US health authorities confirm Zika causes birth defects

The mosquito-borne Zika virus causes birth defects, including a syndrome in which babies are born with unusually small heads, US health authorities confirmed after months of debate and uncertainty.

"Scientists at the Centres for Disease Control and Prevention have concluded, after careful review of existing evidence, that Zika virus is a cause of microcephaly and other severe fetal brain defects," said a statement from the federal agency yesterday.

The decision was based in part on a series of studies in Brazil, where thousands of babies were born last year with birth defects, coinciding with a spike in Zika virus infections among the general public.

"This study marks a turning point in the Zika outbreak," said CDC chief Tom Frieden.

"It is now clear that the virus causes microcephaly."

There was no "smoking gun," or single piece of evidence that offered conclusive proof, said the full report, published in the New England Journal of Medicine.

Rather, the decision was made based on "increasing evidence from a number of recently published studies and a careful evaluation using established scientific criteria," said the CDC.

Further studies are being launched to "determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems," Frieden added.

Zika virus was first identified in 1947 in Uganda, but the virus was poorly understood and symptoms have typically been mild, including rash, joint pain and fever. Most of those infected report no symptoms at all.

Zika was first identified in Brazil in early 2015, and a surge in infants born with microcephaly began nine months later, in September.

Brazil has confirmed 907 cases of microcephaly and 198 babies with the birth defect who have died since the Zika virus outbreak started, officials there said last month.

The Brazilian health ministry said it is still investigating 4,293 suspicious cases.

"The severe microcephaly and other brain anomalies that have been observed in many infants are consistent with an infection occurring in the first or early second trimester of pregnancy," said the report in the New England Journal of Medicine.

Until now, no mosquito-borne virus has been known to cause birth defects, and the last infectious pathogen known to cause an epidemic of birth defects was the rubella virus more than 50 years ago.