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Zika virus kills the type of tissue found in the developing brain, researchers have shown.

It was able to destroy or disrupt the growth of neural progenitor cells, which build the brain and nervous system, in lab tests.

The discovery, published in the journal Cell Stem Cell, adds weight to claims that Zika is causing brain abnormalities in babies.

However, the US researchers caution this is not yet the conclusive link.

There have been more than 4,800 confirmed and suspected cases of babies born with small brains - microcephaly - in Brazil.

It is widely thought that the Zika outbreak is to blame, but this has not been scientifically confirmed.

The team from the Johns Hopkins, Florida State and Emory universities infected a range of tissue samples with Zika virus for two hours and then analysed the samples three days later.

The virus was able to infect up to 90% of neural progenitor cells in a sample leading to nearly a third of cells dying and the growth of the rest being disrupted.

A similar effect in a developing brain could have devastating results.

Image copyright Fiocruz Image caption One of the first images of the Zika virus taken from a patient in the outbreak in South America

The virus was able to infect only 10% of other tissue types tested including more advanced brain cells, kidney cells and embryonic stem cells.

Prof Guo-li Ming, one of the researchers, said the findings were significant and represented a first step to understanding the link between microcephaly and Zika.

She told the BBC News website: "Neural progenitor cells are especially vulnerable to the Zika virus.

"They are giving rise to the cortex - the primary part [of the brain] that shows reduced volume in microcephaly.

"But this research does not provide the direct evidence that Zika virus is the cause for microcephaly."

She said studies looking at brain organoids or animal studies were still needed.

Analysis

David Shukman, Science editor, BBC News

In the overcrowded hospitals of Recife, the teeming Brazilian city at the epicentre of the crisis, medics have long assumed that the Zika virus is to blame.

In a sweltering waiting room last month, where anxious mothers were clutching babies with abnormally small heads, Dr Angela Rocha told me that she was convinced of the link.

In a normal year, she said, she might see five suspected cases of microcephaly while in the previous few months she had seen more than 200.

When I asked whether it would be wise to wait for a full scientific analysis with peer-reviewed conclusions, she looked at me with scorn.

With so many cases, and more in prospect after the next rainy season, we do not have time to wait, she said.

It is unclear why these cells should be so vulnerable, but it appears they do not mount an immune response to Zika infection.

While not definitive, the study adds to mounting evidence including Zika being discovered in the brains of dead babies as well as in amniotic fluid.

Brain development researcher Dr Madeline Lancaster, from the MRC Laboratory of Molecular Biology, said the study was a "significant step forward".

She told the BBC News website: "The effect they see could well explain the surge in microcephaly and it opens the door for many further studies into how the virus is affecting stem cells and whether this affects their ability to generate neurons in the developing brain.

"I think it's a very important contribution and is extremely timely."

But she agreed with the researchers that more research was needed to "test whether Zika does indeed affect neuron generation and brain size" as well as how it crosses the placenta.

Dr Bruce Aylward, from the World Health Organization, said the evidence was mounting that Zika was causing microcephaly and another condition - Guillain-Barre syndrome.

He said: "Since the public health emergency of international concern was declared back in February, the evidence that there may be a causal relationship has continued to accumulate."

Meanwhile a study in Rio de Janeiro has added extra evidence to the microcephaly link.

It looked at pregnant women coming to hospital with a rash between September 2015 and February 2016.

Out of the 42 testing positive for Zika, ultrasound scans showed 12 had babies with abnormalities.

There were no abnormalities in the babies of the 16 women who did not have Zika.

The study published in the New England Journal of Medicine concluded: "Zika virus infection in pregnancy appears to be associated with grave outcomes."

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