Scientists in Brazil are warning that the Zika virus may cause far more birth defects than was earlier believed - and the current wave of babies born with neurological damage could be the "tip of the iceberg". 

Although the infection rate has slowed down in parts of Brazil mostly because of a large public education campaign and mosquito-abatement programs, the Zika virus is still spreading.  More than a thousand babies have been born with microcephaly, to about one percent of women known to have been infected with Zika during pregnancy.  But doctors are warning that as many as 20 percent of Zika-affected pregnancies will result in a range of other forms of brain damage to the baby in the womb.  An earlier study in the new England Journal of Medicine said that "29 percent of scans showed abnormalities in babies in the womb, including growth restrictions, in women infected with Zika".

Meanwhile, US and Brazilian researchers speaking at a conference of pediatricians in Baltimore over the weekend said doctors might have overlooked many of these cases, because the babies appeared to be normal at birth and didn't exhibit signs of an abnormally small skull. 

"The microcephaly and other birth defects we have been seeing could be the tip of the iceberg," said Dr. Sonja Rasmussen of the US Centers for Disease Control and Prevention.

"The true burden of congenital disease with Zika virus is probably underestimated," according Dr. Marco Safadi of Sao Paulo's Santa Casa Medical School.

He relayed the case of a baby whose mother was infected during pregnancy.  The boy appeared to be normal.  But the doctors followed up two months later with MRI and CT scans, which showed that the baby had brain damage - areas of hardened brain tissue called calcifications, which happen when the cells die.  Other parts of the brain filled with fluid, a condition called ventriculomegaly. 

The case was especially troubling to researchers, because the family was from Sao Paulo in the country's south - and the Zika explosion had been going on in the northern, equatorial regions, where people are poorer.  The family of baby boy in Sao Paulo had access to modern medical scans.  Many children who weren't diagnosed as infants could start having problems as they growing, missing important development milestones.  Researchers worry the children will have learning deficiencies, vision and hearing problems, and physical disabilities.