The Ebola virus can linger for a year or even longer in the semen of some men, researchers reported Tuesday.
That means that Ebola survivors could be an important source for re-igniting outbreaks of the deadly virus, which killed more than 11,000 people in Guinea, Sierra Leone and Liberia in a two-year epidemic that ended earlier this year.
"Before this outbreak, scientists believed that Ebola virus could be found in semen for three months after recovery," said Dr. Moses Soka of the Liberian Ministry of Health, who helped conduct the study.
"With this study, we now know that virus may persist for a year or longer."
Before the West African epidemic, Ebola had only caused isolated, small outbreaks of at most a few hundred people. There was not much hint the virus stayed in the bodies of survivors.
This epidemic infected more than 28,000 people before it came under control. Fresh outbreaks - the latest one just this March - have been traced to sexual transmission from male survivors.
"We had, in March, an outbreak in Guinea," Centers for Disease and Control director Dr. Thomas Frieden told reporters Tuesday.
It was traced to a man who'd recovered more than 15 months before from Ebola. He infected a woman sexually. Thirteen people became infected and nine of them died, Frieden said.
"We have now seen very long persistence in semen, which means we'll see a risk of cases for years to come," he said.
Frieden said it's important to keep studying Ebola and keep up the defenses against it.
"We don't want to see this come roaring back after all the hard work and after all the lives that have been lost," he said.
The CDC is among the agencies working to help Liberia, Guinea, Sierra Leone and other countries build up their public health systems so they can catch Ebola and other diseases as soon as they break out, before they spread.
The team working in Liberia studied 429 men who survived Ebola infection. They found that 9 percent of the men had pieces of Ebola virus in their semen.
"Of those, 63 percent had semen samples that tested positive for Ebola fragments a year after recovering from disease and, in one man's case, at least 565 days after he recovered from illness," the CDC said in a statement.
Men over the age of 40 were more likely than younger men to have Ebola in their semen, the team reported in the journal Lancet Global Health.
In one man, the virus could be found 565 days after he recovered.
"We now have many more Ebola survivors than ever before," Soka said.
"This work demonstrates the importance of providing laboratory testing and behavioral counseling to empower survivors to make informed decisions to protect their intimate partners."
It's made by Gilead Sciences, which specializes in antiviral drugs. It's protected animals against Ebola and has been safe in early tests in people.
The World Health Organization advises Ebola survivors to abstain from sex for three months or use a condom. Ebola spreads through bodily fluids — vomit and diarrhea, blood and even sweat. Infection requires close, sustained contact, however, and caregivers are most at risk, as are the people who prepare the bodies of people killed by Ebola.
But the virus can linger in parts of the body that are called immune privileged sites. These include semen but also the eyes, spinal cord and elsewhere. Scottish nurse Pauline Cafferkey is one example. She got infected while helping fight the Ebola epidemic in West Africa. She was taken back to the hospital at least twice when the virus began replicating in her spinal fluid, causing symptoms.