Washington, DC - Infectious Zika virus was isolated from the brain of a 21-week-old fetus after causing extensive damage to brain tissue – despite ultrasounds that showed no sign of microcephaly at weeks 13, 16, and 17, according to a report published online March 30, 2016 in The New England Journal of Medicine (NEJM).
“While this is a single case, it poses troubling questions that could inform future research,” said the study’s co-senior author, Adré J. du Plessis, MBChB, Director of the Fetal Medicine Institute and Chief of the Fetal and Transitional Medicine Division at Children’s National Health System. “Evidence is mounting that the Zika virus can persist in pregnant women’s bloodstreams weeks after their initial infection, arguing for changes to how these pregnancies are monitored,” Dr. du Plessis said.
Six of the named authors are affiliated with Children’s National, where the pregnant woman sought more thorough assessment after testing positive for the Zika virus herself following international travel.
According to the Centers for Disease Control and Prevention (CDC), Zika viral transmission is occurring extensively throughout Central and South America; the agency now asks pregnant women to consider delaying travel to those regions. The mosquito-borne infection can be passed by pregnant women to developing fetuses and has been implicated in a growing number of cases of Brazilian infants born with microcephaly, a condition characterized by undersized heads and severe brain damage. The World Health Organization has declared the Zika epidemic a Public Health Emergency of International Concern.
The 33-year-old Finnish woman whose case was summarized in the NEJM was 11 weeks pregnant when she and her husband traveled on vacation to Mexico, Guatemala, and Belize from November 22 through 29, 2015. The pair told researchers that they had been bitten by mosquitoes during their trip, particularly in Guatemala. One day after returning to their home in Washington, DC, the woman got sick, experiencing eye pain, muscle pain, and a mild fever (37.5 C) that lasted for five days. On the second day of her fever, she developed a rash. Her husband suffered similar symptoms at the same time.
A series of fetal ultrasounds that began one week after her symptoms subsided (13, 16, and 17 weeks of gestation) showed no evidence of microcephaly or brain calcifications that can signal Zika infection in the womb. The woman’s blood, however, tested positive for viral fragments (RNA) that further testing confirmed to be a strain of the Zika virus circulating in the current epidemic. More sophisticated testing, also conducted by the CDC, corroborated that the virus was closely related to Guatemalan strains.
By the 19th week, an ultrasound revealed significant abnormalities in the fetus’ brain. This timeframe is a critical period in the development of the fetal brain, as the brain normally grows larger and more complex as it builds the neural foundation that will be relied upon for life. In this fetus, however, the cerebral cortex appeared thin and there was extra space between the cerebral cortex and the cranium, an indication that while the skull continued to grow, the brain was not keeping pace.
By the 20th week, fetal magnetic resonance imaging (MRI) showed severe atrophy throughout the brain, especially in the frontal lobe, involved in decision-making, and the parietal lobe, where all incoming sensory information is processed and stored to facilitate learning, vision, hearing, touch, and taste. The MRI imaging technology peers into the womb, providing a clear view of soft tissue, and showed that the corpus callosum, which should be fully grown by the 20th week, was significantly shorter than normal. The anterior to posterior length was 14 mm, compared with the normal length of 18 to 22 mm for that age.
“The Zika virus infection, which resulted in extensive cell death, occurred at a time when the brain was most vulnerable,” said study co-author Roberta DeBiasi, MD, Chief of the Division of Infectious Disease at Children’s National. Given the grave prognosis, the couple elected to terminate the pregnancy at 21 weeks gestational age.
On the day the woman terminated the pregnancy, her blood surprisingly still tested positive for Zika RNA, some 10 weeks after her initial exposure. Eleven days after terminating the pregnancy, no evidence of Zika was found in the woman’s blood, saliva, or urine.
Imaging that was done from 19 to 20 weeks of gestational age showed significant structural abnormalities in the brain. Indeed, from the 16th to 20th week, the fetal head circumference decreased from the 47th to 24th percentile. Yet, this fetus did not meet the threshold – a head circumference below the 3rd percentile – to be diagnosed with microcephaly.
The aborted fetus’ brain, however, weighed 30 grams, less than the normal size of 49 grams. And high concentrations of Zika virus were found in the fetal brain using two different detection methods. Direct viral infection of neurons in the neocortex – the thin layer on the surface of the brain that plays a role in sight, hearing, and language – resulted in “significant” cell death, according to the study authors. “The neurons that were partly mature were the most affected; the less developed and the more developed brain cells were spared,” said Cheng-Ying Ho, MD, PhD, a Children’s National neuropathologist and co-lead study author.
While the concentration of virus at 21 weeks was highest in the fetal brain, umbilical cord, and placenta, lower concentrations of Zika virus also made its way to the developing fetus’ muscle, liver, lung, and spleen tissues as well as the amniotic fluid, the research team writes. Zika virus isolated from the fetus’ brain remained infectious when tested, growing easily in a human neuroblastoma cell line.
“The high amount of virus in the fetal brain and the placenta are concerning and suggest that the virus may be able to hide from the immune system there,” Dr. DeBiasi said. “Equally concerning is Zika’s apparent preference for fetal brain cells, where it replicates efficiently and can do damage silently,” Dr. du Plessis added.
Indeed, the research team hypothesizes that the woman’s persistent Zika infection was a “consequence of viral replication in the fetus or placenta, which had high viral loads.” Thus, early ultrasounds that appear to be “normal,” and a single blood test performed after the pregnant woman’s symptoms subside both have the potential to provide a false sense of security about the health of the mother and her offspring.
“What our paper suggests is that physicians should use caution in reassuring patients who have normal fetal ultrasound examinations early in their pregnancies, as these single snapshots in time may not capture infection-associated fetal brain abnormalities that may worsen as the pregnancy progresses,” Dr. du Plessis said.
“A number of viruses have the capability to pass from an infected pregnant woman to her fetus and to damage the brain. However, Zika is unique compared with other mosquito-borne viruses, such as dengue and chikungunya, in its capacity to infect and seriously harm the developing fetal brain,” said Dr. DeBiasi. “More research is needed to determine if serial measurements and blood tests could more accurately detect and, ultimately, predict fetal abnormalities following Zika infection. Learning more about the specific brain cells that Zika infects and the stage of pregnancy when this is most damaging may help us to identify therapeutic targets and provide clearer guidance for pregnant women.”
Contact: Diedtra Henderson at 202-476-4500