A new study shows that the Zika virus can stay in semen for up to six months after the onset of symptoms.

The Centers for Disease Control and Prevention and World Health Organization recommend that men with Zika symptoms should wait at least six months before having sex. The recommendation came with researchers not knowing how long the virus remains in semen.

Now doctors are saying that the recommendation may need to be even longer, according to a report from the BBC.

The study, conducted by the Laboartory of Virology of the Institute of Infectious Diseases Lazzaro Spallanzani in Rome, followed a 30-year-old man who got Zika after visiting Haiti in January. Researchers found that the man's sperm continues to test positive for Zika six months after symptoms popped up.

"Previous studies have shown that the Zika virus had been detected in semen until 62 days after the onset of symptoms," the lab said.

The lab said that it still needed to investigate the persistence of Zika in asymptomatic people and the risk for sexual transmission. Only one in five people show Zika symptoms, which include a fever and nausea. However, Zika has been found to cause microcephaly, a birth defect in which babies are born with underdeveloped brains and abnormally small heads.

As of Wednesday, 22 cases of sexual transmission of the Zika virus have been found in the U.S., according to the CDC. The virus is spread mainly through mosquito bites.