Liz Szabo

USA TODAY

The Centers for Disease Control and Prevention announced Wednesday that it now considers Zika to be the definitive cause of the birth defects seen in Brazil and other countries affected by the outbreak.

The declaration marks a "turning point" in the scientific understanding of the virus and its link to microcephaly, a condition in which babies are born with abnormally small heads and incomplete brain development, said CDC director Thomas Frieden.

"There is no longer any doubt that Zika causes microcephaly," Frieden said. "Never before in history has there been a situation where a bite from a mosquito could result in a devastating malformation" in a fetus.

Study: Zika may affect babies even in later stages of pregnancy

Scientists around the world have suspected for months the mosquito-borne virus causes birth defects. The World Health Organization announced Thursday that "based on a growing body of preliminary research, there is scientific consensus that Zika virus is a cause of microcephaly and Guillain-Barré syndrome," a rare condition in which the body attacks its own nerve cells, causing paralysis.

Although scientists diagnosed the first case of Zika in a human in 1954, no one suspected it might cause birth defects until 2015, after the virus was detected in Brazil and reports of microcephaly began to climb.

CDC officials said they waited to make the announcement until they could be certain of the science. The new CDC report, published in The New England Journal of Medicine, is based on studies published as recently as Sunday.

The agency has taken a cautious stance on Zika since January, when the CDC warned pregnant women not to visit countries or territories with Zika outbreaks. The CDC issued that alert after tests found the Zika virus in the brains of two full-term infants with microcephaly who died just after birth.

That travel advice hasn't changed, CDC officials said.

The type of microcephaly seen in babies affected by Zika is particularly severe, said Sonja Rasmussen, director of the CDC's division of public health information and dissemination.

The babies' small heads are likely to be the "tip of the iceberg," Rasmussen said, noting that babies infected with Zika could face additional health problems that are less visible. Babies affected by Zika appear to have "fetal brain disruption sequence," which occurs when normal growth in a fetus is interrupted by a destructive virus or parasite.

In addition to microcephaly and brain abnormalities, babies with Zika infections also have had eye problems that could cause blindness, extra scalp skin, clubfoot and arthrogryposis, a condition in which newborns' joints are stuck in either a flexed or extended position, according to the CDC report.

Frieden said the lifelong cost of caring for a baby with microcephaly is estimated to be $10 million.

Scientists have a number of unanswered questions about Zika. They don't know, for example, how often Zika virus causes microcephaly, Rasmussen said.

Doctors also don't know if Zika poses a greater risk during some periods of pregnancy than in others, or if babies are at greater risk if their mothers exhibit symptoms of Zika than if they don't. About 80% of Zika infections cause no symptoms. The other 20% cause symptoms such as a fever, itchy rash, joint pain and pink eye.

Public health experts disagree about the effect the CDC announcement will have on the public.

"The CDC is the scientific gold standard," said Lawrence Gostin, director of the O’Neill Institute for National and Global Health Law at Georgetown University. "It's a wake-up call to the United States."

Others note that the public isn't paying close attention to Zika.

Four in 10 Americans have heard only a little or nothing at all about the virus, according to a survey released last week from the Associated Press and the NORC Center for Public Affairs Research at the University of Chicago.

Infectious disease researcher Michael Osterholm predicts Americans won't really care about Zika until the disease begins spreading here. Osterholm, the director of the University of Minnesota's Center for Infectious Disease Research and Policy, noted Americans showed little interest in Ebola until a patient was diagnosed in Dallas.

Now that the scientific debate over the Zika virus is over, "we can get on with the debate about how to stop it," Osterholm said.

Gostin said both individuals and political leaders need to take Zika seriously.

"We think we’re immune from this kind of thing because we have a highly developed health care system," Gostin said. "But this is not a case where a doctor can fix your baby."