Brazilian researchers studying the Zika virus say they've found evidence it may have evolved into a new form that's more likely to damage brain cells and cause birth defects.
Tests in mice, on cells in lab dishes and on "mini-brains" show the Brazilian strain seems to be more damaging to human brain cells, they report in the journal Nature.
Much more research needs to be done to prove this — but the findings mesh with other findings that show mutations that make the Zika strain now circulating in Latin America and the South Pacific are different from older strains first seen in Africa in the 1940s and 1950s.
"Our findings support the hypothesis that microcephaly is a distinctive feature of recent Zika virus Asian-lineage virus, which originated in the Pacific and is now spreading in South and Central America," Patricia Beltrão-Braga of the University of Sao Paulo and colleagues wrote.
Also Wednesday, several other teams of scientists published a batch of studies showing how the virus can get into the brain and cause the catastrophic birth defects for which it's been blamed.
Together, the studies nail the case that Zika is definitely causing the birth defects, and it seems to be acting alone — not helped by other factors, such as other viruses or poor nutrition.
"It is sufficient to cause birth defects on its own," Alysson Muotri of the University of California San Diego, who worked on the study, told reporters in a telephone briefing.
"The data in mice also suggest that microcephaly is only the tip of the iceberg," he added in a statement.
"The animals have extensive intra-uterine growth arrest, which essentially means poor fetal development in the womb. Media covering the Zika story have focused upon affected babies with small heads because such images are profoundly dramatic, but the true health impact is likely to be more widespread and devastating."
Zika started hitting Brazil hard about a year ago. Doctors there also raised the alarm about a troubling increase in birth defects, notably microcephaly, which is marked by damaged brain development and a small head.
But Zika had not been known to cause any kind of birth defects before, and was in fact a fairly obscure virus because it didn't even make most people sick. So there were doubts about whether the birth defects in Brazil were related to Zika.
In recent months doctors have found the virus in the brains of fetuses killed by Zika and in the placentas of women infected by the virus. The Centers for Disease Control and Prevention says there's no doubt Zika causes birth defects, and it almost certainly also causes rare neurological damage in adults such as the paralyzing Guillain-Barre syndrome.
One mystery had been why Zika wasn't ever known to do this before. But when scientists went back and looked at outbreaks in French Polynesia in 2013, they did find an increase in birth defects and Guillain-Barre. Genetic tests have shown two distinct strains of Zika — one that circulated in Africa, and one that moved from the South Pacific and into Brazil.
Those differences may explain why Zika now wreaks so much havoc, Muotri said. They experimented with both strains and the African strain seemed to be far less fatal to cells and to mice, they reported.
It's also possible that the birth defects and neurological damage are only being seen now because Zika is infecting so many people — more than a million and counting — so fast.
Other teams did experiments that show how the current Zika strains wreak devastation.
Dr. Michael Diamond and colleagues at Washington University in St. Louis infected pregnant mice directly with Zika. It killed the unborn pups, they reported in the journal Cell.
And there were huge amounts of viral genetic material in the placenta, which suggests the virus is attracted to placentas.
"This is the first demonstration in an animal model of in utero transmission of Zika virus, and it shows some of the same outcomes we've been seeing in women and infants," Diamond said in a statement.