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New evidence confirms Zika virus transmissible by sex

Genetically identical virus in man’s semen and infected woman who had never been to Zika area

Louise Prime

Tuesday, 19 April 2016

The Zika virus can be sexually transmitted, a new study* published in the New England Journal of Medicine has confirmed. Researchers have found an exact genetic match in Zika virus samples from a woman who had never visited an endemic area, and in the semen of a man with whom she had sex who had caught the virus in Brazil. They have called for guidelines to clarify for how long men returning from endemic areas should continue to use condoms when having sex with pregnant women and those of child-bearing age, as well as for recommendations regarding the possibility of transmission by oral sex.

Although the Zika virus is transmitted to humans almost exclusively by the Aedes species of mosquito, there had been previous suggestions, after two studies found high loads of the virus in semen, that it might also be possible for it to be transmitted sexually. Researchers led from INSERM (Institut National de la Santé et de la Recherche Médicale) in Paris have reported a case that, they said, confirm this hypothesis.

A 24-year-old woman living in Paris, who had never travelled to a region where Zika was endemic or to tropical or subtropical areas, became ill on 20th February. Between 11th and 20th February she had had seven episodes of both vaginal sexual intercourse, without ejaculation and without the use of a condom, and oral sex with ejaculation, with a man who had been ill in the first week of February while in Rio de Janeiro, but had no symptoms on the day that he returned to France, 10th February.

The researchers ruled out associated sexually transmitted infections in both patients by HIV, syphilis, hepatitis B and hepatitis C serology, and by urine chlamydia and gonococcal PCR. However, they found Zika virus RNA in the woman’s urine and saliva, but not in her blood plasma (though antibodies were detected) or a vaginal swab. They also found Zika virus RNA in the man’s urine and semen, but not in his plasma or saliva, making it unlikely that they were the route of transmission.

The team sequenced a complete Zika virus genome from the woman’s saliva and from the man’s semen, and found 100% correlation between the two genetic sequences – they reported: “Apart from four mutations, all of them ‘synonymous’, the nucleotide sequences both encoded an identical form of the virus.”

Because high viral loads were still present in the man’s semen 3 weeks after his return from Brazil, the study authors have called for clarified guidelines to prevent transmission of Zika virus.

They concluded: “We need to better define recommendations to prevent transmission of the virus. In particular, guidelines regarding how long men who are returning from an area where active ZIKV transmission is occurring should continue to use condoms during sexual contact with pregnant women and those of child-bearing age are lacking. In addition, recommendations regarding the possibility of oral transmission of the virus through semen are needed.”


* D’Ortenzio E, Matheron S, de Lamballerie X et al. Evidence of sexual transmission of Zika virus. New England Journal of Medicine, 13 April 2016. DOI: 10.1056/NEJMc1604449.

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