[Update: Rio Grande do Sul, Brazil’s southernmost state, suspended the use of the pyriproxyfen—a pesticide that stops the development of mosquito larvae in drinking tanks—to combat the spread of the Zika virus. Read more here.]

As the Zika epidemic "spreads explosively" around the world, pregnant travelers have been put on pause due to the virus's suspected association with microcephaly, the congenital condition in which a baby's head is abnormally small.

While the link between the mosquito-borne virus and microcephaly has yet to be scientifically proven, Argentinian and Brazilian doctors have suggested an alternate culprit: pesticides.

The report, written by the Argentine group Physicians in the Crop-Sprayed Towns (PCST), suspects that pyriproxyfen—a larvicide added to drinking water to stop the development of mosquito larvae in drinking water tanks—has caused the birth defects.

The authors said that the pesticide, known by its commercial name SumiLarv, is manufactured by Sumitomo Chemical, a Japanese subsidiary of Monsanto.

According to PCST, in 2014, the Brazilian Ministry of Health introduced pyriproxyfen to drinking-water reservoirs in the state of Pernambuco, where the proliferation of the Zika-carrying Aedes aegypti mosquito is very high.

"Malformations detected in thousands of children from pregnant women living in areas where the Brazilian state added pyriproxyfen to drinking water is not a coincidence, even though the Ministry of Health places a direct blame on Zika virus for this damage, while trying to ignore its responsibility and ruling out the hypothesis of direct and cumulative chemical damage caused by years of endocrine and immunological disruption of the a acted population," PCST said.

Public health officials in Brazil inspect points of standing water, a breeding ground for the Zika-carrying Aedes mosquito. The virus has been linked to microcephaly, but the theory has not been scientifically confirmed. Photo credit: Flickr/Twitter

As it happens, the northeast state has registered the largest number of cases of microcephaly. Pernambuco, the first state to detect the problem, has roughly 35 percent of the total microcephaly cases across Brazil.

"Previous Zika epidemics did not cause birth defects in newborns, despite infecting 75 percent of the population in those countries," the paper said. "Also, in other countries such as Colombia there are no records of microcephaly; however, there are plenty of Zika cases."

Indeed, as The Ecologist observed from PCST's report, Colombia has so far diagnosed 3,177 pregnant women with the Zika virus, but these women gave birth to healthy babies or are carrying healthy fetuses.

Sumitomo Chemical says on the Sumilarv website that the product "not only poses minimal risk to mammals, birds and fish; it also allows it to be applied to drinking water. Application to the aquatic breeding sites of mosquitoes ensures a rapid decline of the adult mosquito population significantly reducing bite rates."

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However, GM Watch has cited a report from Abrasco, an organization of Brazilian doctors and public health researchers, that links the pesticide to the abnormality. GM Watch wrote:

Abrasco also names Pyriproxyfen as a likely cause of the microcephaly. It condemns the strategy of chemical control of Zika-carrying mosquitoes, which it says is contaminating the environment as well as people and is not decreasing the numbers of mosquitoes. Abrasco suggests that this strategy is in fact driven by the commercial interests of the chemical industry, which it says is deeply integrated into the Latin American ministries of health, as well as the World Health Organization and the Pan American Health Organization.

PCST also observed that the highest concentration of microcephaly cases are found in the poorest areas of northeastern Brazil. These areas, "with intermittent water supply, led these communities to store water at home in an unsafe manner, due to the inadequate protection of tanks intended for human consumption" leading to very favorable conditions for the Aedes mosquito.

While the Internet is abound with a number of possible theories on the origin of the virus—genetically modified mosquitoes, for one—the World Health Organization (WHO) has been careful to not explicitly link Zika to microcephaly.

"Although a causal link between Zika infection in pregnancy and microcephaly has not, and I must emphasize, has not been established, the circumstantial evidence is suggestive and extremely worrisome," WHO General Director Margaret Chan announced last month.

Zika-related microcephaly can be difficult to confirm. The Washington Post reported last month that after experts scrutinized 732 of the cases out 4,180 cases of Zika-related microcephaly, they found more than half either weren’t microcephaly or weren't even related to Zika at all. In fact, according to the ministry bulletin, just 270 were confirmed as microcephaly that appears to be linked to Zika or other infectious diseases.

However, as PBS's Frontline reported last week, Pernambuco researchers have identified clear evidence of the microcephaly-Zika link. According to the report, "researchers here tested the spinal fluid of 12 babies with microcephaly, all of whom were born to mothers who reported having symptoms of Zika early in their pregnancies. In all 12 cases, the researchers found evidence of Zika."

Secretary of Vigilance and Health Luciana Albuquerque said in a press conference at the Pernambuco Health Department that “this is not a scientific study,” noting that the results do not indicate a definitive causal link but added, “it’s a very important suggestion of the link.”

The WHO officially declared Zika a global health emergency and officials are carefully monitoring the spread. Researchers are also currently racing to develop a vaccine.

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