HOW EBOLA ADAPTED TO US
The Nebraska Medical Center in Omaha has practiced Ebola treatment after two patients were from flown in from the affected country in West Africa
During the recent record-breaking outbreak, the virus picked up a mutation that made it better at infecting human cells
In December 2013, in a small village in Guinea, the Ebola virus left its traditional host—probably a bat—and infected a young boy. That leap triggered what became the largest Ebola outbreak in history. At first, the virus stayed within Guinea’s borders and, as in every previous epidemic, affected just a few hundred people. But in the spring of 2014, that gentle simmer came to a disastrous boil.
Cases skyrocketed as the virus spread to Sierra Leone, Liberia, and other countries. By the time it was finally brought to heel in 2016, more than 28,000 people had been infected and 11,000 of them were dead.
The unprecedented scale of the outbreak gave the virus ample opportunities to adapt to its new human hosts—and it took advantage of them. Two independent teams of scientists have shown that in early 2014, Ebola virus picked up a mutation called A82V, which made it worse at infecting bat cells, but better at infecting human ones.
And once viruses with that mutation appeared, they quickly took over. They were the ones that spread beyond Guinea, the ones responsible for the vast majority of cases, the ones that landed in a Texan emergency room.
Both groups emphasize that the mutation may not have caused the virus’s exponential spread. “Its appearance coincided with the virus taking off, but others factors were probably more important, like the movement of infected people into urban areas and lack of proper burials,” says Jonathan Ball from the University of Nottingham, who led one of the teams.
“If we hadn’t seen that mutation, there would probably still have been a big outbreak.” Still, it’s clear that the virus can adapt to humans if given half the chance. “That mutation happened several months into the outbreak, and would never have happened if we had stopped the virus early,” says Pardis Sabeti from the Broad Institute, who co-led the other group.
“It’s a reminder of the importance of working fast, and not letting these viruses have a lot of opportunities to reproduce in humans and adapt to them.” Until recently, many researchers doubted that Ebola was adapting to people at all.
“The word on the street was no. The conclusion has been that Ebola is Ebola is Ebola,” says Jeremy Luban from the University of Massachusetts Medical School, who worked with Sabeti. “There was no obvious evidence that there was anything special about the viruses that recently killed so many people.”
He found such evidence by comparing the genomes of almost 1,500 Ebola viruses from the outbreak. These belonged to two main lineages—one that began the epidemic and stayed within Guinea, and another that replaced it and spread further afield.
The latter strains evolved from the former and differed from their ancestors by a couple of mutations. One of these, A82V, looked like it was especially important because it affected small molecules called glycoproteins on the virus’s surface.
If you look at Ebola under a microscope, you’ll see a long, knotted tube, covered in tiny studs. Those are the glycoproteins. They fit into a host cell like a key into a lock, and in doing so, they open the door to an infection. And the A82V mutation seemed to change the shape of the key. “It’s in exactly the part of the glycoprotein that interacts with the host cell,” says Luban. “That’s an extraordinary coincidence. It’s hard to brush off.”