Paris (AFP) - Two studies of newborns in Zika-stricken Brazil yielded meagre clues Wednesday about the mysterious workings of the virus, and prompted researchers to call for better tests to identify brain-damaged babies.
Some infants with brain abnormalities may not be diagnosed because they have normal-sized heads instead of the tell-tale small skulls of those born with Zika-linked microcephaly, said one of the papers published by The Lancet.
More than 100 babies who had "definitely or probably" been infected with Zika in the womb, turned out to have normal-sized heads in a recent study, researchers said.
The skull is fully developed by about week 30 of pregnancy, which lasts some 40 weeks.
This meant that "newborns infected with the virus late in pregnancy may go unreported due to their head size being within normal range," said study co-author Cesar Victora of the Federal University of Pelotas.
Also, many of the affected infants' mothers had not had the pregnancy rash sometimes indicative of Zika infection.
Benign in most people, the mosquito-borne virus has been linked to microcephaly -- a shrinking of the brain and skull -- in babies, and to rare adult-onset neurological problems such as Guillain-Barre Syndrome, which can result in paralysis and death.
In an outbreak that started last year, about 1.5 million people have been infected with Zika in Brazil, and more than 1,600 babies born with abnormally small heads and brains.
Existing diagnostics, such as skull measurements or checking for a rash, were not enough to detect all Zika-affected children, said the team.
As a result, some may be living with brain damage that will only become apparent much later.
Doctors should also screen for other signs of brain abnormalities, using ultrasound brain examinations, for example, Victora told AFP.
And, "we need to improve the detection of Zika virus infection on the blood".
- Second wave -
The authors speculated that babies may develop brain damage from an infection that occurs even after birth.
"Zika affects the growing brain, and brain growth does not stop at birth but continues throughout infancy," said Victora.
"If a foetus infected in the last trimester of pregnancy can suffer brain damage, couldn't a newborn who is infected by a mosquito also be affected?"
No such case has yet been reported, "but I think it is possible that this will happen," the scientist said.
With a new wave of Zika virus infections in southeast Brazil early this year, there could soon be a second wave of microcephaly births, the authors added.
A second study added to the growing body of evidence linking the virus to birth defects.
It reported finding Zika in the brain tissue of three dead babies with severe brain damage, and the placenta of two miscarriages.
The US Centers for Disease Control and Prevention has concluding that Zika causes microcephaly, even though there have only been a handful of studies to confirm the presence of Zika in foetuses or babies with birth defects.
A lot more research is needed, said the authors, to determine how the virus works and what it does.