Prevention campaigns are trying to limit the spread of Zika virus ANDINA/Xinhua Press/Corbis

Health officials in the US have confirmed that a person who contracted Zika on the mainland was infected through sex; the virus is present in semen. The infection is generally mild, but if a pregnant woman catches the virus, it can prevent the fetus’s brain from developing properly, leading to potential life-long health issues.

It was widely thought that Zika could only be transmitted via bites from infected mosquitoes. So how does the fact it can be passed on through sex change things?

What just happened?

Yesterday, Texas health officials said a case of Zika had been confirmed in someone who had sex with a returnee from a region where the virus is present, reported to be Venezuela.


Is this the first such case?

No. A US researcher caught Zika in 2008 while on a field trip to study malaria in Senegal. Six days after returning home he fell sick, and four days later so did his wife. Blood samples later revealed they both had Zika.

How do we know these cases weren’t caused by a rogue mosquito carried back in luggage?

Few details have been released about the Texas patient, but in the 2008 case, the timing of the wife’s infection suggests sexual transmission. The researcher and his wife had been having unprotected sex.

What are the chances of sexual transmission?

We don’t know. But the rarity of known cases suggests that in areas where Zika is endemic they are likely to be dwarfed by infections from mosquito bites. However, a study from Yap, an island in Micronesia, hints that sexual transmission might be more common than we thought. There, more women were infected with Zika than men, a pattern that is seen with most sexually transmitted diseases.

How does this change health advice?

It makes a bad situation worse. In regions with Zika, pregnant women are being told to take preventive measures against being bitten by mosquitoes, such as covering up and using repellents, which can be impractical and unaffordable in Central and South America. Now their partners may have to do the same.

And how about people in Europe and the US?

The advice to pregnant women and those trying to conceive is to avoid areas with Zika – this might be expanded to include their partners. Worryingly, we don’t know how long the virus lingers in semen. Men who have travelled to Zika regions are being advised by the UK’s Royal College of Obstetricians & Gynaecologists to use condoms for four weeks after their return home, or, if they experienced Zika symptoms, for six months after recovery.