"Early during pregnancy, if the mother is infected, there is significant impact on the fetus."
PIC: Zika is believed to be transmitted primarily through the bite of an infected mosquito, but sexual transmission is also possible. (AFP)
Zika may replicate in the vagina for several days after infection, researchers said Thursday after using lab mice to study sexual transmission of the virus blamed for serious birth defects.
Infection with Zika via the vaginal tract may be a robust source of infection "with potentially dire consequences," said the study by a Yale University team, published in the journal Cell.
When pregnant mice were infected vaginally with Zika, the virus amplified and spread from the genitals to the fetal brain.
"We saw significant virus replication in the genital tissue, up to four to five days," said Akiko Iwasaki, professor of immunobiology and an investigator with the Howard Hughes Medical Institute.
When mice were infected early in pregnancy, scientists found evidence of the Zika virus in the fetal brain. Such infections were associated with fetal weight loss.
"Early during pregnancy, if the mother is infected, there is significant impact on the fetus, even in wild-type mice," she said.
While findings of mice studies often do not directly translate to humans, Iwasaki said the findings shed some new light.
"The finding may be important for women, not only pregnant women," she said.
"The vagina is a site where the virus can replicate and possibly transmit to partners. In pregnant women, vaginal transmission of Zika virus may have a significant impact on the developing fetus."
Zika is believed to be transmitted primarily through the bite of an infected mosquito, but sexual transmission is also possible.
There is at least one known case of a woman infecting her partner. Multiple other cases have been documented in which men spread the infection during sex to either male or female partners.
Zika has been found to persist in semen for as long as six months.
If a pregnant woman is infected with Zika, she faces a higher risk of bearing an infant with brain deformities, a condition known as microcephaly.
Pregnant women are urged to use condoms or abstain from sex if they live in or travel to areas where Zika is circulating, mainly in the Caribbean and Latin America.
Jill Rabin, a doctor and co-chief of women's health at Northwell Health in New Hyde Park, New York, described the study as "interesting."
"Ascending infection from the vagina to the fetus in humans may provide a more direct route for infection," she said.
"Consequently, this study may help provide a model to study the impact of therapeutic treatment of vaginal Zika virus infection during pregnancy."
Tomer Singer, director of reproductive endocrinology and infertility at Lenox Hill Hospital in New York, said the study "raises concerns about Zika becoming a sexually transmitted disease rather than just a virus that affects pregnant women."
Viruses tend to replicate more readily in areas that are warm and moist, he added.
"More studies and larger ones -- in non-pregnant humans -- are needed to confirm or rule out these concerns, before we can give concrete recommendations to our patients."