Some male survivors of the Ebola disease may carry the virus in their semen for months after they are discharged from hospitals, and could potentially infect others, a new study revealed Wednesday.

It has been known for decades that men who survive Ebola and related viruses may on rare occasions infect female sexual partners in the weeks or months after their illness. The first such suspected case was recorded in 1967, when a woman contracted the Marburg virus — a cousin of Ebola — six weeks after her husband survived the infection.

But the new study, along with a separate paper that documented an Ebola case in Liberia in March that was almost certainly sexually transmitted — represent the strongest evidence so far that the Ebola virus can linger in testicles and be present in semen long after a patient recovers. This occurs not just rarely, but in a substantial portion of survivors, the study found.

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Thousands of men have survived the most recent outbreak of Ebola, raising questions about the risk their sexual partners may face.

The World Health Organization recommends the Ebola survivors forgo sex for the first three months after they recover. Barring that, safe sex is advised.

At three months, semen from male survivors should be tested for Ebola and if it is positive, abstinence or safe sex should be extended to six months, said Dr. Nathalie Broutet, an author of the study and a researcher on sexually transmitted diseases at the WHO.

Ebola experts believe it is rare for the virus to be spread sexually. But even a few such cases could seriously complicate efforts to finally bring the nearly two-year-old West African outbreak to an end.

“The challenge with sexual transmission is not that it would be a source of many new EVD [Ebola virus disease] cases but that it may be a source of late EVD cases,” Dr. Armand Sprecher, an Ebola expert with Doctors Without Borders, wrote in an editorial that accompanied the two scientific reports in this week’s issue of the New England Journal of Medicine.

The report on the sexually transmitted case focuses on a Liberian woman who contracted Ebola in March.

The woman had no known contact with active Ebola patients, but had had unprotected sex with a man who survived Ebola the previous autumn — six months earlier. She died from her infection.

Her sexual partner agreed to give a semen sample, which tested positive for the virus. A comparison of the genetic sequences of viruses from the woman and the man showed distinct mutations not seen elsewhere in the outbreak.

The accompanying study looked for evidence of Ebola virus in the semen of 93 men in Sierra Leone. It was conducted by scientists from the country’s ministry of health, the WHO, and the Centers for Disease Control and Prevention.

Semen samples from nearly half — 49 percent — tested positive for Ebola. Of those, 65 percent of the men were still positive four to six months after getting sick, and 26 percent were still testing positive seven to nine months after the onset of their illness.

One man’s semen had an inconclusive result at 10 months. The study organizers did not enroll men who had gotten sick more than 10 months before the start of the research, so they cannot say with certainty that nine months is the outside limit for a positive semen test. It is, however, the longest period recorded so far in scientific literature.

Even though these men had Ebola virus in their semen, it’s not clear whether they could have infected sexual partners. The test used in the study detects segments of the Ebola virus, but cannot discern whether the virus is alive.

Additional work is underway at the CDC in Atlanta to try to see whether the samples contained live infectious viruses, Broutet said.

Ebola experts said they need answers to the questions raised by these studies if they are to learn how to better contain Ebola outbreaks.

“We have to do our best to understand what we can about how frequently do people retain virus in the semen and the gonads and what risk does that present?” said Dr. Daniel Bausch, a veteran Ebola outbreak responder who is currently working at the WHO as a technical consultant.

Bausch, who was not involved in this study, said he thinks sexual transmission of Ebola must be rare, or more such cases would come to light.

He and others acknowledge they are concerned the latest findings could exacerbate the stigma survivors already face.

Sprecher urged people to remember what Ebola patients went through. Many lost multiple members of their families, lost jobs, and still experience health problems.

“If they are then treated as pariahs and threats, we add a terrible unkindness on top of their suffering,” he wrote.

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