Ebola in survivors can trigger outbreaks years after infection

Scientists already knew Ebola could lie dormant in survivors, who test negative because the virus is in tissue rather than circulating in the blood.

But analysis of an outbreak this year in Guinea, published Wednesday in the journal Nature, found these "virus reservoirs" appear able to awaken and cause new infections and transmission years on.

To trace the source of the Guinea outbreak, which involved 16 confirmed cases, 12 of whom died, researchers analysed the genomes of samples from several patients.

Ebola outbreaks are usually thought to result from the virus "spilling" from an animal host to a human.

But the analysis showed the Guinea strain was virtually identical to that from a 2013-16 wave.

If the virus had been circulating actively in the community since then, it would have accumulated a certain number of mutations as it spread.

Instead, the 2021 virus had just 12 changes, "far fewer than would be expected... during six years of sustained human-to-human transmission".

That strongly suggests the source was a reactivated virus that had lain dormant in a survivor, said Alpha Keita, a researcher at the University of Montpellier who led the study.

"This is the longest known time between the declared end of an epidemic and a viral resurgence," he said.

"It's a new paradigm: the possibility that transmission from an individual infected during a previous epidemic could be the source of a new outbreak."

How and why a dormant Ebola virus suddenly awakens and sickens a person remains something of a mystery, though there are some tantalising clues.

Sometimes a spike in Ebola antibodies can be detected in survivors at a given time -- a possible sign that the body is responding to a resurgent virus.

Around two-thirds of Ebola survivors have high antibody levels even five years after infection, but "the question to pose is what happens if there's a resurgence in the people whose immunity has dropped", said Keita.