Origin of the Ebola Outbreak

With today's unprecedented outbreak of Ebola virus (EBOV) disease in West Africa (NEJM JW Infect Dis Jul 1 2014), understanding how the outbreak began and how the virus may be evolving can benefit prevention and control efforts. To obtain this information, a multinational research team sequenced EBOV genomes from 78 patients in Sierra Leone.

The first case of EBOV disease in Sierra Leone was confirmed on May 25, and epidemiologic analysis linked this case to the burial of a traditional healer who had treated EBOV disease patients in the neighboring country of Guinea. Tracing revealed 13 additional cases in women attending the burial. Investigators obtained 15 inactivated EBOV samples from 12 of these patients, as well as 84 samples from 66 other EBOV disease patients, totaling >70% of cases diagnosed in Sierra Leone through mid-June.

Phylogenetic comparison of the Sierra Leonean genomes with 20 from previous outbreaks in central Africa and 3 from cases in Guinea suggests that the strains responsible for the three most recent outbreaks diverged from a common ancestor in about 2004. The genetic similarity among EBOV strains from the current outbreak suggests a single transmission from a natural reservoir to humans, then ongoing human-to-human transmission. It appears that two distinct viral strains that had likely diverged in mid-April in Guinea were introduced into Sierra Leone at the time of the burial ceremony. The observed substitution rate within the 2014 outbreak is roughly twice as high as that between outbreaks.