A resurgence of Ebola in Liberia is likely to have originated in a survivor still carrying the virus, scientists said Wednesday as the country announced a second death in the new outbreak.

The World Health Organization (WHO) said the genetic sequence of the virus sampled from the 17-year-old victim at the centre of the first cases for three months did not match variants circulating in neighbouring Guinea or Sierra Leone.

"The origin of the cluster of cases remains under investigation," the agency said in it latest situation report on an epidemic that has killed more than 11,250 people in the three countries.

"Preliminary evidence from genomic sequencing strongly suggests that the most likely origin of transmission is a re-emergence of the virus from a survivor within Liberia."

The country was declared free of transmission on May 9, six weeks after the funeral of Ruth Tugbah, a 44-year old fruit seller who is thought to have contracted the virus through sex with her boyfriend, an Ebola survivor.

A new cluster of six cases has emerged in the last two weeks, however, since the 17-year-old tested positive after his death in the coastal county of Margibi.

The source of the new outbreak had been a mystery, with Liberian health experts considering several hypotheses, including that he contracted the virus abroad or from the flesh of a dead dog he had cooked and eaten.

Liberian health authorities announced on Wednesday a second patient had died in the resurgence, the first case to be reported in the capital, Monrovia.

"Ebola is no longer confined to Margibi County. A case has been reported in Monrovia, but has been reported expired," chief medical officer Francis Karteh said on state radio.

"The case was carried in a critical condition to the (Ebola treatment unit) and later died."

Ebola persistent in survivors

The WHO report did not mention a second death but confirmed that "one of the cases reported in the week to July 12 had symptom onset in a quarantined home in Montserrado County, near to the capital, Monrovia".

The man, a health worker, was being monitored as a known contact of one of the previous cases, but hid his illness from the authorities by taking medication to bring down his temperature, Karteh said.

The world's worst Ebola epidemic spread to Liberia from Guinea in March 2014, killing more than 4,800 Liberians in a year.

Tests on the 17-year-old showed the variant which killed him was genetically similar to the 2014 outbreak, according to the WHO.

Scientists have already demonstrated that Ebola can persist in semen for several months after a patient is declared healthy, and the virus has been detected in an eye of a patient months after it vanished from his blood.

Ian Crozier, an American doctor, was diagnosed with Ebola in September 2014 while working in Sierra Leone and was sent to an Ebola unit in Atlanta, Georgia.

He left the hospital in October when Ebola was no longer detected in his blood, but two months later fluid from an inflamed eye was found to contain the virus, according to the New England Journal of Medicine.

Besides problems with his vision and swelling, his iris changed colour, going from blue to green 10 days after the symptoms were first detected.

There were 30 confirmed Ebola cases reported in the week to Sunday, according to the WHO —13 in Guinea, three in Liberia and 14 in Sierra Leone.

For the first time in months, most cases were reported from Conakry and Freetown, the capitals of Guinea and Sierra Leone, the agency said.

Explore further Liberia announces second Ebola death as new outbreak spreads

© 2015 AFP