Scientists raised the possibility that the Zika virus can be transmitted by oral sex — perhaps even by kissing — today in a letter to The New England Journal of Medicine describing one such case in France.

A single incident may seem trivial. But until early this year, there was only one known instance of sexual transmission of the Zika virus — a 2008 case in which a mosquito researcher just back from Africa infected his wife in Colorado.

Now scientists believe that sexual transmission is an important driver of the Zika epidemic in the Americas. Cases have been reported in 10 countries where no mosquitoes carry the virus, including France, Germany, Italy, Portugual and New Zealand.

In the French case, a 46-year-old man returned to Paris from Rio de Janeiro on Feb. 10, shortly after experiencing Zika symptoms in Brazil — fever, headache and a rash — that had just ended when he reached France.

He and a 24-year-old partner had sex seven times between Feb. 11 and Feb. 20, each involving vaginal sex without ejaculation and oral sex with ejaculation.

The woman fell ill on Feb. 20. Both were tested for Zika infection on Feb. 23. The man had high levels of the virus in his semen and urine, but none in his blood or saliva. The woman had the virus in her urine and saliva, and antibodies to the virus in her blood. But a vaginal swab was negative for the infection.

The two were using oral sex as a form of birth control, said Dr. Yazdan Yazdanpanah, an infectious disease specialist at the French National Institute of Health and Medical Research in Paris and one of the report’s authors.

“I don’t think this changes anything, but it shows you how elaborate the number of avenues of possible transmission can be,” said Dr. William Schaffner, head of preventive medicine at Vanderbilt University Medical School.

He agreed that the most likely transmission route was oral sex, although he said it was possible that the woman was infected by pre-ejaculate during vaginal sex, or that the couple’s recollections of each sex act were imperfect.

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