The Zika virus causing an epidemic in Brazil and spreading through the Americas can infect and alter cells in the human nervous system that are crucial for formation of bones and cartilage in the skull, a study found on Thursday.

The research, published in the journal Cell Host & Microbe, may help explain why babies children born with to mother who have had the virus can have smaller-than-average skulls and disproportionate facial features.

Zika has already been shown to attack foetal brain cells known as neural progenitor cells - a type of stem cell that gives rise to various kinds of brain cells.

The death of these cells is what disrupts brain development and leads to microcephaly, the severe birth defect seen in babies whose mothers were infected with Zika during pregnancy.

American scientists who conducted this latest study by infecting human cells with Zika in the lab, found the virus can infect another type of cell known as cranial neural crest cells - which give rise to skull bones and cartilage - and cause them to secrete signaling molecules that alter their function.

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In the lab, the increased levels of these molecules were enough to induce premature differentiation, migration, and death of human neural progenitor cells, the researchers said.

"In addition to direct effects of Zika virus on neural progenitors and their derivatives, this virus could affect brain development indirectly, through a signaling cross-talk between embryonic cell types," says Joanna Wysocka, a chemical and systems biologist at the Stanford University School of Medicine who co-led the study.

She added that neural crest cells may be just one example, and that the same mechanism may also be relevant to other tissues that come in contact with a foetus' developing brain during head formation and could be infected by Zika.

The Zika virus has spread to almost 60 countries and territories since the current outbreak was identified last year in Brazil, raising alarm over its ability to cause microcephaly as well as other neurological disorders in infants and adults.

Brazil has been the country hardest hit so far, with more than 1,800 reported cases of microcephaly.

Wysoka's team cautioned that their work did not provide direct proof that Zika virus infects cranial neural crest cells in live animals or humans. Follow-up research would also be needed to look for any evidence that the virus' effect on these cells would be enough to cause microcephaly, they said.