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The findings indicate that many children with Zika-related brain damage may go undiagnosed and are at increased risk for learning disorders, mental illness and dementia, according to researchers from the University of Washington School of Medicine.
However, researchers recently realized that even children with a normal head size at birth may develop serious Zika-related eye problems or late-onset microcephaly, when the head fails to grow normally after birth.
"Current criteria using head size to diagnose Zika-related brain injury fail to capture more subtle brain damage that can lead to significant learning problems and mental health disorders later in life," study lead author Dr. Kristina Adams Waldorf said in a university news release. She's a professor of obstetrics and gynecology who specializes in maternal and fetal infections.
"We are diagnosing only the tip of the iceberg," Adams Waldorf said.
For the study, the researchers monitored five macaque fetuses in mothers that were infected with Zika during pregnancy. Macaque monkeys are considered one of the closest animal models to human pregnancy, although research in animals may not produce the same results in humans.
Weekly ultrasounds revealed no obvious brain abnormalities in four of the fetuses. Though the brains of the fetuses did grow slower than normal, they did not meet the criteria for Zika-associated microcephaly used by the U.S. Centers for Disease Control and Prevention.
Under those criteria, 91 to 96 percent of children in the United States born to mothers infected with Zika during pregnancy are also not considered to have microcephaly, meaning they might not be checked regularly for Zika-related brain injury, according to the researchers.
Although ultrasound showed no problems, MRI scans revealed brain abnormalities in four of the five macaque fetuses. Certain areas of the brain were not growing as quickly as others, the study found.
"Subtle damage caused by this virus during fetal development or childhood may not be apparent for years," study co-author Dr. Lakshmi Rajagopal said in the news release.
Even so, it "may cause neurocognitive delays in learning and increase the risk of developing neurological disorders such as schizophrenia and early dementia," said Rajagopal, an associate professor of pediatrics and an expert on newborn infectious diseases.
Zika is typically transmitted by infected mosquitoes. But it can also be transmitted through sex from a person with Zika to his or her partners.
Last month, U.S. researchers reported that birth defects likely caused by Zika rose 21 percent between the first half of 2016 and the last half of 2016 in regions that saw local transmission of the virus that summer: southern Florida, a part of south Texas, and Puerto Rico.
Brazil was the epicenter of a large Zika outbreak that began in 2015.
Adams Waldorf said her study's findings indicate that "all children exposed to Zika virus in utero should be followed long-term for problems with learning and development, regardless of head size at birth.
"We should also be worried about children and young adults becoming infected with the Zika virus because they have the same vulnerable stem cells in their brains as the fetus," she added.
The research was published Feb. 5 in the journal Nature Medicine.
-- Robert Preidt
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