SEATTLE (FRONTLINE MEDICAL NEWS) – Zika virus RNA lingered longer in semen than in any other body fluid but was cleared by 95% of men after 3 months, according to investigators from the Centers for Disease Control and Prevention.

The findings support the agency’s recommendation that men abstain from sex or use a condom for 6 months after coming down with symptoms. There had been concern following case reports of two men with semen positive for Zika RNA beyond 180 days, but further study supported the recommendation.

Many wondered if 6 months was too short, creating “a lot of anxiety and some confusion. It was good to find that [6 months is fine],” said senior CDC epidemiologist and lead investigator Gabriela Paz-Bailey, MD . “Based on our findings, late detection seems rare,” and the longest any infectious virus has been found in semen is 69 days.

The investigation team collected body fluids for 6 months from 150 patients in Puerto Rico who had confirmed Zika infection. Samples included semen samples from 55 men and vaginal secretions from 95 women in the study. Most of the participants presented to a hospital with symptoms, but some were household contacts who didn’t remember symptoms until asked.

“We found that two out of every three people who have evidence of infection reported symptoms,” which was higher than what was expected for an infection thought to be asymptomatic in many who remembered symptoms only when drilled by persistent epidemiologists. “You have to ask the questions,” Dr. Paz-Bailey said at the annual Conference on Retroviruses & Opportunistic Infections in partnership with the International Antiviral Society.

The samples were tested by reverse transcriptase polymerase chain reaction to see how long viral RNA stayed in various body fluids. Urine often is used to screen patients for Zika RNA, but the team found that virus evidence lasts longer in serum. While half of patients still had Zika RNA in their serum 2 weeks after symptom onset, and 5% were positive after about 2 months, urine was positive for Zika RNA in half of patients for only about a week. After 39 days, only 5% had RNA in their urine.

“It was thought that you could detect the virus more often in urine. We found the opposite. This is new. Serum may be a superior diagnostic specimen compared to urine,” Dr. Paz-Bailey said.

Additionally, RNA was found in semen longer than any other fluid, with about half of men positive after a month and 5% at about 3 months. The maximum for detection of Zika RNA in semen was 125 days. Meanwhile, RNA was largely undetectable in saliva and vaginal fluids after a week.

Until now, the 6-month window for men has been based on case reports and cross-sectional observations, mostly of travelers returning to the United States. The new findings bolster the 6-month abstinence or condom use recommendation but also support advice that women avoid pregnancy for 2 months following symptom onset, although the evidence for a 2-month window in women isn’t as strong.

The serum findings “suggest that the risk of intrauterine transmission … is small” in women trying to conceive toward the end of the 2-month period, but “we will continue to monitor women of reproductive age to inform evaluations of these recommendations,” Dr. Paz-Bailey and her colleagues said in a report published after the presentation (N Engl J Med. 2017 Feb 14. doi: 10.1056/NEJMoa1613108 ).

It’s unclear if finding Zika RNA in body fluids means that there’s still active virus that can be transmitted. The team is plating out their specimens to see if they grow live virus.

The results are from an interim analysis. CDC plans to recruit at least 300 people into the study.

Dr. Paz-Bailey had no relevant financial disclosures. CDC funded the work.