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Ebola Is Not Mutating As Fast As Scientists Feared

In November, the Ebola virus found in Mali was surprisingly similar to strains circulating in Sierra Leone six months earlier.
Courtesy of NIAID
In November, the Ebola virus found in Mali was surprisingly similar to strains circulating in Sierra Leone six months earlier.

Back in August, scientists published a worrisome report about Ebola in West Africa: The virus was rapidly changing its genetic code as it spread through people. Ebola was mutating about twice as fast as it did in previous outbreaks, a team from Harvard University found.

The study spurred a bunch of concerns. Could the virus evolve into a more dangerous pathogen? Could it start spreading through the airborne route?

Virologists said neither of the scenarios were likely. Past outbreaks showed that pathogens don't easily change their mode of transmission. And sometimes they become less deadly as they adapt to people.

But there was one legitimate concern: Diagnostic tests and experimental treatments could stop working if Ebola changed its genetic code too quickly.

Now scientists at the National Institute of Allergy and Infectious Diseases have new data that alleviate these concerns.

Nine months into the epidemic, Ebola wasn't mutating any faster than in previous outbreaks, Heinz Feldmann and his colleagues report Thursday in the journal Science.

In fact, Ebola's genes remained relatively stable between June and November 2014, the team found, despite the extensive human-to-human transmission taking place during that time.

"Our data indicate that EBOV [Ebolavirus] is not undergoing rapid evolution in humans during the current outbreak," Feldmann and his colleagues write. At the same time, there's no evidence the virus has become more deadly.

The team came to these conclusions after sequencing the genomes of four Ebola viruses taken from patients in Mali from October and November. They compared the genetic codes of these strains to those sequenced in June from Sierra Leone and from other outbreaks in the past. The strains weren't as different as previous studies predicted they should be.

"This is some good news for the development of interventions," says Dr. Anthony Fauci, who directs the NIAID but wasn't directly involved with the study. "The data also indicate it's quite unlikely the virus will mutate and change its way of transmission."

One study, of course, can't give the whole picture. To date, the world has recorded nearly 25,000 cases. The study analyzed only four genomes — just a tiny slice of the virus strains circulating in West Africa. Other versions of the virus may be evolving more rapidly.

And with viruses, Fauci says, you never know what will happen. "One should not be surprised that RNA viruses, like Ebola, mutate," he says. "They do that all the time. The questions are how much are they mutating and are there functional consequences of those mutations."

The few mutations observed in the Mali sequences don't look like they would effect Ebola tests, potential vaccines or treatments, Fauci says.

It's unclear why the current study disagrees with the previous one from Harvard University. One possibility is that the two teams used different computer models to estimate the mutation rate. When the NIAID team applied its model to the earlier data from Harvard, the team also came up with the lower mutation rate — the one that matches the rate observed in previous outbreaks in Central Africa.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Michaeleen Doucleff, PhD, is a correspondent for NPR's Science Desk. For nearly a decade, she has been reporting for the radio and the web for NPR's global health outlet, Goats and Soda. Doucleff focuses on disease outbreaks, cross-cultural parenting, and women and children's health.