Does Zika really cause microcephaly?

While reporting on the spraying of naled in Miami Beach, I encountered tons of protesters who said they were more afraid of the insecticide naled that was being sprayed to kill mosquitoes than they were of the virus itself. Many didn’t believe Zika was THAT harmful or that the virus actually caused microcephaly, as the CDC said.

The truth is, there’s a lot we still don’t know about Zika. And scientists are studying it in real time, so we’re not going to have the kind of definitive information we’re used to having.

That leaves a lot of non-science nerds left scratching our heads and getting nervous amid the dearth of information. These are a few questions I’ve heard you asking. I talked to a few experts to get to the bottom of them.

Zika’s been around since 1947. How come we’re only saying it causes microcephaly now?

The suspected link between Zika and microcephaly really started to gain traction six to eight months ago, when Brazil saw an unexpected hike in babies born with abnormally small heads around the same time the Zika outbreak took full force.

“We saw an unusual increase in microcephaly that we’ve never seen before,” according to Dr. Charles R. Bauer, a neonatologist at the University of Miami. But Zika isn’t the only cause of microcephaly or other brain defects in infants, it’s just one of many factors.

Scientists did a bunch of statistical mapping to figure out if Zika was the cause of the hike in Brazil, and their conclusion was basically “yup.”

But why is Brazil the only place where Zika has caused microcephaly? Zika was in Colombia too and it wasn’t that bad.

When doctors started seeing these hikes in Brazil, they did a retrospective study — aka looked back at a previous outbreak in French Polynesia — to see if it also caused a similar hike in microcephaly or other birth defects in places it’s been in before.

This was a good thing to do because, as we know hindsight is 20/20. So it really gave enough time for the virus to take its full course, and for the babies that would have been infected to be documented and monitored.

Scientists studied an outbreak in French Polynesia between Oct. 2013 and April 2014, when 66 percent of the general population was infected with the virus.

Here’s what they found: The normal rate of microcephaly in French Polynesia is about 2 out of every 10,000 babies (so that’s your baseline). With Zika, that rate went up to 95 out of every 10,000 babies. So that’s going from 2 babies to 95 babies with microcephaly, at the same time of the Zika outbreak.

Let’s fast forward to today.

As of Sept. 22, 47 countries and territories in the Americas have confirmed Zika transmission since 2015, and 16 of those countries have confirmed cases of birth defects associated with Zika.

In Brazil, there have been close to 2,000 confirmed cases of Zika-related birth defects. In Colombia there have been just 41. Here’s the whole list of cases of Zika-related birth defects in the Americas.

There’s a few reasons for the difference.

First: Scientists suspect that the interaction between Zika and another disease in Brazil might make it particularly aggressive.

Second: Timing. There’s not enough time to see the whole picture. Zika started circulating in Colombia after it got to Brazil, so those numbers were clocked in earlier. Zika is the most detrimental during the first trimester of pregnancy, so the babies that are being born right about now will give the best estimates of its impact on newborns in Colombia.

Third: Reporting. The data across countries is imperfect. Colombia was only reporting babies born alive, whereas other countries were reporting stillborns, miscarriages, and abortions.

I heard that they were putting pesticide in the water in Brazil. Could that cause microcephaly?

In Brazil, an insecticide called pyriproxyfen was added to water to stop mosquito larvae from breeding. A group of doctors in Argentina issued a report saying that the chemicals were put in drinking water a few months before the hike in birth defects. Even though there’s skepticism about the report, Brazil stopped using that pesticide in the drinking water because better to be safe than sorry.

But most experts say it’s pretty unlikely that pyriproxyfen caused any birth defects because it doesn’t interfere with the development of mammals — it only disrupts the development of insects. And it’s in a really small concentration in the drinking water.

Is there any evidence at the cellular level that Zika causes microcephaly?

Yup. And to be clear, microcephaly is the worst of it. But the interaction between the Zika virus and nerve cells can also cause a whole bunch of birth defects and brain calcifications that aren’t quite as bad as microcephaly. It can even affect adult nerve cells.

When you’re trying to prove a linkage at the cellular level, you need to prove something called causality, which basically means X causes Y. Lots of people were attributing the birth defects to other things like pesticides.

Causality was really important in this case because it eliminated those environmental factors, Muotri said.

“We proved the Zika virus can cause microcephaly with fetuses,” said Alysson Muotri, associate professor Pediatrics and Cellular & Molecular Medicine and the University of California, San Diego.

A Brazilian native, Muotri previously did autism research and realized he could use similar techniques to study Zika. He infected a pregnant mouse and saw birth defects in the pup, including microcephaly. He also used a structure called a “mini-brain” (which is exactly what it sounds like) and infected it with the virus and saw that the virus attracted to brain cells (something called neurotropism). It seeks them out, binds to them, and wrecks them.

“You can define what is the agent responsible for the damage — we see that microcephaly babies were born with the Zika virus, it was not the environment or a pesticide and we were able to eliminate environmental factors,” he said.Zika is enough and sufficient to cause microcephaly.”