One mutation. A simple tweak in the Ebola gene — a C got turned into a T. That's all it took to make Ebola more infectious during the West Africa epidemic, scientists report Thursday.
Two studies, published in the journal Cell, found that a single mutation arose early in the epidemic. It allows Ebola to infect human cells more easily than the original version of the virus — way more easily.
"The largest difference we saw was about a fourfold increase in the number of cells infected," says Jeremy Luban, a virologist at the University of Massachusetts Medical School, who led one of the studies. "When you're talking about a virus that could kill you, this is a pretty scary number."
When Ebola appeared in West Africa late in 2013, it spread faster and further than any previous Ebola outbreak. One reason was because it hit densely populated cities. And countries in West Africa didn't have the tools to stop the epidemic.
But early on, computational biologists at Harvard University also noticed the virus was changing. Its genes were mutating.
That's not surprising, Luban says, on its own. It's what viruses do, especially when they first start spreading in people.
But the mutations raised a big concern: "Is this virus somehow becoming more transmissible, more dangerous or more deadly?" Luban says.
The computational biologists were stumped. They could see the mutations, but they didn't know what they did.
So Luban — and another team led by Jonathan Ball at the University of Nottingham in the U.K. — started experimenting with the mutations in the lab. Quickly one mutant jumped out as peculiar. So Luban set up a video conference call with the Harvard biologists.
"I remember, we asked them, 'So what's significant about this particular mutation?' And they all started jumping up and down," Luban says.
The Harvard team got excited because they had been watching this mutation. It cropped up in the outbreak when there were only a hundred or so cases in Guinea. And then it spread like gangbusters.
It went to Sierra Leone, Liberia. It showed up in Nigeria, Mali. And then it came here to the U.S., Luban says.
"It's the form of the virus that made its way to an emergency room in Texas," he says, when Thomas Eric Duncan brought Ebola to Dallas from Liberia.
Although the mutant appears to be more infectious than the original version of Ebola, Luban and his colleagues don't know if it makes the virus more deadly.
Luban has some evidence that it's linked to an increase in mortality rate. "But that increase is very, very slight. So that alone doesn't prove anything," says Vincent Racaniello, a virologist at Columbia University, who wasn't involved with the studies.
Also, all the experiments have been in cells with a model of the Ebola virus. Racaniello says. So the findings need to be confirmed in animals, with a real virus.
For these reasons, Racaniello doesn't find the findings frightening. "No, not at all," he says. "What's frightening is the outbreak, which affected tens of thousands of people and killed many thousands of people. I think that's very scary."
Besides, he says, we stopped this mutant. The outbreak is over. There hasn't been a case of Ebola in months.
"That [mutant] virus, as far as we know, it's not circulating anymore. It's not causing infections anymore in people in West Africa," Racaniello says.
In fact, that mutant hasn't just stopped infecting people, it's probably actually been completely eradicated.
"As far as we know that virus is gone," he says.
Why? Because here's something I forgot to mention about the mutant virus: The same mutation that helps the virus infect human cells more easily also prevents it from infecting animal cells. So the mutant virus probably can't hide in bats — or other animals — and threaten to trigger another outbreak.
In other words, once the epidemic in West Africa was over, so was this mutant.
ARI SHAPIRO, HOST:
Scientists reported today they have discovered something about the Ebola virus that might explain why the outbreak in West Africa a few years ago was so explosive. The virus has a mutation that seems to make it easier to spread. NPR's Michaeleen Doucleff reports.
MICHAELEEN DOUCLEFF, BYLINE: When Ebola appeared in West Africa late in 2013, it spread faster and further than any previous Ebola outbreak. One reason was because it hit densely populated cities, but, also, a group of geneticists noticed the virus was changing. Its genes were mutating. Jeremy Luban at the University of Massachusetts Medical School says that raised a big concern.
JEREMY LUBAN: There was this obvious question to them. Is this virus is somehow more transmissible or more dangerous or more deadly?
DOUCLEFF: And was that why this outbreak was so bad? The geneticists had no idea. They could see the mutations, but they didn't know what they were doing. So Luban started experimenting with the mutations in the lab, and one mutant was behaving in a curious way. So he set up a video conference call with the geneticist.
LUBAN: I remember we asked them - so what's significant about this particular mutation? And they all started jumping up and down.
DOUCLEFF: Because they had been watching this mutation. They didn't know if it was doing anything, like making people even sicker or the virus more contagious, but they had been watching it spread across the world. It appeared early on and spread like gangbusters. It became the dominant strain from Guinea to Sierra Leone, Liberia, showed up in Nigeria, Mali. And...
LUBAN: It's also the form of the virus that made its way to an emergency room in Texas.
DOUCLEFF: That was the case of Thomas Eric Duncan, the man who brought Ebola to the U.S. from Liberia and infected several other people here. Now Luban and another team have figured out that this wasn't a harmless mutation. They're reporting in the journal Cell that this mutation likely spread so far because it makes it easier for the virus to infect human cells - way easier.
LUBAN: The largest difference we saw was about four-fold, which, when you're talking about a virus that could kill you - it's a pretty scary number, I'd say.
DOUCLEFF: Even scarier, it was just one tiny change that created this mutant version. This shows how easily Ebola can evolve and become more dangerous, but here's the thing. They don't know yet if the mutation makes the virus more deadly. Vincent Racaniello is a virologist at Columbia University who wasn't involved in the research. He says there's a hint the mutant might make people more sick.
VINCENT RACANIELLO: It's associated with a slightly increased mortality, but it's very, very slight. So that alone doesn't prove anything.
DOUCLEFF: This work was done in human cells. The next step would be to test the mutant in animals. Right now, Racaniello actually isn't worried about this mutant. He doesn't find it scary at all because, he says, we stopped it.
RACANIELLO: So that virus - that virus that spread in people, as far as we know, it's not circulating. It's not causing infections anymore in people in West Africa.
DOUCLEFF: There hasn't been a case in months.
RACANIELLO: As far as we know, that virus is gone.
DOUCLEFF: Done, dead, finished - because here's the other thing about this mutant. The same mutation that helps the virus more easily infect human cells also now blocks it from infecting animal cells. That's important because it's thought Ebola spread from bats to people in West Africa, and that triggered the outbreak, and that the virus hides out in bats, waiting to spark another outbreak. But this mutant strain can't do that, so once the outbreak was over in West Africa, so was this mutant. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.