Zika virus
Matheus Lima, 22, holds his two-month-old son, Pietro, who has microcephaly, at his home in Salvador, BrazilChristophe Simon/AFP

Scientists believe the Zika virus is causing eye abnormalities in newborns that could result in vision loss. They spotted damage to the retinas or optic nerves of more than a third of Brazilian babies affected by the virus whom they examined in December 2015, and ruled out other known potential infections that could have caused them.

Zika has already been linked to microcephaly — unusually small head and brains — in babies born to mothers with the disease. The virus could be responsible for some 4,000 cases of microcephaly in Brazil. But the discovery of the eye abnormalities indicates brain and nerve damage from Zika could be even more extensive than microcephaly.

The new findings involve only 29 infants, but they were carefully examined and have had a number of tests, said Dr Bruno de Paula Freitas and colleagues at Federal University of São Paulo.

The babies all had microcephaly. "Twenty-three of 29 mothers (79.3%) reported suspected Zika virus infection signs and symptoms during pregnancy, 18 in the first trimester, four in the second trimester, and one in the third trimester," the team wrote in the Journal of the American Medical Association's Jama Ophthalmology.

Ocular abnormalities were present in 10 (34.5%) of the children. Seven out of the 10 newborns had defects in both eyes, while three infants had damage in a single eye. The most common problems were black speckled lesions in the back of the eye, large areas of tissue damage in the retina itself, or damage in the layer of blood vessels and tissue below the retina.

It's not yet known exactly how the discovered lesions will effect vision as the children grow.

"Exactly how much these babies can see is unknown at this point. It is very difficult to measure visual function in a newborn baby," Northwestern University opthamology professor Dr Lee Jampol said in an editorial accompanying the study. But "when we can see these lesions, that means there's damage."

It's not yet clear whether a baby with a normal-sized head who was exposed to Zika in utero might develop ocular damage.

A geneticist at Seattle Children's Hospital, Dr William Dobyns, has said his examination of some brain scans of Brazilian children with microcephaly show "very severe, destructive injury".

The US Centers for Disease Control has advised pregnant women to stay away from countries where Zika is spreading, and the World Health Organization has declared the outbreak of microcephaly to be an international public health emergency