New Zika Threat to Infants: Late-Onset Microcephaly

Brenda Goodman

August 11, 2016

CDC researchers have made a startling discovery about microcephaly, a devastating birth defect caused by the Zika virus: It can affect babies late into a mother’s pregnancy and not become apparent until months after they are born.

Up to now, researchers have said babies were most susceptible to the virus when a woman was infected early in a pregnancy. Babies who have the devastating condition generally have abnormally small heads at birth.

But in the newly described form of microcephaly -- dubbed late-onset microcephaly -- babies appear to have normally sized heads at birth, but their damaged brains have stopped growing. By about 6 months of age, they have developed microcephaly because their head size hasn't kept up with normal growth, said William Dobyns, MD, professor of pediatrics and neurology at Seattle Children's Hospital. Dobyns has been studying brain injuries in Zika-infected babies.

The condition was observed by CDC researchers who are following more than 1,200 Zika-affected pregnancies in Brazil, said Ted Pestorius, an assistant incident manager for the CDC's Zika response. The finding has not been published in a medical journal.

Previously, scientists have described a fetal brain defect associated with Zika that causes the brain to shrink and the skull to collapse while a baby is still in the womb. Published case reports have also described a kind of late-onset microcephaly that doctors can’t detect on ultrasound scans but diagnose shortly after birth.

Pestorius shared the discovery with media attending a Zika briefing arranged by the Georgia Department of Economic Development on Wednesday. The CDC did not immediately provide more details about the study.

Cesar Victora, MD, PhD, an epidemiologist with the Universidade Federal de Pelotas, in Rio Grande do Sul in Brazil, and part of the team that’s been carefully documenting the outcomes of babies born to Zika-infected mothers in Brazil, confirmed that these cases were occurring. He said his colleagues have treated some of the babies.

The babies with late-onset microcephaly have been born to mothers infected during the third trimesters of their pregnancies, Pestorius said.

Dobyns said he's wary of that. "Connecting that to a third-trimester exposure is very premature," he said. He said he's familiar with the unpublished data, and it's based on relatively few cases.

Researchers had initially thought that Zika was most damaging when infection occurred during the early weeks of pregnancy, when a baby’s organs are still forming. The possibility that Zika may continue to cause severe damage later in pregnancy means unborn babies may be vulnerable to its effects at any time during their development.

However, Dobyns said based on his research, it's clear that Zika-infected babies can be born with normal head sizes and severe brain injuries.

"There's a lot of things to worry about here," he said.

In his briefing, Pestorius also updated reporters on the investigation into the puzzling case of nonsexual Zika transmission in Utah, where a dying man apparently passed the virus to a son who was taking care of him.

Pestorius said the case was “an outlier” because the older man had such high amounts of virus in his body. He said investigators have assessed the various ways the son might have come into contact with his father’s bodily fluids, and it appears that the son may have caught the virus as he wiped his father’s tears.

He said the CDC is considering whether it needs to revise its guidelines for medical professionals and others who may care for people infected with Zika.


Media briefing, CDC.

Ted Pestorius, management officer, National Center for Emerging and Zoonotic Infectious Diseases, CDC, Atlanta.

Cesar Victora, MD, PhD, epidemiologist, Universidade Federal de Pelotas, Rio Grande do Sul, Brazil.

William Dobyns, MD, professor of pediatrics and neurology, Seattle Children's Hospital.


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