In most people, Zika lives in the blood for about seven days and then resolves. It can last a bit longer in urine and saliva, but even that goes away after a couple of weeks, leaving antibody testing of the blood as the only way to determine whether you've had the virus.
But a letter published Wednesday in the New England Journal of Medicine
tells the story of a Brazilian baby who continued to have Zika in his blood for more than two months after his birth. He wasn't diagnosed with microcephaly when he was born, nor did he have any obvious developmental issues at his two-month checkup, even though his mother had symptoms of the virus during the third trimester of pregnancy.
But the baby's blood tested positive for Zika 54 days after birth and then again at day 67, and by the time he turned 6 months of age, he had severe muscle contractions and other signs of neurological decay
So when was the baby's brain damaged, before or after birth? No one knows. The case also raises questions about how long the virus can hide within the body and where.
Researchers have known for some time that Zika can be transmitted via semen from men to women, hiding in the testes, where it can avoid the immune system; in fact, there is mounting evidence that it can continue to replicate in the testes -- and even increase the viral load -- for months after infection. That's why the Centers for Disease Control has issued strict safe-sex precautions
for pregnant women and their partners, which also apply to anyone trying to conceive.
"All pregnant women with sex partners who live in or traveled to an area with Zika use barrier methods during sex or abstain from sex for the remainder of their pregnancy," the CDC said, adding that men should wait at least six months after infection to attempt to conceive
Then, in July, scientists found evidence that Zika can hide in a woman's cervical mucus
, quickly followed by the first report
of a woman giving Zika to her male partner during sex. There were only two cases, but the CDC quickly responded, revising its safe-sex guidelines
to include female-to-male and female-to-female sexual transmission.
To determine whether vaginal transmission might be more common than previously thought, a team of researchers at Yale University began vaginally infecting virgin and pregnant mice with Zika. The results, published today in the scientific journal Cell
, are alarming: Zika could hide and reproduce in vaginal tissue of mice for up to seven days, with an average of four to five days.
What's more, in the pregnant mice, the virus spreads from the genitals to infect the fetal brain, causing reduced fetal weight gain and impaired development.
But as these are mice, not people, there are some major differences. Wild mice are not typically vulnerable to Zika, so researchers use a genetically altered form of mice that have weakened immune systems, making them more similar to humans.
The study used both wild mice and genetically altered mice, and that's when things got even more interesting. Researchers expected to see a response in only the genetically altered mice. Instead, they found that Zika replicated just as quickly in the vaginas of wild mice.
"We were surprised," said co-author Laura Yockey of the Department of Immunobiology at the Yale University School of Medicine. "We don't see this type of response to other sites of infection in wild-type mice."
And if the wild mice were infected early in their pregnancy, there was a significant impact on the fetus, although it was not as severe as in the immune-compromised mice, who often suffered spontaneous abortions.
If Zika can impact an animal that is not naturally susceptible to this degree, say the researchers, there is reason to worry that Zika "introduced into the human vagina is likely to replicate more robustly than in the vaginal cavity of wild-type mice."
"The finding may be important for women, not only pregnant women," said co-author Akiko Iwasaki, professor of immunobiology at Yale. "The vagina is a site where the virus can replicate and possibly transmit to partners. And for pregnant women, vaginal transmission of Zika virus may have a significant impact on the developing fetus."