of the Zika outbreak is near the place where an Oxitec trial was held.

Oxitec-bred mosquito larvae in Brazil. Nelson Almeida / AFP / Getty Images

The post, in the /r/conspiracy subreddit, noted that Juazeiro, where the 2010 trial was, is in the north-east of Brazil. The outbreak also began in the north-east.

It also says that, in the lab, 3-4% of the supposedly non-viable mosquito offspring survive to adulthood even without tetracycline.

The post asks: "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."

However, there are some major flaws with the theory. Firstly, it's got the wrong Juazeiro.

Google / BuzzFeed

The Reddit post's map points to a place called Juazeiro de Norte. The Oxitech trials were in Juazeiro. It's about 170 miles away. That's the distance from London to Manchester.

(This fairly fundamental error was spotted by Christie Wilcox of Discover magazine.)

It gets worse. The Zika outbreak wasn't centred around Juazeiro (either Juazeiro). It was centred around Recife, on the coast.

Google / BuzzFeed

The Zika virus has existed in Africa and Asia for decades, but the recent outbreak in South America, and its apparently associated microcephaly, was first noticed in Recife in October. That's almost 400 miles from the correct Juazeiro – the distance from London to Aberdeen.


What's more, the Oxitec release in Juazeiro ended in 2012, and the Zika outbreak didn't start until May 2015.

Oxitec-bred GM mosquitoes in a laboratory in Brazil. Nelson Almeida / AFP / Getty Images

Haydn Parry, the CEO of Oxitec, told BuzzFeed Science that there was another release in Jacobina, 120 miles south, which was supplied from the Juazeiro factory, but that ended a while ago, and "nothing has happened in Juazeiro for at least a year". There are still mosquitoes in the factory, he says.

There is an ongoing trial in Piracicaba. But that's more than 1,000 miles away.

The virus doesn't last very long in the body – patients recover within two weeks of being bitten – so it's unlikely that it would survive unnoticed for months or years.

The idea that the genetically modified mosquitoes could increase the risk of a novel disease is false as well. For one thing, the mosquitoes that are released are all male.

Male mosquitoes do not bite – only females do. That means that males don't carry the virus.

Also, the idea that 3-4% of GM mosquitoes could survive without tetracycline is not supported by the evidence.

In laboratory settings, when the tetracycline was withheld, up to 5% of the mosquitoes did survive to adulthood. However, laboratory settings and the wild are very different. "When you rear these mosquitoes in the lab, they have beautiful conditions," Parry told BuzzFeed Science. "It's like a five-star hotel: Constant temperature, abundant food, no predators, mates, resting space."

Under those perfect conditions, still, fewer than one in 20 mosquitoes survived. When the researchers tried varying the temperature, they found no survivors at all. A study found no survivors in the wild eight weeks after release, either, even though the GM mosquitoes (and any offspring) were marked with a fluorescent protein which made them easy to spot.

Even if some do successfully reproduce, which a few may, there's no reason to think that they would be more likely to carry Zika or any other disease.

Manode / Getty Images

"The offspring [of the genetically modified males] die," said Parry. "If some don't – 5%, 1%, 0.1%, or whatever it is – then what have you done? You've put a mosquito into the environment that wasn't supposed to be able to reproduce, but it can. You've created a normal mosquito."

Except that any of the offspring that inherit the modified gene will almost certainly die as well. "If you breed that 5% on, they might have some survivors, but there's no resistance mechanism being bred in," Parry said.


One exponent of the conspiracy theory thought that GM genes could leap from the mosquito to the virus. But that's totally impossible.

The Ecologist piece, by Oliver Tickell, suggested that the method used to put the modified gene into the mosquito, a short piece of DNA known as the "piggyBac transposon", could have allowed genes to jump from mosquito DNA to viral DNA.

But, as Wilcox points out, the Zika virus doesn't have any DNA. It's an RNA virus. The genes simply can't jump across that way.

The sad irony is that Oxitec's methods could be the most powerful tool we have in slowing or stopping the spread of Zika.

Marvin Recinos / AFP / Getty Images

The company was doing it to target dengue, but Zika is transmitted by the same mosquito, and the same techniques would work to interrupt the spread of the disease.

Spraying for mosquitoes only reduces populations by about 30 to 50%, whereas breeding GM mosquitoes and releasing them has reduced them by at least 80% (and depending on how you measure it, over 90%) in every area it's been tried.

So far the efforts have only been small-scale, but Parry says that it's easy to scale up. "We work on the assumption that every female mosquito gives you 500 eggs, half male, half female," he says. Their offspring are ready to breed in five weeks. So one female mosquito could give you 250 more females in five weeks; 62,500 in 10 weeks; 15,625,000 in 15 weeks.

"It took us about three months to get a factory up and running in the UK," he says. "It's probably more complicated in Brazil – we assume six months rather than three. But it's only a question of how many buildings you buy, how many cages and trays. If you want to do this at scale, you can."


The Brazil Zika outbreak began in May 2015. An earlier version of this article said that it began in May this year.

Tom Chivers is a science writer for BuzzFeed and is based in London.

Contact Tom Chivers at

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