A Los Angeles County woman became the fifth person in the US to contract the virus through sexual intercourse in the past year, and the first in her home city.
The woman’s partner had traveled to Mexico earlier this year, and developed signs of the infection in November.
She did not accompany him on the trip, but shortly after her partner’s trip, the woman was also diagnosed.
Zika diagnoses were a fraction as common in 2017 as they were in the previous year, but experts warn that sexually transmitted cases are likely under-reported.
Zika can be passed from person to person through sexual contact, though most are infected through mosquito bites while travelling abroad
So far, an estimated 51 people have gotten the virus from having sex with an infected person, accounting for under one percent of 5,548 diagnoses, according to the Centers for Disease Control and Prevention (CDC).
California alone has has cases of Zika, though no reported infectious mosquitoes.
But travelers who get bitten while abroad can typically pass the Zika virus on, and the CDC advises women who develop symptoms to wait at least eight weeks before trying to get pregnant.
Zika is rarely sexually transmitted and women are more vulnerable to catching the virus that way than men are.
‘The virus is systemic so lots of different tissues become infected including testes, semen,’ says Dr Laura Kramer, director of a lab that studies Zika at the University of New York at Albany.
‘It gets into bodily fluids – including saliva, though there’s been no evidence of transmission by saliva – and into mucus in vaginal tract, which is how women get infected by it,’ in sex she says.
When women come into contact with a man’s infected sperm during intercourse, they may contract the virus.
‘The concentration of the virus is detectable in semen for probably 24 days at longest, though one in one case it was 44 days,’ says Dr Kramer.
The CDC recommends that if a man has been exposed to Zika, he wait six months before having sex without a condom.
‘There are examples in all directions, but most commonly the [person that transmits it] is the man,’ Dr Kramer says.
‘It probably has something to do with the amount of virus in vaginal mucosa, versus in semen, I suspect,’ but Zika has been so rare that the exact concentrations have not been studied.
The virus poses a particular risk to women who contract it sexually because if they get pregnant, there is a 45 percent chance that they will pass Zika to the infant, leading to potentially devastating birth defects like microcephaly and brain development delays.
Zika is often asymptomatic, though its impossible to know how many people are infected if they never show signs.
This makes it even more likely that people could transmit it to partners.
‘I think they will keep on occurring, but we’re not going to have a [Zika] epidemic initiated by sexual transmission,’ Dr Kramer says.
‘The level of immunity is so high in America and there is a very low level of transmission. It’s there, and not going to go away, but I don’t think we’ll see another large outbreak [unless] immunity drops to levels like in Latin American countries, where the virus is more endemic,’ she adds.