Side-view illustration of a baby with microcephaly (left) compared to a baby with a typical head size. Credit: Centres for Disease Control and Prevention
Report says pesticide is to blame for microcephaly outbreak - not Zika

But let’s be clear - there is no scientific evidence to support that link.

BEC CREW
16 FEB 2016
 

Right now, the Internet is abuzz with questions over a report that states it’s more likely that the outbreak of microcephaly - a rare neurological disorder that causes newborns to develop abnormally small skulls and brains - is linked to pesticides rather than the spread of Zika virus. The pesticide in question is pyriproxyfen - a larvicide that targets the Zika-spreading Aedes aegypti mosquito, produced by Sumitomo Chemical. 

"In the area where most sick persons live, a chemical larvicide producing malformations in mosquitoes has been applied for 18 months, and that this poison (pyroproxifen) is applied by the State on drinking water used by the affected population," the report, published by Argentinian group Physicians in Crop-Sprayed Towns (PCST) last week, states. 

 

The group adds that it’s "not a coincidence" that cases of microcephaly have proliferated in Brazil where pyriproxyfen is being used, while in Colombia, Zika infections have not been linked to microcephaly, despite the fact that it has the second highest incidence of the virus after Brazil. 

And even within Brazil, not every case of microcephaly has been linked to presence of Zika. "After experts scrutinised 732 of the cases, they found that more than half either weren’t microcephaly, or weren’t related to Zika," The Washington Post reported last month.

In response to the report, one Brazilian state has suspended use of pyriproxyfen until further notice.

"We decided to suspend the use of the product in drinking water until we have a position from the Ministry of Health, and so, we reinforce further still the appeal to the population to eliminate any possible mosquito breeding site," Joao Gabbardo dos Reis, state health secretary in Rio Grande do Sul, told the press over the weekend.

So what exactly is going on here? While some say the evidence supporting a link between pyriproxyfen and microcephaly is "overwhelming", others are calling it a "conspiracy theory", and the World Health Organisation (WHO) is insisting that pyriproxyfen is safe in drinking water at the recommended levels. 

The reality is that, right now, we have circumstantial evidence suggesting that pyriproxyfen in Brazilian drinking water could be increasing the risk of microcephaly, and this link is made stronger in the absence of a definitive causal link between the neurological disorder and Zika virus. People want answers, so it’s understandable that new evidence or hypotheses are given credence as we try to make sense of what is going on.

But is the lack of scientific evidence linking microcephaly to Zika equal to the lack of scientific evidence linking microcephaly to pyriproxyfen? Not exactly. 

While the WHO has explicitly said that the link between the microcephaly and Zika outbreaks has not yet been confirmed, this is less about a lack of scientific evidence and more about giving scientists the time they need to carry out their investigations properly. 

If they’re going to explicitly state that there is a link, they want to be 100 percent sure that this is backed by solid, causal evidence showing how an infection by Zika virus can cause biological changes in a foetus that severely disrupts growth. 

“Unlike the relationship between the Zika virus and microcephaly, which has had its confirmation shown in tests that indicated the presence of the virus in samples of blood, tissue and amniotic fluid, the association between the use of pyriproxyfen and microcephaly has no scientific basis," the Brazilian government said in response to the PCST report

"It’s important to state that some localities that do not use pyriproxyfen also had reported cases of microcephaly."

According to Donna Bowater at The Telegraph, the WHO says a definitive link between Zika and microcephaly is within weeks of being confirmed. "Two separate studies last week also found evidence of Zika virus in the brain tissue of aborted foetuses or babies who died soon after birth, who had microcephaly," she reports.

The director of Disease Control and Diseases of the Health Department of Pernambuco in Brazil, George Dimech, pointed out to the BBC that the city of Recife currently has the highest reported amount of cases of microcephaly, and yet pyriproxyfen is not used in the region - they use another insecticide altogether. Neurologist Vanessa van der Linden added that, "Clinically, the changes we see in the scans of babies suggest that the injuries were caused by congenital infection and not by larvicide, drug, or vaccine."

Orac over at the Respectful Insolence blog points out that "humans do not make or use sesquiterpenoid hormones (aka insect juvenile hormones), which is what pyriproxifen targets", and over the years, great deal of research has been carried out on the pesticide's physiochemical properties, toxicology, and safe levels.

If we take the circumstantial evidence linking microcephaly and pyriproxyfen at face value, it’s a wonder why microcephaly isn’t a much bigger, global problem. The pesticide is approved for use in the US and Europe, and has been for many years - albeit not in the drinking water, as it has been in some regions in Brazil.

There are still a whole lot of questions about why cases of microcephaly appear to have exploded in the recent months and years - many of which will not immediately be answered if scientists do come out and say there is a definitive link between it and Zika virus. But until we have causal evidence for a link between the disease and a certain type of pesticide, it’s unscientific to jump to conclusions based on inconsistent circumstantial evidence. 

What we’re about to say is a total cliche, but it’s a cliche for a reason: keep calm and carry on until we have all the facts.

Editor's note: We have removed mention of Monsanto in this article, because while Sumitomo Chemical and Monanto have worked together in the past, Monsanto was not involved in the production of this chemical, so the link between the two is not relevant to the story.

More From ScienceAlert

Scientists are meeting to discuss looking for life on Enceladus

This is our best shot at finding life in the Solar System.

58 minutes ago
The ozone hole is finally closing up

We didn't ruin *everything*.

2 hours ago
WATCH: Could we actually create artificial gravity?

Yes, and it could change everything.

3 hours ago

More From ScienceAlert

107 Nobel Laureates just signed a letter slamming Greenpeace over GMOs

"How many poor people must die before we consider this a 'crime against humanity'?"

1 day ago
A huge internet cable connecting the US and Japan is about to go live

It's promising to deliver 60 Terabits per second.

1 day ago