As questions swirl around the Zika virus outbreak in South and Central America, a group of doctors in Argentina lobbed a grenade of a hypothesis into the discussion: Could a chemical meant to kill mosquitoes, introduced in 2014 to Brazilian water supplies, be the actual reason rates of local birth defects are increasing?

Globally, health officials like those at the World Health Organization acknowledge a possible correlation between Zika infections during pregnancy and a spike in microcephaly. A definitive link, however, between children born with smaller heads and the mosquito-borne disease remains elusive.

Into this gap jumped Physicians in Crop-Sprayed Villages, cautioning that larvicide pyriproxyfen might be the root of the congenital problems — while also raising the specter of agriculture giant Monsanto as boogeyman. (The involvement of Monsanto with pyriproxyfen supplier Sumitomo was later clarified to Tech Times as simply a business partnership).

The evidence is sparse — but then again, one might argue, so is what we know about Zika.

A city worker fumigates, attempting to eradicate the mosquito that transmits the Zika virus.

If the virus seems understudied for a disease that scientists were aware of since the ’40s, Zika has historically been eclipsed by more severe mosquito-borne illnesses, like dengue and yellow fever. In a snapshot addressed chiefly to pregnant women, the Centers for Disease Control and Prevention laid out what we do and don’t know about Zika: We don’t know when it’s safe for pregnant women to visit areas where Zika-infected mosquitoes might live (the CDC released an unprecedented travel advisory to this effect); we don’t know the rates of infection if bitten, or the likelihood of birth defects if a mosquito bites a pregnant woman.

But the desperate international focus on Zika has produced more details, and those details appear to weaken the position the Argentine doctors staked out. Viruses can, in fact, cause birth defects — rubella, for instance, has caused deformities in the brain and other organs. In a February 10 study in The New England Journal of Medicine, scientists described an association between Zika and birth defects — but, to be clear, not absolute proof — by examining a microcephalic fetus. What they found was that fetal brain tissue contained the “complete genome” of the virus. There is no such comparable evidence for pyriproxyfen, a chemical that mimics a mosquito hormone not found in the human body.

Responding to the physicians’ cautions on the potential link to larvicide, local health officials in Rio Grande do Sul ceased use of the chemical agent. This, in turn, sparked consternation among national experts.

In a statement, via The Telegraph, the federal government declared, “The association between the use of pyriproxyfen and microcephaly has no scientific basis. It’s important to state that some localities that do not use pyriproxyfen also had reported cases of microcephaly.”

As Zika continues to spread — 1.5 million Brazilians might have Zika, though some 80 percent won’t show symptoms — it’s unclear if chemical contingencies will be a sufficient way to combat the virus. Countries are exploring new options to tackle Zika — the WHO recently came out in support of releasing genetically modified mosquitoes that cause wild species populations to collapse.

Although uncertainty remains — and questioning the indiscriminate application of pesticides has been a smart move in the past — it’s very unlikely in this case that the link between larvicides and birth defects will prove stronger than the one being forged with Zika.

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