Why Women Are At Greater Risk For Zika

Sexual transmission of Zika may be worse than mosquito bites

Illustration: Tara Jacoby
Nov 17, 2016 at 5:11 PM ET

Mosquitoes have gained most of the attention as carriers of the Zika virus, but they might not be the ones primarily responsible for Zika’s worst effects. Infected humans, particularly those who have sex, could represent a much greater risk, new research has found. That’s because the same weakened immune response in the vagina that makes reproduction possible also leaves women uniquely vulnerable to the virus.

Recent studies have indicated Zika infects women more frequently than men, though the scale of a global epidemic makes it difficult to track the precise roles gender and sexual transmission might play in its spread. To get a better understanding, researchers at the Gladstone Institutes and the University of California, San Francisco infected mice with Zika and another virus that is also built out of the genetic material RNA.

They infected mice in ways that replicated either a mosquito bite or sexual transmission. In the former case, the immune system kicked into gear as it ought to. But when Zika entered the mice’s body through the vagina, the immune system failed to release the interferon proteins needed to fight off the virus. While this finding can’t prove that human vaginas are similarly ill-equipped to deal with Zika and other RNA viruses, this tracks with the data that shows the virus infects women more frequently.

“Don’t worry about the mosquito, worry about your partner more,” researcher Shomyseh Sanjabi told Vocativ. That’s especially true because only one out four people infected with Zika actually show symptoms. “So there’s three out of four infected individuals going around not even knowing they’re infected. But the virus remains in semen for up to six months.”

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In the absence of interferon, the immune system would struggle to remove Zika from the vagina, which would make it possible to spread further up the reproductive system. Sanjabi said that this could explain why Zika has caused so many cases of microcephaly in the children of pregnant women infected with the virus.

“If the virus is actually transmitted sexually during pregnancy, it might have stronger consequences for the unborn baby,” she said. “It may have a higher chance of highly replicating in the female reproductive tract and getting to the baby versus if it were transferred to the mother through a mosquito.”

To support this idea, Sanjabi pointed to the relatively low incidence of microcephaly in Colombia, despite the large number of Zika cases there. It’s possible greater awareness of the Zika risk reduced the number of sexually transmitted cases, meaning a larger percentage were relatively less severe mosquito transmissions. However, she stressed this was just speculation for the time being.

As for why the vagina would be so vulnerable to the Zika virus, the answer lies in its intended purpose. Unlike the rest of the body, which guards against any invasion from an unfamiliar organism, a women’s reproductive tract has to be able to handle an alien presence growing inside it for nine months.

“If you think about it, a fetus is only 50 percent made up of the mother’s genetic material and 50 percent made up of the father’s genetic material,” said Sanjabi. “It’s almost like putting a mismatched organ inside of you. It would normally get immediately rejected.”

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Sanjabi said it’s for now unclear why RNA viruses like Zika appear particularly adept at taking advantage of the reproductive tract’s so-called immunological tolerance. In all likelihood, Zika is among the unlucky – for us, at least – winners of an ancient evolutionary tradeoff that makes it possible for us to have children.

“Evolution chose to make us more susceptible to certain pathogens to allow us to reproduce,” said Sanjabi. “So there is a loophole in the system, and the virus is now taking advantage of that loophole.”

There’s no easy way to close that loophole, as all the treatments we currently have to boost the immune response against Zika are, unsurprisingly, unsafe for pregnant women to take. Solving this medical problem may well require developing a new treatment that can remove Zika from the vagina without damaging that crucial immunological tolerance, but such a treatment is likely a long way off. In the meantime, Sanjabi said people ought to be careful when planning a pregnancy or having unprotected sex, particularly in regions affected by the Zika epidemic.

“Sexual transmission [of Zika] is real, and it may cause more harm than appreciated at this point,” she said. “That’s the point I want to get across: that it is extremely important to have the partner tested before you go for a pregnancy.”