Zika can linger months longer in the blood of pregnant women than in other patients infected with the virus, according to a report published online Wednesday.
Although most people clear the virus from their blood within a week, one pregnant woman’s blood tested positive for Zika for 10 weeks after being infected, according to a report in The New England Journal of Medicine. The 33-year-old woman, who was infected with Zika while traveling in Central America when she was 11 weeks pregnant, only tested negative after the pregnancy ended. She had an abortion 21 weeks into the pregnancy, after tests showed extensive brain damage to her fetus.
Doctors can’t say for sure why the woman harbored Zika in her blood for so long, in spite of having developed antibodies against the virus.
Although antibodies protect the body from foreign invaders such as viruses, the immune system doesn’t usually attack a fetus. That could have allowed the Zika virus to survive and multiply in the fetus’ body. Authors of the study found large amounts of Zika virus in the fetus’ brain, as well as in the placenta and umbilical cord.
Viruses coming from the fetus or placenta could have reinfected the mother throughout her pregnancy, said study coauthor Rita Driggers, medical director of maternal and fetal medicine at Sibley Memorial Hospital in Washington. That might also explain why the woman’s blood tests for Zika were negative after the pregnancy ended and there was no longer a source of virus in her body.
It’s also possible that the woman cleared the Zika virus from her blood just as quickly as others, but that genetic material from the virus remained behind, said Peter Hotez, director of the National School of Tropical Medicine at the Baylor College of Medicine in Houston, who was not involved in the new study.
Scientists don’t know whether the woman’s case is unique or if other pregnant women would also test positive for Zika throughout pregnancy, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, who was not involved in the new study. The only way to know for sure would be to take blood samples from many pregnant women throughout their pregnancies.
“You can’t get all the answers from one case,” Fauci said.
Doctors in Brazil are currently conducting large studies of pregnant women, comparing those infected with Zika to those who are uninfected, to see how infection affects the health of the women and their babies.
The woman’s case could make other pregnant women with Zika infections uneasy.
The woman, who originally hailed from Finland but lived in Washington, had ultrasounds during the 13th, 16th and 17th weeks of pregnancy. When the results were normal, the woman’s doctors reassured her the her fetus was developing normally, Driggers said.
During the 19th week of pregnancy, however, an ultrasound showed abnormal brain anatomy in her fetus. The next week, doctors performed an MRI, a sophisticated scan that provides more detail than an ultrasound but doesn’t involve radiation. The MRI found that much of the fetus’ brain had wasted away, according to the study.
“Normally, you would expect the brain to fill up the skull,” said Driggers, who is also an associate professor of gynecology and obstetrics at the Johns Hopkins University School of Medicine in Baltimore. “There was fluid around the brain, filling up the spaces where the brain wasn’t.”
Zika has been strongly linked to microcephaly, a birth defect in which babies are born with abnormally small heads and, in most cases, incomplete brain development. In this study, the fetus’ brain didn’t meet the standard definition of microcephaly.
Driggers said her study shows that that fetuses can have extensive brain damage, even if their brains are normal size.
Doctors should consider performing an MRI “if there are any doubts about the diagnosis,” said study coauthor Adre du Plessis, director of Children’s National Health System’s Fetal Medicine Institute and chief of the fetal and transitional medicine division.
But it’s not practical to order an MRI for every pregnant woman, said Laura Riley, a spokeswoman for the American College of Obstetricians and Gynecologists.
That’s because the procedures can cost thousands of dollars. Also, there are relatively few experts who specialize in interpreting the results of MRIs in fetuses, which aren’t commonly performed.
Even the most sophisticated tests won’t answer all of a pregnant woman’s questions, Riley said. It’s possible that babies born after being exposed to Zika could have subtle problems that aren’t obvious on imaging tests.
“MRI may give you a better look at brain structure, but nothing is available to tell you about the brain’s function,” Riley said.