By Randy Dotinga
Still, guidelines from the U.S. Centers of Disease Control and Prevention recommend that infected men use condoms or abstain from sex for six months after infection with the Zika virus.
Infectious disease experts said those guidelines should stay that way.
"Better to err on the long end," said Matthew Aliota, an assistant scientist who studies viruses at the University of Wisconsin's School of Veterinary Medicine.
The Zika epidemic began nearly two years ago in Brazil and has since spread around the world, causing severe birth defects in thousands of babies born to women infected with the virus during pregnancy.
The most common birth defect has been microcephaly, which causes an abnormally small head and brain. More subtle sensory and neurological problems have been surfacing as doctors study these babies for longer periods of time.
According to CDC figures from late January, the virus has been detected in 1,394 pregnant women in U.S. states and 3,071 pregnant women in American territories. Puerto Rico has been especially hard hit. So far, 38 babies have been born there with Zika-related birth defects, the agency says.
Scientists believe that infected mosquitoes are primarily responsible for the spread of the disease, but it can also be transmitted through sexual activity and blood transfusions.
As to how long the virus remains in semen, "we have anecdotal reports of a long duration, four, to five, to six months, and that has had people worried," said Philippe Lagace-Wiens. He's an assistant professor and studies tropical diseases at the department of medical microbiology at the University of Manitoba in Canada.
In the new study, CDC researchers led by Dr. Gabriela Paz-Bailey tracked the levels of the Zika virus in 95 women and 55 men in Puerto Rico. All participants had recently been infected.
The scientists found little virus in saliva and vaginal secretions, but it lingered in blood serum and urine, sometimes for weeks. By 81 days, the virus had disappeared from semen in 95 percent of men, the study found.
Aliota said the results are important even if they're not surprising.
"More data like these are necessary to understand the risks associated with Zika virus in different body fluids," he said.
Men who visited areas with Zika but haven't shown symptoms "should consider using condoms or not having sex for at least eight weeks after their return in order to minimize risk," the CDC says.
Two researchers who study viruses -- Aliota and Uriel Kitron, chair of the department of environmental sciences at Emory University in Atlanta -- said the six-month recommendation to abstain from sex or use condoms should remain in place.
"Given the severity of the potential outcome for the fetus, six months remains logical," Kitron said.
What's next for scientists trying to understand the virus?
Researchers want to learn more about when men are most infectious in terms of transmitting the disease, Lagace-Wiens said. Currently, it seems to be within the first few days and weeks after they're infected, he added.
The study findings were published Feb. 14 in The New England Journal of Medicine, to coincide with a planned presentation at the annual Conference on Retroviruses and Opportunistic Infections, in Seattle.