In case there were any lingering doubts, three new studies published May 11 could cement the theory that Zika virus infection in utero causes birth defects.
One shows that mice engineered to be susceptible to Zika can pass the virus to offspring via the placenta. In these pregnant mice, which have severely crippled immune systems, Zika infection can kill fetuses and developing brain cells, too, viral immunologist Michael Diamond of Washington University in St. Louis and colleagues report in Cell. But the researchers can’t say for certain whether the virus itself snuffs out cells, or whether damage to the placenta starves cells of oxygen.
Answers might come from two other mouse studies. Injecting Zika virus straight into the brains of fetal mice halts cell growth and kills cells, Cui Li of the Chinese Academy of Sciences in Beijing and colleagues report in Cell Stem Cell. Just five days after infection, embryonic mice already have smaller-than-normal brains. The results draw a direct link between Zika infection and microcephaly, the authors write.
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An even stronger link comes from Fernanda Cugola of the University of São Paulo in Brazil and colleagues. They infected a strain of mice called SJL with the Brazilian Zika virus — and the researchers didn’t have to tinker with the mice’s immune systems to do it. InfectedSJL mice then transmitted the virus from placenta to pups, and newborn animals showed signs of microcephaly, Cugola’s team reports in Nature. But mice of a genetically different strain, called C57BL/6, resisted Zika’s brain-damaging handiwork.
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The results suggest genetic differences could help explain why Zika strikes the babies of some pregnant women but not others, the authors propose.
The U.S. Centers for Disease Control and Prevention had officially confirmed the link between Zika and microcephaly and other birth defects last month, a few weeks after the World Health Organization reached the same conclusion. But the evidence weighed by both groups relied on clinical evidence, not experimental proof in animals.