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TUESDAY, Feb. 21, 2017 (HealthDay News) — Working with mice, researchers have learned more about how exposure to Zika virus early in pregnancy may increase the risk for miscarriage.

Normally, the placenta protects a developing fetus from viral infections. But, somehow, Zika seems able to cross the placenta in early pregnancy, the study authors said.

The mouse study also found that Zika-exposed fetuses that survive are more likely to be born with thinner-than-normal brain tissue, as well as brain cell inflammation.

The researchers believe that their findings highlight a point of vulnerability that could be a potential target for future Zika interventions.

“We need to find a way to stop transmission of Zika through the placenta into the fetus, because that is where the damage is being done,” said study co-leader Sabra Klein. She is an immunologist and microbiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

“In the placentas of our mice, we’re seeing a defense against Zika being mounted but falling short, especially in early pregnancy, a time that corresponds to the first trimester in humans,” Klein said in a school news release.

It’s important to note that the study by Klein’s team was with mice, and research with animals doesn’t always turn out the same in people.

The current study involved injecting Zika directly into the reproductive tract of pregnant mice, during a timeframe considered equivalent to the first trimester among women.

About 94 percent of the mouse pregnancies carried to term if they weren’t infected with Zika.

But only 56 percent to 71 percent of mice infected with Zika were able to carry a pregnancy to term, depending on the type of Zika strain, the investigators reported.

The miscarriage rate dropped, however, when the mice were exposed to Zika only during the equivalent of the late second trimester. This suggests that perhaps the placenta barrier is stronger by this point. As the pregnancy progresses, it’s possible that the more developed layers of protection are better able to block the passage of the virus, the researchers suggested.

In the same vein, mice born after Zika exposure during the first trimester equivalency were much more likely to have thin brain cortex tissue than those exposed during the late second trimester equivalency, the findings showed.

The researchers said it’s unclear whether maternal exposure to Zika at any point might also pose a risk to future mouse or human pregnancies down the road.

“We don’t know if the effects persist in future pregnancies,” Klein said. “We’re just dealing with the here and now. We have no idea what the long-term consequences are for the mother.”

The study was published in the Feb. 21 issue of Nature Communications.

Scientists have been scrambling to find a vaccine against the mosquito-borne Zika virus ever since an outbreak began in April 2015 in Brazil.

Thousands of babies in that country have been born with severe birth defects after their mothers were infected during pregnancy. The most common defect seen has been microcephaly, where the head and brain are abnormally small. But other birth defects have also been spotted with increasing frequency, including ones that damage vision, hearing and the nervous system.

More information

There’s more on Zika at the U.S. Centers for Disease Control and Prevention.

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