A big-picture look at the world’s worst Ebola epidemic: West Africa, 2013-2016

between cities also played a role, with the shorter the distance, the more likely that infected travelers would arrive and seed an infection. Distance was key to sparing nearby Guinea-Bissau, Senegal, Mali, Cote d’Ivoire and northern Guinea from severe and protracted epidemics. Some of these regions had large cities in which Ebola would likely have exploded had the virus been introduced.

“Essentially, it was entirely down to chance that the outbreak didn’t spread further and cause an even bigger crisis,” Dudas said.

Other variables, such as shared languages, economic output and climate were not found to be significantly associated with speeding or slowing the epidemic.

The analysis did see correlations between border closure dates and virus traffic reduction; once the borders were closed, virus movement occurred mostly within countries rather than among them.

But by the time Sierra Leone, Liberia and Guinea closed their borders, cross-border travel had already seeded outbreaks in each country. And although international traffic of viruses was reduced after the closures, it didn’t stop completely.

“That was part of the problem in Sierra Leone and Guinea in the final stages of the epidemic, where a particularly mobile chain [of infected people] was moving back and forth between the countries,” Dudas said.

What the genome knows
In previous genome analyses, scientists traced the epidemic’s origin to December 2013, when a two-year-old who had been playing near a bat-filled tree died in a small village in the southeastern part of Guinea. It took until March 2014 for hospital workers to detect and report the spread of a disease with an unusually high death rate. By later that month it was identified as Ebola.

Bats are the suspected — but not proven — reservoir for Ebola virus. (A reservoir refers to an animal that harbors a virus, allowing the virus to live and multiply between outbreaks in humans.) The virus spreads to humans and then from person to person through direct contact.

Sequencing virus genomes from even a fraction of people infected in an epidemic and comparing mutation patterns can give researchers valuable information about how big the epidemic is, how long it has been spreading and where transmissions chains start and end, said Dr. Trevor Bedford, a Fred Hutch evolutionary biologist and one of the paper’s authors.

Some of this information can be and was obtained the old-fashioned way, by public health workers going door-to-door, tracing contacts of those infected. But in West Africa and other