A report last week has raised questions about a pesticide used to kill mosquito larvae, linking its use to the increase in cases of microcephaly in Brazil. But researchers say that's an unlikely connection.
The insecticide, called pyriproxyfen, is added to water to prevent mosquito larvae from hatching and growing properly.
Doctors in Argentina with a group called "Physicians Against Fumigated Towns" came out with a report last week that said Brazil had started using the chemical in drinking water a few months before health officials began noticing an increase in newborns with small heads.
"It's a hypothesis, a probability," says Dr. Medardo Avila Vazquez, a pediatrician in Cordoba, Argentina, and the main author of the report. "And for us, it's more likely that it's the chemical larvicide and not Zika."
The message quickly spread in Brazilian media, and last Saturday health officials in one Brazilian state announced that they would stop using pyriproxyfen in drinking water.
Even though there is a lot of skepticism about this report, the idea that the chemical is responsible has taken off. One of the main arguments is that if the compound can interfere with insect development, then it might also interfere with human development.
But that's not the case, says Bruce Gordon, coordinator of the Water, Sanitation, Hygiene and Health group at the World Health Organization.
"Pyriproxyfen actually mimics a hormone found in invertebrates, so it basically interferes with their development, but mammals don't have that development process," Gordon says. "There's absolutely no concern for reproductive effects that have been raised for this chemical."
Plus, the Brazilian Ministry of Health directs state and city officials to use a mere 0.0003 ounces of the chemical per gallon of water. That concentration is enough to kill mosquito larvae, but it's much lower than what WHO has deemed to be a safe concentration in drinking water.
"It's miles below the sort of acceptable recommended doses or the maximum doses you would apply," Gordon says. "You'd have to drink hundreds of liters of water to get to anywhere near potentially risky level."
In a statement Tuesday, Sumitomo Chemical, the company that manufactures pyriproxyfen, said that the product "has shown no effects on the reproductive system or nervous system in mammals, and has been approved and registered for use in the past 20 years by the authorities of around 40 countries around the world."
It's unclear how many countries actually use pyriproxyfen in their drinking water. For example, it's approved as a minor-use pesticide in the U.S. but only for crops and lawns, or to control fleas and ticks on pets, not for human ingestion. A study in a Cambodian village used it in water storage jars but did not follow up on long-term health effects.
Toxicologists have fed the chemical to guinea pigs, dogs, mice, lactating goats, laying hens, and pregnant rats and rabbits. At very high doses, it gave some animals mild anemia and liver and kidney problems. When given to the goats and hens, extremely small amounts of pyriproxyfen were found in their milk and egg yolks. (You can read about the toxicology studies here.)
Before pyriproxyfen, the Ministry of Health in Brazil distributed a different larvicide, temephos, to treat drinking water. It switched to pyriproxyfen in 2014, when mosquitoes became resistant to temephos.
"So far there is no evidence suggesting that [pyriproxyfen] could be the problem," says Dr. Marcos Espinal, director of communicable diseases and health analysis at the Pan American Health Organization.
The Zika virus is, he says, guilty until proven innocent.
He and other health officials maintain that it's hard to ignore the mounting evidence that Zika is responsible for microcephaly. The virus has been found in the placentas of infected women and in the brains of babies born to mothers who had the virus while pregnant.
Still, some doctors say it's better to be safe than sorry.
Shortly after the Argentine report came out the Brazilian Association of Collective Health, known as ABRASCO, put out its own statement calling for "immediate suspension of pyriproxyfen" and similar chemicals in drinking water.
"We need more information, since it's something new," says Dr. Gustavo Bretas, an epidemiologist with ABRASCO who worked for 15 years with the Pan American Health Organization.
"The position of ABRASCO is that we should try to minimize the use of larvicides and insecticide," he says, and instead focus on improving people's access to clean water.
Many households store water in tanks or buckets, in areas where piped water is intermittent. Those create perfect mosquito breeding sites.
"I think that's the most important message ... that we should prioritize access to water and sewage system, which is too deficient in the country," says Bretas. By investing in infrastructure, the government can ensure that mosquitoes don't have the chance to breed in stagnant tanks and buckets in the first place.
In the end, the most compelling point against pyriproxyfen's role in Brazil's health issues is this: Health officials in the state of Pernambuco, the so-called epicenter of microcephaly, say that in the three cities reporting the most cases — Recife, Jaboatao and Paulista — pyriproxyfen is not in use.
Valdemar Geo and Vanessa Rancaño contributed to this report.