No, a Monsanto-linked pesticide is not to blame for more microcephaly birth defects

A viral story is making the rounds online that the upsurge in cases of the birth defect microcephaly, a disorder where babies are born with abnormally small heads, is not linked to the Zika virus, but to a pesticide used to kill mosquito larvae.

It's almost certainly not true.

The number of babies born with microcephaly in northeast Brazil, the country hardest hit by the Zika outbreak, has increased about 40-fold from baseline levels in the last six months, according to reports from Brazil's Ministry of Health. While a link between microcephaly and Zika infections has not been proven, there's evidence to suggest the virus may be passed from mothers to their babies via the placenta.

But a story now claims that a chemical mosquito larvicide called Pyriproxyfen, manufactured by Japanese company Sumitomo Chemical, a business partner of agriculture giant Monsanto, was added to the drinking water in 2014, and is to blame for the recent rise in Brazilian cases of microcephaly.

The Brazilian state of Rio Grande do Sul has reportedly already stopped using the larvicide. Even Star Trek actor George Takei seems to be buying it.

Why a larvicide isn't to blame

But experts say that Pyriproxyfen cannot plausibly be responsible for the birth defects for several reasons.

• The chemical works by interfering with the hormones that help insect larvae hatch, which doesn't happen in humans.
• The pesticide is not easily absorbed by our bodies, but instead is quickly flushed from our digestive tracts.
• Animal studies of the chemical have found it has no health effects on reproduction or development at doses up to at least 100 mg per kg of body-weight per day.

"In terms of how much is present in water reservoirs that have been sprayed with pyriproxyfen to control mosquito larvae, a person would have to drink well over 1,000 liters of water a day, every day, to achieve the threshold toxicity levels seen in animals," Ian Musgrave, a senior lecturer in the medical school at the University of Adelaide in Australia, said in a statement.

The link between Montsanto and Sumitomo is also weak - Montsanto does not manufacture or sell larvicides, and does not own any part of Sumitomo, although the companies do partner in the area of weed control in Latin America.

While the levels of pyriproxyfen in drinking water may be harmless to humans, the effect of banning the larvicide may not be.

Andrew Batholomaeus, a consultant toxicologist at Australia's University of Canberra and the University of Queensland, states that "the potential human health consequences of discouraging the use of pyriproxyfen in drinking water storage and other mosquito-reduction programs is catastrophic with potential deaths and serious disease from otherwise avoidable malaria, dengue and other mosquito-borne diseases numbered in at least the hundreds of thousands."