Birth Defects in Brazil Are Not Caused by Pesticide, Say Zika Experts

The rapidly spreading rumors linking larvicide use with microcephaly have no biological basis.

Getty/Victor Moriyama

The least-insane of the craziest Zika conspiracies — that a mosquito-killing larvicide, and not the virus itself, is responsible for microcephaly afflicting patients around the world right now — has been debunked by experts calling the claim “ridiculous.”

Rumors that the larvicide pyriproxyfen, used to kill off mosquito babies in Brazilian water supplies, is the real cause of the country’s rising rate of birth defects spread like wildfire through the information-dry social landscape. The lack of understanding about the Zika virus, combined with a rumored link between the larvicide and environmental darling Monsanto and a co-sign from Mark Ruffalo (which he later rescinded), fanned the flames, eventually leading health officials in one Brazilian state to halt its use in the local water supply.

But there’s no biological evidence to support their claims.

Enraged scientists have called the baseless claims — largely promoted by a group of Argentine doctors known as the University Network of Environment and Health — alarmist, irresponsible, and counterproductive.

University of Pittsburgh microbiologist Ernesto Marques, Ph.D., who is studying microcephaly in Brazil, said the larvicide-microcephaly link was false, and reiterated the Brazilian Ministry of Health’s statement that pyripyroxyfen has been used for decades with no reports of increased birth defects. Even the World Health Organization has testified to its safety.

Addressing the Argentine doctors’ claim that there is an increased incidence of microcephaly in areas where the larvicide is used, the director of the public health entomology laboratory at the University of Kentucky in Lexington, Grayson Brown, Ph.D., explained that the larva-killing toxin is applied to areas where microcephaly already occurs, because that’s where it’s most important to keep mosquito numbers down.

“The whole notion,” said Brown, “is misplaced cause and effect.”

Pyripyroxyfen works by interfering with a mosquito growth hormone that prevents larvae from becoming adults. What it does not do, as U.S. Defense Department consultant Graham B. White, Ph.D. has made clear, is interfere with the human nervous system, which is the part of the fetus that is affected by microcephaly.

It’s true that our knowledge about Zika is, at this point, painfully sparse, which creates the perfect conditions for such fear-based rumors to spread — and there are a lot of them.

While we wait for scientists to fill in the gaps, it’s crucial that government leaders encourage frightened citizens to stay rational and consider their sources. To sort out biological disasters, we need to trust our biological experts — and not Mark Ruffalo.

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