Three different studies published on Wednesday offered proof that the Zika virus can reach and damage brain cells in pregnant mice's fetuses, strengthening the links between the mosquito-borne infection and birth defects.

The World Health Organization (WHO) declared the virus a global health emergency in February, based on the thousands of cases in Brazil of microcephaly, a birth defect characterized by small heads and severe developmental disorders.

Scientists were left to wonder: how can a virus that leads to mild adult symptoms wreak so much havoc on a developing fetus?

The new studies offer insight into how Zika can attack brain tissue and injure fetuses. In the first one, published in the journal Nature, researchers tainted laboratory mice with very high doses of Zika from a Brazilian patient, and found that the said strain penetrates the placenta, inhibits fetal mouse growth, and leads to microcephaly.

The research also showed that Zika could affect brain cell clusters, either causing cell growth disruption or death.

In the second study published in the journal Cell, Dr. Michael Diamond from Washington University and his team studied immunocompromised mice. In one experiment, they bred mice with genetically weakened immunity, where Zika exposure killed most fetuses in a week. Those that survived were not left unscathed: they had notable abnormalities including severely disrupted growth.

There was genetic material from Zika in the placentas that was 1,000 times higher than in the mothers’ blood – a mark that the virus had been developing in the placenta.

In the third study published in Cell Stem Cell, Chinese scientists injected Zika directly into the brain of fetal mice in utero, and found that during birth they demonstrated microcephaly symptoms. Once the lab mice were given the virus during what could be second trimester in human, fetal brains shrank as the rate of virus increased.

Diamond argued that Zika’s association with fetal abnormality showed to be direct and consistent, which could put to rest speculations that it needs to interact with other factors – such as a past dengue infection – to result in fetal brain damage.

"That’s not to say there aren't other factors that may predispose people to get Zika. But you don’t need all those things,” Diamond explained of the ensuing brain damage.

He said these mouse experiments will help in the testing of new drugs and vaccine and in looking at whether immunization can prevent Zika transmission to the fetus.

National Institute on Allergies and Infectious Diseases director Dr. Anthony Fauci, on the other hand, warned that a mouse model is evolutionarily different from a human. He called for caution when extrapolating findings to humans, although confirming the experimental value of pregnant mice being infected with Zika and to sire damaged babies.

The WHO asserts a strong scientific consensus that Zika can also lead to a rare neurological disorder called Guillain-Barre, which results in temporary paralysis among adults.