The link between the zika virus and birth defects is uncertain, so:
CDC, Brazil Start Big Study To Test Zika Link To Birth Defects, Reuters, 19 February 2016
The study has two groups: Babies with and without microcephaly. Babies and their mothers will be tested for zika, then the groups compared. This is still an epidemiological study, not a study that can discern a mechanism. But it’s a good one because it has matched controls.
The communities that are most affected by zika and by birth defects (thus, the link) are poor. Some households lack indoor plumbing. The water is dirty. There’s not enough of it. Windows, without screens, are left open for ventilation. Trash is piled in the streets.
The National Geographic posted several photos of the area in northeastern Brazil where there is a cluster of babies born with microcephaly. Here are a few:
What I have learned is that zika is a window into a bigger story.
By the way, those spraying photos remind me of when the insecticide DDT (manufactured by Monsanto, among others) was sprayed liberally in communities across the US in the 1950s. Do you remember the trucks? This is an actual photo of DDT spraying:
DDT was banned here in 1972, but not without a fight from agribusiness. (I see a similar fight going on now with the herbicide glyphosate, used in Monsanto’s Roundup.)
In 1962, Rachel Carson published the book Silent Spring. It cataloged the environmental impacts of widespread DDT spraying in the United States and questioned the logic of releasing large amounts of potentially dangerous chemicals into the environment without understanding their effects on the environment or human health. The book claimed that DDT and other pesticides had been shown to cause cancer and that their agricultural use was a threat to wildlife, particularly birds. Its publication was a seminal event for the environmental movement and resulted in a large public outcry that eventually led, in 1972, to a ban on its agricultural use in the United States.