Zika Virus Might Still Pose a Silent Threat to Pregnant Women

The Zika virus might be a more common cause of miscarriages and stillbirths in women who show no symptoms than previously thought, according to a study published on Monday in journal Nature Medicine.

An infection known to cause birth defects, Zika gained national attention in 2015when a cluster of cases in Brazil produced an uptick in babies born with abnormally small heads. Known as microcephaly, this brain malformation is connected to developmental delays, seizures and other serious neurologic issues. Now, through new research conducted on monkeys, scientists found that the Zika virus leads to miscarriage or stillbirth in 26 percent of infected monkeys, animals who often don't exhibit symptoms. This could be a clue that the rate of pregnancy loss in humans is much higher than previously thought since women who lose their babies during pregnancy might not know that they are infected, researchers said.

To conduct the study, scientists infected 50 monkeys with Zika virus during the beginning of their pregnancies and monitored the primates with ultrasounds and blood tests.

The researchers found a much higher rate of pregnancy loss than the nearly eight percent rate found earlier this year by a study of women infected with Zika early in pregnancy. Three more baby monkeys in the new study also died shortly after birth.

The actual rate of human miscarriage and stillbirth in Zika-infected pregnancies is likely much higher than human studies show, lead author Dawn Dudley, a researcher with the Wisconsin National Primate Research Center, said in a statement.

"There are limitations to the human studies, which rely on symptomatic infections," said Dudley. "Women get enrolled in the studies because they have Zika symptoms, but we know that up to half of people who have Zika don't show any symptoms at all. So, the pregnancy studies are probably missing half of the people who have Zika."

There are also cases that go undocumented. Some women may have Zika-caused miscarriages before they know they are pregnant. Others avoid follow-up care after miscarriages for personal reasons.

"You could never account for those women having a miscarriage due to Zika virus infection," said Dudley.

The researchers found that the virus took a toll on the tissues connecting the mother to the developing fetus.

"The placenta provides the blood flow and nutrients to the fetus. If you have problems with that aspect of pregnancy, that can lead to intrauterine growth restriction and poor outcomes—smaller babies all the way to fetal demise if the damage is substantial enough," said Dudley. "We're seeing evidence of damage to the placenta, cell death and areas where the cells are damaged to the point where you don't have proper function."