Male survivors of Ebola may carry the virus in their semen even months after they recover from the infection, according to a recent study.

In the study, researchers looked for genetic material from the Ebola virus in semen and found that 100 percent of the specimens sampled between two and three months following an Ebola infection showed signs of the virus. Among the samples taken four to six months after an Ebola infection, 65 percent carried signs of the virus, and 26 percent of the samples taken at the seven- to nine-month mark also tested positive for the virus.

Current guidelines from the World Health Organization (WHO) say that people who recover from Ebola should practice safe sex for at least six months after the onset of their Ebola symptoms, said Dr. Amesh Adalja, a senior associate at the University of Pittsburgh Medical Center (UPMC) who was not involved in the new study.

"Now, it's clear that six months is probably not enough for at least a quarter of the people," said Adalja, who is also a spokesman for the Infectious Disease Society of America (IDSA). "So there's going to have to be some revision of the guidelines," Adalja told Live Science. [The 9 Deadliest Viruses on Earth]

The Centers for Disease Control and Prevention recommends that survivors use condoms indefinitely.

The new study included 100 male survivors of the virus in Sierra Leone. The participants reported information about their Ebola episode, their current health status and sociodemographic characteristics.

Ninety-three participants provided a semen sample for analysis, and the researchers tested the samples by looking for certain gene sequences of the virus.

The findings suggest that the persistence of Ebola in semen declines over time after a person recovers from his or her infection, the researchers said. However, researchers still don't know how infectious the virus in semen may be, or how easily Ebola could spread to other people during sex, in the months following a person's recovery.

Researchers have documented one case of the sexual transmission of Ebola in Liberia, in mid-March. The woman who contracted the virus had not had contact with anyone who had Ebola symptoms, nor had she traveled to other areas where people were infected with the virus. However, she'd had unprotected sex with a man who had been declared Ebola-free six months earlier.

Of course, the researchers can't say for certain whether the woman contracted Ebola through sexual activity. Yet the information suggests that it may be possible.

"What we do know is that if the virus is in someone's semen, they can probably pass it sexually, and a survivor could spread the virus even after recovery," Adalja said.

Previous studies have shown that the main mode of Ebola transmission is through direct contact with the blood or other bodily fluids of a person with Ebola, or through direct contact with the remains of a person who died from the virus. However, although the virus is detectable in the bloodstream only during the acute stage of the illness, it may remain for longer periods in places in the body that are "immune-privileged sites," the researchers said. In these sites — which include breast milk, vaginal secretions, the fluid of the eyes and semen — the body tolerates foreign materials, and does not produce the usual immune system response to combat them.

The researchers said they are still looking into what factors may determine how long the virus stays active in survivors' bodily fluids. The severity of a person's infection, the amount of virus in his or her body, and the person's genetics all may play a part, they said.

The study was published today (Oct. 14) in the New England Journal of Medicine.