Zika’s million-dollar question: Where are the birth defects?

For experts studying the mosquito-borne virus Zika, the data just got even more confusing.

A curious conclusion is emerging: While the virus has spread like wildfire across the Americas, outside Brazil, an epidemic of birth defects has not.

Tens of thousands of Zika cases have been confirmed or suspected in countries like Colombia, Venezuela and Nicaragua since late 2015. The infected have included thousands of pregnant women. But those infections have not led to a dramatic rise in reported birth defects like microcephaly, in which a baby develops an abnormally small head and brain.

This is confounding to researchers, who have suspected that Zika may lead to birth defects, especially based on Brazil's experience of the disease.

“Researchers are absolutely curious,” said Julie Fischer, co-director of the Center for Global Health Science and Security at Georgetown University. “It’s an enormous relief that the surge in microcephaly cases that was first noticed in northern Brazil has not spread everywhere that the Zika virus has been detected.”

Scientists are looking hard for answers. Their hypotheses range from "perhaps there have been more abortions in Colombia" to "a potential local Brazilian co-factor could have mixed with Zika to worsen the impacts."

We created three graphics for a better look at the latest numbers. They offer a glimpse at why researchers are so perplexed by Zika’s uneven impact across the Americas — what University of Texas Zika expert Nikos Vasilakis calls Zika’s “million-dollar question.”

1. The spread of Zika

As you can see above, Zika has now shown up in most of the largest countries in the Americas, though the number of suspected cases in Brazil is still higher than anywhere else. That’s according to the latest data from the World Health Organization’s regional office, the Pan American Health Organization, or PAHO.

In total, PAHO reports about 164,000 confirmed cases and more than 500,000 suspected cases of Zika in more than 50 countries and territories across the hemisphere. We’ve included only the 22 most populous countries and territories in our chart.

If you click on the tab at the top, you’ll see something even more interesting: the rate of Zika infection per 100,000 people.

A quick caveat: Zika is notoriously difficult to identify and quantify. Most people infected with the virus display no symptoms, and there still isn’t a cheap, widely distributed test to diagnose Zika infection. So overall infection totals, and incidence rates, need to be taken with a generous pinch of salt.

Interestingly, Brazil has the largest number of total cases, but it doesn’t top the list of per capita infection. Little Puerto Rico has an infection rate almost eight times higher than Brazil, according to PAHO data. Colombia, Venezuela and Honduras all have higher per capita rates of infection than Brazil.

This is important because of what comes next.

2. The uneven spread of Zika-linked congenital syndromes

Most Zika experts say we need to stop using the term “microcephaly” alone when talking about possible impacts from the virus. Zika is now suspected of being a factor in several types of fetal abnormality, as well possibly contributing to the onset of Guillain-Barre disease in adults. So, we’re sticking with the PAHO definition of “congenital syndromes,” which basically means a condition such as a deformity or birth defect found in the nervous system of a baby or fetus.

According to the PAHO data (and more on this in a minute), since late 2015 there have been a total of 2,175 confirmed cases of congenital syndromes associated with the Zika virus across all the countries and territories of the Americas. Of the total, 2,033 — 93.5 percent — have been reported in Brazil.

“The issue of microcephaly in Brazil is truly remarkable,” said Vasilakis, who was among the first researchers to rush down to Brazil to study the virus more closely in late 2015.

However, while researching, we noticed another curious thing about the data.

A footnote on the data bulletin from PAHO points out that Brazil reported “confirmed” cases of congenital defects differently from other countries in the Americas.

It reads (our emphasis): “Brazil Ministry of Health case definition for confirmed cases of congenital syndrome associated with Zika virus infection includes confirmed and probable cases per PAHO’s case definition.”

That struck us as a bit weird.

After all, PAHO lists “confirmed congenital syndrome associated with Zika virus infection” for each country. It states clearly that Brazil has 2,033. But most of those cases aren’t “confirmed,” by PAHO’s standards.

So we called Daniel Epstein, a spokesman at PAHO, to find out why they’re listed that way.

“Brazil has, in essence, cast a very wide net and it’s whittling down the numbers,” Epstein said. “The other countries have different definitions. There’s not a unifying standard, so we are kinda in the position of collecting the information that the countries give us.”

In other words, the numbers are kind of all over the place.

But one thing is for sure: The Brazilian government is at least claiming that there are 2,000-plus cases of babies born with congenital syndromes that are possibly linked to Zika.

The accuracy of Brazil’s data collection has been a big concern throughout this crisis. Fundamental statistics, like how many cases of microcephaly Brazil had pre-2015, have been called into question by experts. Back in December 2015, the Latin American Collaborative Study of Congenital Malformations, which monitors birth defects in the region, concluded that doctors in northeastern Brazil reported excessive microcephaly cases “probably due to active search and over-diagnosis.”

Meanwhile, Colombia and other neighbors had a bit more time to prepare. Colombian doctors and officials also say an increase in abortions may be a factor in the country’s lower number of Zika-linked birth defects.

But researchers in Brazil are looking into the possibility of local factors at work.

3. Within Brazil: A highly concentrated cluster of cases

The cluster of birth defects reported in northeastern Brazil remains one of the big mysteries of the Zika story. Researchers are still looking for answers.

Related: Brazil now has doubts that Zika alone causes birth defects

This week, Brazilian Health Ministry official Fatima Marinho told The Washington Post, “We don’t believe that Zika is the only cause.”

A team of experts working in Brazil on behalf of the government is now studying whether the reported surge of birth defects was caused by other factors, including a possible cross-reaction of Zika and another mosquito-borne disease like dengue fever or Chikungunya, said Oliver Brady, an epidemiologist on the team from the London School of Hygiene and Tropical Medicine.

We’ll continue following this story. Check back for updates.