DALLAS » The vagina is a welcoming home for Zika. In a new study published today, researchers at Yale University found that Zika reproduced in the vaginas of pregnant mice four to five days after infection and that the virus spread from the vagina to the fetal brain.

Studying Zika in mice is tricky. A mouse’s immune system fights off the infection, making the animal quite resistant to Zika. When researchers want to study the infection in mice, they first tweak the animal’s immune system to make it more susceptible.

But in this case, researchers using a pipette to infect regular mice with Zika in the vagina found that even in mice with a robust immune system, the virus managed to reproduce.

We already know that Zika can spread through sex. The second documented case of sexually transmitted Zika was in Dallas earlier this year. In that case, the virus was spread through sex between two men. Last month, the first case of a woman spreading Zika to a man through sex was reported.

This study adds to the evidence that the virus can thrive in the vagina. A previous study showed that Zika persists in a woman’s vaginal mucus 11 days after infection. It persists a lot longer in men: the virus can linger in semen up to six months after infection.

Not only did Zika thrive in the vagina, according to the latest study, but it spread to the fetal brain and caused weight loss, the researchers said. That was true even in mice whose immune systems were not tweaked and therefore should have had a natural resilience to the infection.

Zika is known to cause a wide range of birth defects in humans including microcephaly, vision loss and joint problems. A study released Tuesday by Brazilian and American doctors showed just how broad that range of defects can be. The researchers said Zika can lead to enlarged ventricles — the fluid-filled cavities in the brain — as well as brainstem malformation in babies.

Experiments on mice should not be applied broadly to humans but the Yale researchers do end their paper by asking: “How might our findings apply to humans?” They say that since we are more susceptible to Zika than mice, Zika “introduced into the human vagina is likely to replicate more robustly than in the vaginal cavity of (regular) mice.”

Advice on how to avoid Zika has already been expanded to include messages about safer sex. Some countries have even called on women to delay pregnancy until the epidemic is under control. This new study adds more uncertainty to how long the virus can hang out in various parts of the human body after infection.

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