Ebola virus. Micrograph from CDC/Cynthia Goldsmith
Ebola virus. Micrograph from CDC/Cynthia Goldsmith

Is It Worth Imagining Airborne Ebola?

Back in September, when the West African Ebola outbreak was getting worse with every passing week, a lot of people began to worry that the virus could spread by air. And even if it couldn’t spread by air yet, they worried that it might be on the verge of mutating into an airborne form.

When I talked to virus experts, they saw little ground for either concern. The epidemiology of the outbreak, like previous ones, had the sort of pattern you’d expect from a virus that spreads mainly through contact with body fluids. A look at the evolutionary history of viruses indicates that a fluid-adapted virus would be unlikely to switch to going airborne with just a couple mutations. (I wrote in the New York Times about these conversations here and here.)

The anxiety over airborne Ebola has faded. The outbreak itself has dwindled down dramatically, although driving it down to zero may prove hard. But a new “Opinion/Hypothesis” piece published in the journal mBio, called “Transmission of Ebola Viruses: What We Know and Do Not Know,” has breathed some new life into the old worry.

The piece was written by Michael Osterholm of the University of Minnesota and a number of other researchers. Back in September, Osterholm wrote a controversial op-ed in the New York Times, declaring, “If certain mutations occurred, it would mean that just breathing would put one at risk of contracting Ebola.”

In the new mBio piece, Osterholm and his colleagues survey a number of past studies on how Ebola spreads. These studies don’t tell us as much as we’d like. We know less about Ebola than we do about, say, influenza, because it’s a lot rarer and a lot deadlier. Scientists thus have fewer opportunities to study it, and when they do, they have to take enormous precautions. But the evidence we do have offers a pretty clear picture, Osterholm and his colleagues write: “Available data indicate that direct physical contact and exposure to infected body fluids are the primary modes of Ebola virus transmission.”

Those fluids may be the blood of a sick patient, or diarrhea, vomit, or sweat. People can get infected by touching those fluids, but it’s also conceivable that the virus can reach a new victim in a spray of fluid. The droplets in these fluids don’t travel far, so they don’t create airborne transmission in the same sense that a virus like measles is airborne–with tiny aerosols drifting on air currents. Some animal studies have shown that Ebola can spread without direct contact, but they don’t demonstrate clear evidence that aerosols delivered the virus. Still, Osterholm and his colleagues note that when Ebola victims are autopsied, the viruses sometimes turn out to be present in their lungs. A cough or a sneeze could conceivably deliver virus-laden aerosols.

While that’s theoretically possible, Osterholm and his colleagues acknowledge that this route has never been documented in humans. “This could be because such transmission does not occur or because such transmission has not been recognized, since the number of studies that have carefully examined transmission patterns is small,” they write.

There are other factors in Ebola outbreaks that we still don’t understand well. Some evidence suggests that certain people may become “superspreaders,” transmitting Ebola to many more people than usual, but we don’t know what’s responsible for these differences. It’s also possible that different strains of Ebola have genetic differences that cause some to spread faster than others. Some preliminary studies suggest that people who got sick in the West African outbreak build up more viruses in their bodies than people in earlier outbreaks.

After surveying what we do and don’t know about Ebola transmission, the authors offer what they call a hypothesis: Ebola might indeed be able to become airborne. Infected people might cough up virus-laden droplets, which other people might then breathe into their lungs, setting off an infection. Mutations could make this route easier for the virus to take. “We agree this is an improbable (although not impossible) scenario,” Osterholm and his colleagues acknowledge, but they point out that Ebola has sprung many surprises on us in the past. “We should not assume that Ebola viruses are not capable of surprising us again at some point in the future,” they conclude.

I got in touch with some other experts to see what they thought about this new piece. The most positive of them was Pardis Sabeti, a Harvard scientist who has been analyzing the genes of Ebola viruses to track their evolution. “I think that overall it is a really nice and thorough review,” she told me. She agreed it was important to figure out whether different Ebola lineages spread differently. As for airborne Ebola, she considered it unlikely although not impossible. “We should continually monitor its properties as it continues to evolve,” she said.

But other researchers were less enthusiastic. “I don’t see any new data that really sheds any new light on things in terms of the outbreak,” said Thomas Giesbert of the University of Texas. Most of the scientists I reached out to found the hypothesis of airborne Ebola even less impressive. “I guess you can make hypothesis about anything, and a ‘hypothesis’ about ‘potential’ isn’t very strong,” said Edward Holmes of the University of Sydney. “It fails to deliver,” said Vincent Munster of the National Institutes of Health.

Vincent Racaniello of Columbia was even harsher: “It can be viewed as a scare tactic, although to what ends I do not know,” he said.

Racaniello and the other critics note that there’s no evidence of that the Ebola virus has evolved in any significant way during the latest outbreak. And the fact that no one has found compelling evidence of aerosol transmission since the virus was discovered in 1976 suggests that shifting to that route is a major challenge, not an easy evolutionary maneuver.

In fact, viruses in general don’t show the massive evolutionary potential that Osterholm and his colleagues see in Ebola. Smallpox and influenza have infected billions of people by airborne transmission for thousands of years, and there’s no evidence that they have evolved a new route. Poliovirus and norovirus take the oral route, as they always have.

“No human virus has ever changed the way it is transmitted – at least in the 100 years or so we have been studying them,” said Racaniello. “There is no reason to believe that Ebolaviruses will become respiratory pathogens.”

(For more, see this blog post Racaniello published this weekend. For more on viruses generally, see my book A Planet of Viruses.)