In Britain, a man who was infected with Zika virus while traveling to the Cook Islands showed evidence of the mosquito-borne virus in his semen for two months, health officials said.
The finding raises new questions for health authorities as they scramble to learn more about Zika -- linked to a surge in birth defects in Brazil -- and the risk of transmission through sex.
The case involved a 68-year-old man who was infected with Zika in 2014 while traveling. He complained of a fever, rash and lethargy upon return to Britain, where he was tested and the results came back positive for Zika.
"Although we did not culture infectious virus from semen, our data may indicate prolonged presence of virus in semen, which in turn could indicate a prolonged potential for sexual transmission," it said.
Speaking at a American Association for the Advancement of Science meeting in Washington on Friday, the National Institute of Allergy and Infectious Diseases (NIAID) chief Anthony Fauci said more work needs to be done to understand how long Zika may persist in a man's semen. "We don't now know," he told reporters.
"We had said, 'Perhaps it is just during the acute infection.' Well, that is obviously not the case." With Ebola, which comes from the same family of viruses as Zika, research on persistence in semen has shown that it could last up to nine months in some men. "What we need to do are natural history studies" for the Zika virus in order to determine how long men should use condoms or refrain from sexual contact after a Zika infection, Fauci said.
The CDC last week urged condoms or abstinence for men who live in or have traveled to the more than two dozen countries and territories in South America and the Caribbean where the virus has been detected, especially if they have pregnant partners, in which case protective measures should persist until the end of the pregnancy.