Zika may replicate in the vagina for several days after infection, said researchers who relied on lab mice to study sexual transmission of the virus blamed for birth defects.
Significant impact on foetus
Infection with Zika via the vaginal tract may be a robust source of infection "with potentially dire consequences," said the study by Yale University, published in the journal Cell.
Pregnant mice were infected vaginally with Zika, which then amplified and spread from the genitals to the foetal brain.
"We saw significant virus replication in the genital tissue, up to four to five days," said Akiko Iwasaki, professor of immunobiology and investigator of the Howard Hughes Medical Institute.
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When mice were infected early in pregnancy, scientists found evidence of Zika virus in the foetal brain. Such infections were associated with foetal weight loss.
"Early during pregnancy, if the mother is infected, there is significant impact on the foetus, even in wild type mice," she said.
Important finding for women
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.
Read: Zika may persist for months in newborns
"In pregnant women, vaginal transmission of Zika virus may have a significant impact on the developing foetus."
Zika is believed to be primarily transmitted through the bite of an infected mosquito, but sexual transmission is also possible.
Persists in semen
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.
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