NEW YORK (Reuters) - The Zika virus is capable of quickly infecting and harming developing fetal brain cells, scientists said on Friday in a study that provides insight into how the virus might cause the birth defect microcephaly in fetuses exposed in the womb.

Slideshow ( 2 images )

The researchers said their study, published in the journal Cell Stem Cell, does not provide proof of a direct causal link between Zika and microcephaly, but it does identify where the virus may be inflicting the most damage in developing fetuses.

The mosquito-borne virus infects a kind of neural stem cell that goes on to form the cerebral cortex, the brain’s outer layer responsible for intellectual capabilities and higher mental functions, the study showed.

Researchers found that these cells, exposed to the virus in laboratory dishes, were infected within three days, turned into “virus factories” for viral replication and died more quickly than normal.

“Our study shows once the virus gets to the brain it can reach these very important cells,” researcher Hengli Tang, the study’s lead author from Florida State University, said in an interview.

Tang said the study suggests the virus would be capable of doing the damage seen in microcephaly, a condition defined by unusually small heads that can result in developmental problems.

Zika has been linked to numerous cases of microcephaly in Brazil and is spreading rapidly in Latin America and Caribbean nations, prompting the World Health Organization to declare a global public health emergency.

Much remains unknown about Zika, including whether the virus actually causes microcephaly. Brazil said it has confirmed more than 640 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. Brazil is investigating more than 4,200 additional suspected cases of microcephaly.

“By determining whether Zika virus infects cells in the brain and what happens to a cell that is infected, this paper begins to tackle questions surrounding how a virus that had previously been known to cause a mild illness could be linked to microcephaly,” Amelia Pinto, a Saint Louis University expert on viruses transmitted by arthropods such as mosquitoes and ticks, said in a statement.

Tang said future studies will be needed to prove whether or not Zika causes microcephaly.

“We know people would be interested in knowing this information, but a lot still needs to be done,” Tang said. “Ultimately the proof would need to come from the clinical side and animal studies.”

The researchers are currently growing in the laboratory what they called “mini-brains” composed of the stem cells to see how the virus may affect development over a longer period of time.

Traces of Zika virus have been found in the bodily fluids and tissue of mothers and babies affected by microcephaly.

Dr. Lyle Petersen, director of the division of vector-borne diseases at the U.S. Centers for Disease Control and Prevention, told a news briefing on Wednesday at the Pan American Health Organization in Washington that there are numerous lines of evidence now linking Zika with microcephaly.

“I don’t think there is any question about that any longer,” Petersen said.