Zika sexually transmitted: How and why can the virus become an STI?

On 2 February, the Centres for Disease Control and Prevention (CDC) officially declared that the Zika virus has been transmitted through sexual contact in Dallas, Texas. The virus is usually transmitted by the Aedes mosquito, but scientists discovered the Zika virus can be passed through sexual contact after a patient became infected in the US – a country that has no known Zika cases contracted by mosquitoes.

The CDC said that the patient became infected after engaging in sexual activity with a traveller returning from Zika-infected Venezuela. This is the first recorded case of the Zika virus being passed from one person to another in the USA.

This is not the first time it has been reported the virus can be transmitted sexually. In 2011, malaria researcher Brian Foy released a scientific report claiming that he had passed on the virus to his wife, following infection during a field study in Senegal. However, this could not be officially confirmed as no semen samples could be analysed properly to check for the vaccine.

In February 2015, a report published in the journal Emerging Infectious Diseases that showed the virus was found in semen. The researchers said a 44-year-old man from Tahiti endured two periods of Zika-related symptoms, and tested positive for the virus in his semen following the second series of symptoms.

"Our findings support the hypothesis that ZIKV (the Zika virus) can be transmitted by sexual intercourse," said the researchers. "We detected a high ZIKV RNA load and replicative ZIKV in semen samples.

"These results suggest that viral replication may have occurred in the genital tract, but we do not know when this replication started and how long it lasted."

They suggest that infectious organisms tend to be found in blood in the semen of patients. That is exactly where they found the Zika virus of the Tahitian man.

"Zika is a virus which is not restricted to one part of the body, as you would not find in Hepatitis for example," Ian Jones, expert in virology at the University of Reading, told IBTimes UK. "With that in mind, it's not entirely surprising that the virus could be sexually transmitted, but I must stress this is probably a one-off.

"The virus has been around for more than 50 years, and we would have noticed if it could have been sexually transmitted long before now. It is not in the same league as other sexually transmitted infections, like HIV."

Jones says that the entire situation surrounding the Zika virus is an "open question right now", suggesting that there are still a lot of unknowns. That not only includes the risk of sexually transmitted infection, but also the link to microcephaly, which he says is still yet to be proved.

The World Health Organization says that preventing mosquito bites is still the best way to make sure the infection does not spread. Public Health England recommends that men use condoms when travelling back from areas afflicted with the Zika virus – regardless of if they're expressing symptoms or not.

As of 3 February, 28 countries have confirmed cases of the Zika virus. Birth defects have been linked to the infection, predominantly microcephaly, of which 4,000 cases have been reported in Brazil alone since October 2015.