By Emma Farge and Tom Miles

DAKAR/GENEVA (Reuters) - Ebola probably remained latent in Liberia even after it was officially eliminated in May, health officials said on Friday, after test results showed the strain of the virus in newly detected cases closely resembled earlier infections.

The preliminary findings, based on a new technique analysing the genetic sequence of the virus, suggest that transmission was either reactivated by an Ebola survivor, perhaps through sexual intercourse, or continued for weeks undetected, the sources added.

"It indicates that the virus is closely related to one that was circulating in Liberia in that particular area," World Health Organization spokesman Tarik Jasarevic said, confirming results given to Reuters by a health official.

"So it's either (from) a survivor or non-identified transmission in the community."

Liberia's first Ebola case in nearly two months was reported on June 30 when the body of a 17-year-old boy, Abraham Memaigar, tested positive for the virus in Margibi County. Since then, a further four cases have been confirmed.

The discovery of a new case in Liberia was a major setback in the fight against the worst recorded outbreak of Ebola, which has killed more than 11,200 people across West Africa.

The case baffled officials since Memaigar lived in central Liberia's Margibi County, far from hotspots of the epidemic in neighbouring Guinea and Sierra Leone, and was not known to have been in contact with any travellers.

"This (test) suggests that the virus has remained latent within Liberia," the health official said.

With a mortality rate of around 50%, thousands of people have survived the deadly virus, raising the prospect that they could reactivate transmission through sexual intercourse or other unknown means.

Ebola is thought to be able to survive no more than 21 days in most body fluids, such as blood and vomit, which are the primary means of transmission. But it is known to have the ability to lurk in semen and in some soft tissues, such as the eye, for up to several months after recovery.

"MUCH TO TEACH US"

Ben Neuman, a virologist at Britain's Reading University, said it was possible that other parts of the body could harbour the virus in a way that scientists are not yet aware of.

"Ebola still has much to teach us," he told Reuters. "Maybe this investigation will lead us to yet another bodily fluid where the virus can persist for months."

Liberia, the country worst hit by the virus, was declared Ebola-free on May 9 and was hailed as an example for neighbouring Guinea and Sierra Leone, which are struggling to eliminate the virus. Together, the two countries reported 27 cases in the week to July 5.

In an effort to control the epidemic, Sierra Leone said this week it would extend a curfew in affected areas by 30 days.

The WHO declares a country Ebola-free 42 days after the last case - or twice the broadly accepted maximum incubation period. But the new test findings suggest it will take longer to be sure the cycle of transmission has been broken.

WHO Liberia representative Alex Gasasira said its investigation would focus on interviews with residents to gather more information on the history of the first cases. He said he was not aware of any particular lead on sexual transmission.

"In the first days, you don't get the full picture, but as you keep digging gently, ultimately the picture falls into place," he said by phone from Liberia. "All leads are being followed."

An internal health report showed 132 contacts linked to the original cluster of cases were under surveillance. But it also showed that there were four unidentified missing contacts, thought to be motorbike riders.

Officials had initially thought that the first case might have been infected by eating the carcass of a local dog, but the remains tested negative for the virus, sources told Reuters.