NEW YORK • The Zika virus damages many foetuses carried by infected and symptomatic mothers, regardless of when in pregnancy the infection occurs, according to a small but frightening study by Brazilian and American researchers.

In a separate report, other scientists suggested a mechanism for the damage, showing that the virus targets and destroys foetal cells that eventually form the brain's cortex.

The revelation came as Laos joined 41 countries that have reported local transmissions of the virus since the start of the year, according to the World Health Organisation.

The research reports are far from conclusive, but the studies help shed light on an epidemic that has swept across more than two dozen countries in the Western Hemisphere.

In the first study, published in The New England Journal of Medicine on Friday, researchers found that 29 per cent of women who had ultrasound examinations after testing positive for infection with the Zika virus had foetuses that suffered "grave outcomes". They included foetal death and tiny heads.

"This is going to have a chilling effect," said Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "Now there's almost no doubt that Zika is the cause."

The small size of the study - which looked at 88 women at one clinic - was a limitation, Dr Fauci said. From such a sample, it is impossible to be sure how often foetal damage may occur in a larger population. Still, the high percentage of foetuses damaged is "very concerning", he said.

In the second study, published in the journal, Cell Stem Cell, researchers at Johns Hopkins and elsewhere cultured types of cells present in early foetal development, including so-called cortical neural progenitor cells, which form the cortex, the outer brain layer. Three days after they infected all the cells with the Zika virus, 90 per cent of the progenitor cells were damaged.

NEW YORK TIMES