It’s winter in America, but that doesn’t mean scientists are letting their guard down when it comes to the Zika virus. If anything, the virus is becoming even more concerning. A new report from federal health workers reveals the Zika virus can replicate itself thousands of times in the placenta of pregnant women and in the brains of fetuses, which may explain why the virus appears to cause more health complications than the now well-recognized birth defect, microcephaly.

Scientists at the U.S. Centers for Disease Control and Prevention (CDC) tested tissues from 52 women and infants with a suspected Zika infection. This included brain tissue samples from eight infants from Brazil or Colombia who were born with microcephaly and later died. The researchers also tested the placenta tissue of 22 women who either had a miscarriage, still birth, abortion or a baby born with microcephaly. They also tested the placenta tissue of another 22 women who had babies that appeared healthy. The majority of the women in the study were American residents who had traveled to places where Zika was spreading while they were pregnant.

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The researchers discovered that the presence of Zika genetic material was 1,000 times higher in the brains of the fetuses than in the women’s placenta, and the virus continues to replicate in tissue seven months after the initial infection.

The virus’ ability to persist for over half a year could explain why some infants who were born without microcephaly began to show developmental problems as time went on.

“Our findings show that Zika virus can continue to replicate in infants’ brains even after birth, and that the virus can persist in placentas for months—much longer than we expected,” study author Julu Bhatnagar, lead of the molecular pathology team at the CDC’s Infectious Diseases Pathology Branch told HealthDay.

Throughout the ongoing Zika virus outbreak, government and academic researchers have been scrambling to learn as much as possible about the virus and when it’s most likely to cause developmental problems. Another report from the CDC released on Tuesday evening revealed that among pregnant women in the U.S. who contracted Zika, 6% had a fetus or infant that had Zika-related health complications. This was especially prominent among women who were infected in their first trimester: 11% of of these women had a fetus or infant with a birth defect.

Both reports underline advice from medical and government groups that pregnant women and their partners not travel to places where Zika virus is actively spreading. Pregnant women should also be tested for the Zika virus to make sure women and their physicians are aware of any possible risks.

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