The New York City Department of Health and Mental Hygiene (DOHMH) today announced that the first case of a woman transmitting the Zika virus to a man through sex has surfaced in New York City. The Centers for Disease Control and Prevention (CDC) is now rewriting its advice on how to deal with the public health threat.

Until now, all previously reported instances of sexually transmitted Zika infection involved men spreading the virus to female or male partners.

According to an article published online today in the CDC's Morbidity and Mortality Weekly Report, DOHMH identified a young, nonpregnant woman in her 20s who had condomless vaginal intercourse with a male partner the day (day 0) she returned from a country with ongoing Zika transmission. She began experiencing Zika symptoms, including fever, fatigue, a maculopapular rash, and swelling of the extremities, as well as heavier than usual menses on day 1.

On day 3, she visited her primary care clinician, who collected urine and blood specimens. A DOHMH lab detected Zika virus RNA in both serum and urine using real-time reverse transcription-polymerase chain reaction (rRT-PCR). Serum testing for a Zika antibody at a New York State health department lab was negative.

The woman's male partner experienced Zika symptoms on day 6 and also sought medical care. rRT-PCR performed at the DOHMH lab turned up Zika virus RNA in his urine, but not in his blood serum. As with his female partner, the state lab could not find Zika virus antibodies in his serum.

The CDC confirmed all the rRT-PCR results for urine and serum specimens from the couple, according to the MMWR article, written by DOHMH professionals.

Interviews with the man appeared to rule out other possible causes of his infection. He said he had not travelled outside the United States in the preceding 12 months, nor had he been bitten by a mosquito — the predominant transmission mode for the Zika virus — within the week before his symptom onset. He also said he had not engaged in sex with anyone else recently and that his sexual encounter with the female patient did not include oral or anal intercourse.

The MMWR article stated that Zika virus in the woman's vaginal fluids or menstrual blood may have spread to the man during exposure to his urethral mucosa or undetected abrasions on his penis. The authors pointed to earlier research that found the Zika virus in the female genital tract and vaginal fluid in particular.

More research is needed, they said, to determine how the Zika virus sheds in the genital tract and vaginal fluids, as well as the risk for transmitting the virus from a woman to a sexual partner.

Zika Precautions Extended to Female Partners of Pregnant Women

In a news release, the CDC updated some of its advice on avoiding the Zika virus via sex. The agency reiterated its past recommendations that all pregnant women use barrier methods during sex or abstain from sex if they have a partner who has traveled to an area with active Zika transmission, or who resides there. In light of the New York City case reported in MMWR, however, the CDC is now applying these recommendation to female sex partners of pregnant women, even though no cases of woman-to-woman Zika transmission have been reported.

The CDC said it is updating its recommendations for sexually active couples who are not pregnant, or who are concerned about pregnancy, and people "who want to reduce personal risk of Zika infection through sex."

The Zika virus can cause serious birth defects, most notably microcephaly, characterized by unusually small head size and possibly severe developmental problems. As of today, the CDC has identified nine infants with birth defects as well as six lost pregnancies with birth defects in the United States where there is laboratory evidence of a possible Zika infection.

MMWR Morb Mortal Wkly Rep. Published online July 15, 2016. Full text

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