Microcephaly -- an underdeveloped skull and brain -- is the hallmark birth defect caused by Zika, which is mainly transmitted by mosquito bite.
But the study of 11 Zika-infected infants in Brazil -- the epicenter of the Zika epidemic -- found that microcephaly is only one of many brain injuries caused by the virus.
Brain scans of eight living babies and autopsies of three infants who died found lesions on the brain. These tests also showed excess fluid in the brain, an underdeveloped cerebellum, an absence of folds in the cerebral cortex, and abnormal voluntary and involuntary muscle contractions.
"Zika virus can directly kill neural cells as well as alter their differentiation pathway, making the fetal brain smaller by the end of pregnancy," said senior researcher Dr. Amilcar Tanuri. He's head of the Molecular Virology Laboratory at the Federal University of Rio de Janeiro in Brazil.
That's how microcephaly develops, he explained.
"However, it is not solely microcephaly we see. We can find other alterations that make us think of this as a congenital syndrome associated with Zika infection," he said.
Zika is the first mosquito-borne virus known to cause such devastating birth defects. Thousands of babies have been born with Zika-linked microcephaly, most of them in Brazil, since an outbreak began in South America in April 2015.
Zika infections have also been occurring in south Florida, with 59 cases as of Sept. 28, according to the U.S. Centers for Disease Control and Prevention.
Although the mosquito serves as Zika's main transmission route, researchers are discovering other modes of infection. Sexual transmission via sperm has been identified, and the New England Journal of Medicine last week reported on a case where a 38-year-old man in Salt Lake City appeared to contract Zika through physical contact with the sweat or tears of an infected patient.
For this new study, researchers in Brazil sought women who had confirmed Zika virus diagnoses during pregnancy, as well as ultrasound exams that found abnormal brain development in their unborn children.
Eleven mothers were identified. Three of the 11 babies died within 48 hours of delivery; the rest of the infants were followed until they were 6 months old.
Brain damage and neurological impairments were identified in all the babies. The researchers confirmed Zika in tissues from both mother and child, and they also ruled out other causes of microcephaly, such as genetic disorders and other infections.
Regular ultrasounds during pregnancy are essential for identifying children who might be subject to congenital Zika syndrome, Tanuri said.
The study was published Oct. 3 in the journal JAMA Neurology.
Dr. Amesh Adalja is a senior associate at the University of Pittsburgh's UPMC Center for Health Security in Baltimore. He said, "The [new] study establishes that the full spectrum of damage Zika can inflict on a developing fetus extends beyond microcephaly, and provides a basis for using the term 'congenital Zika syndrome.' "
Adalja likened it to congenital rubella syndrome, in which a pregnant woman infected with German measles can give birth to a child with a number of different birth defects.
The new findings add to mounting concerns of long-term neurological problems for any fetus exposed to Zika virus, even if they are born without microcephaly, said Dr. Raymond Roos. He's a professor of neurology with the University of Chicago Medical Center.
These problems could include learning disorders and behavioral problems that wouldn't become apparent until years later, said Roos, who wrote an editorial that accompanied the new study in the journal.
"I would be very surprised if there were not some long-term effects," Roos said. "Not all Zika virus infection results in clear-cut abnormalities that are obvious at the time of birth."
Roos' editorial called Zika a "public health emergency of international concern." But, he added that action by the U.S. Congress last week to provide $1.1 billion in additional funding to combat the virus "makes me a little bit less anxious about the situation."
New information about Zika and its effects has pointed the way toward vaccines that can prevent the virus' spread and antiviral drugs that can stem the damage done during infection, Roos said.
Zika is very similar to the Dengue virus, and that similarity is giving researchers a leg up, he said.
"We know a lot about Dengue viruses and other viruses related to Zika virus, so we can use that knowledge and extend it," Roos said. "It's not like we're starting from square zero."More information
The U.S. Centers for Disease Control and Prevention provides more information on mosquito-borne diseases.
This Q & A will tell you what you need to know about Zika.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.
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