The Zika virus outbreak is affecting 30 countries and has been declared a global health emergency by the World Health Organization (WHO).

At least five nations hit by Zika have seen an increase in cases of Guillain-Barré Syndrome, a rare nervous system disorder that can cause paralysis.

And in Brazil, the country worst affected by the epidemic, the authorities have reported an increase in babies born with defects, noticeably microcephaly, where the head is smaller than average and the brain fails to develop properly.

Naturally, pregnant women in the Zika regions are worried, although there is some doubt about the accuracy of the reporting of microcephaly in Brazil, and a causal link to the virus has not yet been proved beyond all doubt.

The lack of clarity over some of the facts behind the headlines has created a fertile breeding ground for internet conspiracy theories. Some are too crazy to bother with: the outbreak is divine judgement on the sexually licentious Brazilians; it’s the work of the CIA/the Israelis etc.

Others are slightly more plausible and are getting a lot of traction online. Let’s have a look at some of the theories that have gone viral – and find out why real scientists are not impressed.

Pesticides in drinking water

This is probably the most superficially plausible and popular alternative theory out there.

It began when an Argentinian organisation called Physicians in Crop-Sprayed Towns published this statement on the epidemic in Brazil.

The group claimed that congenital defects multiplied after the Brazilian government began adding an anti-mosquito poison manufactured by a subsidiary of the GM food giant Monsanto to drinking water “where most sick persons live”.

The claim was explosive enough for the southern Brazilian state of Rio Grande do Sul to suspend the use of the chemical – a larvicide called pyroproxyfen – in drinking water.

The move was immediately condemned by Brazil’s federal government, who said rumours linking pyroproxyfen and microcephaly lacked “logic and sense”. The manufacturer, Sumitomo Chemical, – not in fact a subsidiary of Monsanto – said there was “no scientific basis” for the rumour.

The Brazilian government pointed that the central argument set out in the statement was untrue: cases of microcephaly had been reported in areas that did not use pyriproxyfen.

This was confirmed this week when Dr Jailson Correia, health secretary of Recife, the city at the centre of the outbreak, stated the authorities do not use any chemical larvicides against mosquitoes.

Like many of the other alternative explanations Zika and microcephaly, the pyroproxyfen theory rests on a number of other ideas which many scientists disagree with:

“Zika hasn’t caused microcephaly anywhere else”

In fact, there is some evidence that congenital abnormalities of the central nervous system rose in French Polynesia after a Zika outbreak in 2013 and 2014.

Professor Paul Reiter, an expert on insect-borne diseases who has advised the WHO on disease control, said: “There was a rip-roaring epidemic of Zika in Tahiti from September 2013 to May 2014. The population is about 200,000, of which about 70 per cent were infected.

“Among these were 18 cases of microcephaly – 40 times the average – and these were associated with Zika infection in the first three months of pregnancy.  There were also 40 cases of Guillain-Barré Syndrome, also much greater than the usual incidence.”

“The point is, pyriproxyfen is not used in Tahiti (French Polynesia), so it’s absurd to implicate pyriproxyfen with this tragic event.”

Even if Brazil were the only place in the world to have ever experienced a spike in microcephaly following a Zika outbreak, that would not be surprising, said Paul Hunter, Professor of Health Protection at the Norwich Medical School, University of East Anglia.

“Outbreaks elsewhere have not been anywhere near as big as the epidemic in Brazil. In smaller epidemics it would have been statistically impossible to distinguish a small increase in cases of microcephaly from the general background.”

And diseases can behave very differently in places where they suddenly appear for the first time, he added.

“In countries of Africa where Zika has been endemic for years there is a further reason why microcephaly has not been observed. The epidemiology of flaviviruses such as Dengue and Zika are actually very different in endemic settings and when introduced into previously unaffected populations.

“When Dengue fever is endemic, most people have their first infection whilst still children and then develop a degree of immunity, whereas when introduced into a new population, adults and children are equally at risk.

“It does not require any stretch of the imagination that in an endemic setting, pregnancy-related problems are going to be much less prevalent than in a new epidemic.”

“Pyriproxyfen causes birth defects”

Except that the chemical has been used for years without anyone spotting these kind of side effects, and no one has come up with a plausible mechanism explaining why a chemical which interferes with the hormones of insects would hurt humans in this way.

Dr Ian Musgrave, a senior lecturer in pharmacology at the University of Adelaide, has said: “This hormone control system does not exist in organisms with backbones, such as humans, and pyriproxyfen has very low toxicity in mammals as a result.

“A person would have to drink well over 1,000 litres of water a day, every day, to achieve the threshold toxicity levels seen in animals.

“In a variety of animal species even enormous quantities of pyriproxyfen do not cause the defects seen during the recent Zika outbreak. Pyriproxyfen is poorly absorbed by humans and rapidly broken down so even the minute amounts humans would be exposed to via water treatment would be reduced even further.”

“Zika may not be to blame for microcephaly”

People pushing the pyriproxyfen theory tend to play on the lingering doubts over whether Zika really is the cause of microcephaly in Brazil. But a number of scientists are increasingly confident that a probable link has been established.

One recent study carried out on a Slovenian woman who was probably infected with Zika in northeastern Brazil showed the virus present in the brain of the foetus (later aborted), which also showed signs of severe abnormality.

Experts have called this paper a “blockbuster” and “pretty definitive evidence” of a link between Zika and microcephaly.

Professor Hunter said: “The epidemiological evidence points very strongly towards Zika being the cause of the microcephaly and this is backed up by recent papers reporting the presence of Zika virus in affected foetuses, and especially the brain.

“Whilst still not full proof the strength of evidence is remarkably good for this stage of an epidemic.”

It’s interesting to note that several scientists who have commented on the pyriproxyfen story have not said we should dismiss it out of hand – which you might expect if there really was some kind of global conspiracy to gag the truth.

Professor Hunter said: “It would be a tragedy if control measures for Zika stalled because of unsubstantiated and unlikely alternate hypotheses. Nevertheless, that does not mean we should not investigate any proposed hypothesis in the epidemiological investigations into this dreadful epidemic.”

Virologist Leslie Lobel was also quoted in the Guardian today as saying that a possible link with pesticides needed to be studied.

Professor Ira Longini from the Emerging Pathogens Institute at the University of Florida told us that while pyriproxyfen had been “used for a number of years in many places without any increase in birth defects… you cannot rule out at the possibility of an interaction between pyriproxyfen and Zika virus”, so more research would be welcome.

Genetically modified mosquitoes

A single speculative post on Reddit appears to have got this one started.

The idea is that a British company called Oxitec released a batch of genetically-modified mosquitoes close to the epicentre of the Zika outbreak in northeastern Brazil. There must, therefore, be some kind of connection between the two, although the actual explanation varies.

The theory is riddled with holes, as this blog on Discover first pointed out.

First, the Reddit post has confused a city where Oxitec really did carry out a trial with GM mozzies – Juazeiro, Bahia – with a different city with a similar name: Juazeiro de Norte, hundreds of miles away from the disease hotspot.

Second, the timing is wrong. Oxitec were not releasing mosquitoes anywhere close the epicentre in 2015, when the epidemic started.

Third, there is no good scientific reason why genetically engineered mosquitoes should help spread Zika. If anything, they are likely to do the opposite, as trials suggest the GM method is effective at suppressing local mosquito populations.

That may be why the WHO said this week that genetically engineered insects could prove to be a crucial tool in the fight against Zika, and recommended more research.

Looks like we can safely squash the GM mosquito theory.

Vaccines

Vaccine conspiracism is massive online, and it was inevitable that a massive health scare like this would press the right buttons.

Some websites have fingered a whooping cough vaccine given to pregnant women in Brazil as a more likely cause for microcephaly.

Again, no plausible reason is given for why this would happen.

Large studies have found no evidence of any link between the Tdap vaccine and birth defects, and other fact-checkers point out that websites pushing this theory have made basic errors like confusing two different vaccines.

The real whooping cough vaccine used in Brazil has been given to women in the US for years with no ill effects reported.