Survivors of Ebola can carry the virus in their sperm and transmit it sexually for up to nine months, a study has found, prompting questions over the potential spread of the disease on west Africa.

Experts believe at least 20 Ebola cases have been the result of sexual transmission so far. It may also be the reason for the re-emergence of Ebola in Liberia in late June after the country had been officially declared free of the disease in May.

Men who have survived the disease have been offered testing and warned to use condoms as a result of the study’s findings.

The study is published as Pauline Cafferkey, the nurse who had recovered from Ebola, lapsed into a critical condition at the Royal Free hospital in London with what was said to be a late complication of the disease.

Doctors expressed alarm at the development, reflecting that it shows how little is really known about the virus. While it is believed to linger on for months in parts of the body, such as eyes and semen, it is not thought to have rebounded to cause life-threatening illness before.

The study, published online in the New England Journal of Medicine, involved 93 men who had recovered from Ebola in Sierra Leone, whose semen was tested for the virus. All of those tested within three months of recovery had semen that was positive for Ebola virus and 65% who were tested between four and six months after their illness were positive. A quarter of those tested between seven and nine months were also positive.

Researchers will continue to test to find out how long the virus persists. One man still tested positive at nine and a half months.

The journal published at the same time a case study of a suspected sexual transmission in Liberia in March. Researchers used genetic testing to link the Ebola infection of a woman to her sexual partner, a man who had contracted the disease six months earlier and recovered.

Dr Nathalie Broutet, of the department of reproductive health and research at the World Health Organisation (WHO), one of the authors of the main paper, said there had been relatively few cases of sexual transmission, in spite of the findings.

WHO already recommends all male survivors should be offered semen testing and should take precautions, including thoroughly washing their hands with soap and water after any contact with semen, including masturbation. Used condoms should also be disposed of carefully.

“It may be that the survivors are compliant with the recommendations or maybe they are too tired to have sex. Maybe they are using condoms – we are providing condoms. That may be why we don’t see many cases,” she said.

She pointed out that there had been no cases in over 300 days in areas of Sierra Leone that had seen big outbreaks of disease. If sexual transmission were common, there would be many more cases in those areas now.

The study was carried out by the Sierra Leone ministry of health and sanitation, WHO and the US Centers for Disease Control and Prevention. Aware of the imminent publication, Sierra Leone launched a programme of regular screening for all male survivors from Ebola two weeks ago.

“They want to provide care to their survivors and help them to know their status,” said Broutet. “There was a great demand from survivors to know their status – we had no difficulty including survivors in the study. People have a right to know it to protect themselves and their sexual partners.”

WHO says testing should take place at three months and then monthly afterwards for those who are positive. Liberia has also launched a testing programme, but Guinea has not. Further work is going on to find out how much the virus persists in other bodily fluids and to support survivors who suffer after-effects, such as joint pains and eye problems.

Dr Jeremy Farrar, director of the Wellcome Trust, said: “This new research shows that Ebola virus can persist in the semen and be transmitted sexually many months after the original infection has disappeared. The fact that there has so far been only one isolated [official] report of this type of infection suggests that this kind of transmission may be rare, but it highlights just how much we still don’t understand about the Ebola virus, infection and recovery.

“It also reminds us that the Ebola epidemic could be far from over. With more than 17,000 Ebola survivors, it’s possible that further cases of delayed transmission and late complications will occur.

“We must remain vigilant and continue to monitor the long-term health of survivors so that any new infections can be identified quickly and measures put in place to prevent further cases.”