Zika virus is transmitted via the bite of the same mosquito that spreads dengue and chikungunya.
Scientists from the Centers for Disease Control and Prevention (CDC) came to this view after carrying out a careful review of the evidence. Reporting their findings in the New England Journal of Medicine, they note:
"On the basis of this review, we conclude that a causal relationship exists between prenatal Zika virus infection and microcephaly and other serious brain anomalies."
The CDC say the study confirms what mounting evidence has been pointing to, and they reassert earlier guidance to pregnant women and their partners, that they should avoid becoming infected with Zika. They also urge health care professionals to pass on this message to their patients.
Zika virus is transmitted via the bite of the same Aedes mosquito that spreads dengue and chikungunya; it can also be passed on through sexual contact. In many people, the infection causes no symptoms, while others may experience a mild illness, with raised temperature, rash, joint pain or conjunctivitis (itchy or sore eyes).
In October 2015, the Brazilian Ministry of Health reported an unusual surge in cases of microcephaly in newborns following a Zika virus outbreak in the northeastern states.
Since then, the spread of the virus to other parts South America and beyond, plus mounting evidence of a link to birth defects, caused the World Health Organization (WHO) to declare in February this year that the Zika virus is a global Public Health Emergency.
No cases of local transmission in mainland US
At present, Zika virus is thought to be spreading by local transmission - that is, mosquito populations are infected with Zika virus and can transmit it to humans - in the Caribbean, Central and South America, the Pacific Islands, Vietnam, the Philippines and Cape Verde.
However, earlier this year, an international team of researchers suggested that the virus also has the potential to spread to parts of the US.
According to the latest figures from the CDC (as of April 13), the US has had no cases of locally acquired Zika virus and 358 cases of travel-associated infection - that is, where infection took place in a country with local transmission and the traveler returned to the US with the virus.
While nearly every state has reported cases of travel-related Zika infection, more than half are in the four states of: Florida (84 cases), New York (57), California (29) and Texas (27). Thirty-one of the cases relate to pregnant women and seven were thought to be sexually transmitted.
In US territories, so far 471 cases of locally acquired Zika infection have been reported, and three travel-associated cases. The vast majority of cases are in Puerto Rico (445 cases), with the remainder in American Samoa (three) and US Virgin Islands (three). Fifty-eight of the cases relate to pregnant women.
Infection with Zika raises the risk of microcephaly, other defects
The report notes there is no single piece of evidence that conclusively proves Zika virus causes microcephaly and other fetal brain defects. But the mounting evidence, when sifted through a series of rigorous questions that disease scientists use in the absence of definitive proof, led the reviewers to come to this conclusion.
The CDC point out they are not saying all pregnant women infected with Zika virus will give birth to babies with brain defects. In fact, many infected women have healthy babies. Rather, they are saying that infection with Zika virus raises the risk of having a baby with microcephaly or another severe defect.
A paper published in The Lancet recently suggests women infected with Zika virus in the first trimester of pregnancy have a 1 in 100 chance of their child being affected by microcephaly.
The CDC are planning further studies to find out if the cases of microcephaly seen in children born to mothers infected with Zika virus are just the tip of the iceberg - how many other cases of brain damage and other developmental problems may be linked to the virus?
On Wednesday, Dr. Frieden tweeted:
"We're no longer questioning whether Zika causes microcephaly - but rather the spectrum of disorders this devastating virus may cause."
In theory, to find out for sure if Zika causes brain defects you would run a study - like a clinical trial testing a drug - that exposes a group of subjects to the virus and compares them to another identical but unexposed group.
For obvious ethical reasons, such an option is not available in Zika research, but there is a next best thing: use lab models of the developing brain - comprising live cells and tissue - and expose them to the virus.
Medical News Today recently learned of such a study, published earlier this week. Using two lab models of embryonic brain development called neurospheres and organoids, a Brazilian team showed that Zika virus kills developing brain cells and stunts growth.