To overcome this problem that prevented further observation, they dissected the pregnant mice to inspect the pups on the 18.5th day of pregnancy and found their heads were about a third smaller than those of normal pups.

"Therefore, this is the direct evidence that Zika infection causes microcephaly," Xu said, but cautioning: "Mouse is not human, and we need be careful when translate our findings into human disease."

Next, the researchers planned to work with other institutes to use their mouse model to test different drugs and a vaccine against Zika.

In a second study, researchers at University of California San Diego, with colleagues in Brazil and Senegal, also said they found the first "direct experimental proof" that the Brazilian strain of Zika virus can cause severe birth defects.

The findings, published Wednesday in the British journal Nature, were based on research in mouse models, human stem cells and cerebral organoids, or miniature brains grown in vitro.

A third study using wide-type mice, which was published Wednesday in the US journal Cell, said that Zika migrated from the pregnant mouse's bloodstream into the placenta, where it multiplied, then spread into the fetal circulation and infected the brains of the developing pups.