Some of the final cases of Ebola in Sierra Leone were transmitted via unconventional routes, such as semen and breast milk, according to the largest study of the tail-end of the epidemic.
Using real-time sequencing of Ebola virus genomes, researchers led by University of Cambridge and Wellcome Trust Sanger Institute in the UK produced a detailed picture of the latter stages of the outbreak in Sierra Leone.
While the study did not suggest that unconventional transmission was more common than previously thought, researchers describe several instances including a mother who may have transmitted Ebola to her baby via breastfeeding, and an Ebola survivor who passed on the virus sexually a month after being released from quarantine.
The findings suggest that rapid sequencing of viral genomes in the midst of an epidemic could play a vital role in bringing future outbreaks under control, by allowing public health workers to quickly trace new cases back to their source, researchers said.
Sierra Leone was the most widely affected of the three West African countries worst hit by the Ebola epidemic, with 14,124 cases and 3,956 deaths to date.
Without effective vaccines or treatments for the infection, bringing the epidemic under control relied largely on public health measures such as the rapid identification and isolation of Ebola patients, contact tracing and quarantine, as well as encouraging safe burial practices.
By mid-2015 cases in the three most-affected countries had declined, but isolated cases of the disease continued to appear, even though all known transmission chains were thought to be extinguished.
Researchers began investigating these cases in a temporary genome sequencing facility. They generated 554 complete Ebola genome sequences from samples of blood, buccal swabs, semen and breast milk collected between December 2014 and September 2015 from Ebola isolation and treatment centres in the north and west of the country.
These were combined with 1019 samples sequenced by other groups to create a picture of the viral variants present in Sierra Leone.
They found that during 2015 at least nine different lineages of the virus were circulating in Sierra Leone, eight of which evolved from a single variant that introduced Ebola to the country in June 2014.
The remaining viruses came from a separate, geographically distinct lineage that originated in Guinea.
Starting in mid-2015 samples from all new Sierra Leone cases were rapidly sequenced in the facility. The data, combined with the growing reference set, helped field workers locate the source of infection for some of the final Ebola cases in Sierra Leone, they said.
The findings were published in the journal Virus Evolution.
