The patient, a 27-year-old man in France, came down with mild symptoms -- weakness, muscle pain and conjunctivitis -- shortly after returning from Thailand late last year. He had decided to freeze his sperm before an upcoming chemotherapy treatment, which led to a French laboratory testing his semen for Zika on March 9. While the virus was found in his semen, it was not found in his blood or urine.
After taking a detailed medical history, the authors of the letter were unable to identify other ways the man could have been infected. The virus is most commonly spread by a species of mosquito that is found in Thailand, but it may also be spread through sexual contact, contaminated blood or needle sticks.
Thailand does not have active transmission of the virus, according to the U.S. Centers for Disease Control and Prevention, but it has reported sporadic cases of Zika since 2012, according to the Thai Ministry of Public Health.
The CDC currently recommends that men diagnosed with Zika wait six months before having unprotected sex so that they do not infect their partner. Infected men who have a pregnant partner are advised to wear barrier contraception for the duration of the pregnancy. The U.S. Food and Drug Administration does not routinely test donated sperm for Zika, but it does bar anonymous donors for six months if they have received a Zika diagnosis, traveled to an affected area, or had sex with someone who may have been infected.
The authors of the Lancet letter advised that CDC guidelines should “be regularly revised to keep pace with the evolution of scientific knowledge” about Zika virus, especially in light of the finding that it can remain in semen for months.