What we are not afraid to say about Ebola virus

18 SEPTEMBER 2014

By Vincent Racaniello

In a recent New York Times OpEd entitled What We’re Afraid to Say About Ebola, Michael Osterholm wonders whether Ebola virus could go airborne:

You can now get Ebola only through direct contact with bodily fluids. If certain mutations occurred, it would mean that just breathing would put one at risk of contracting Ebola. Infections could spread quickly to every part of the globe, as the H1N1 influenza virus did in 2009, after its birth in Mexico.

Is there any truth to what Osterholm is saying?

Let’s start with his discussion of Ebola virus mutation:

But viruses like Ebola are notoriously sloppy in replicating, meaning the virus entering one person may be genetically different from the virus entering the next. The current Ebola virus’s hyper-evolution is unprecedented; there has been more human-to-human transmission in the past four months than most likely occurred in the last 500 to 1,000 years.

When viruses enter a cell, they make copies of their genetic information to assemble new virus particles. Viruses such as Ebola virus, which have genetic information in the form of RNA (not DNA as in other organisms), are notoriously bad at copying their genome. The viral enzyme that copies the RNA makes many errors, perhaps as many as one or two each time the viral genome is reproduced. There is no question that RNA viruses are the masters of mutation. This fact is in part why we need a new influenza virus vaccine every few years.

The more hosts infected by a virus, the more mutations will arise. Not all of these mutations will find their way into infectious virus particles because they cause lethal defects. But Osterholm’s statement that the evolution of Ebola virus is ‘unprecedented’ is simply not correct. It is only what we know. The virus was only discovered to infect humans in 1976, but it surely infected humans long before that. Furthermore, the virus has been replicating, probably for millions of years, in an animal reservoir, possibly bats. There has been ample opportunity for the virus to undergo mutation.

More problematic is Osterholm’s assumption that mutation of Ebola virus will give rise to viruses that can transmit via the airborne route:

If certain mutations occurred, it would mean that just breathing would put one at risk of contracting Ebola. Infections could spread quickly to every part of the globe, as the H1N1 influenza virus did in 2009, after its birth in Mexico.

The key phrase here is ‘certain mutations’. We simply don’t know how many mutations, in which viral genes, would be necessary to enable airborne transmission of Ebola virus, or if such mutations would even be compatible with the ability of the virus to propagate. What allows a virus to be transmitted through the air has until recently been unknown. We can’t simply compare viruses that do transmit via aerosols (e.g. influenza virus) with viruses that do not (e.g. HIV-1) because they are too different to allow meaningful conclusions.

One approach to this conundrum would be to take a virus that does not transmit among mammals by aerosols – such as avian influenza H5N1 virus – and endow it with that property. This experiment was done by Fouchier and Kawaoka several years ago, and revealed that multiple amino acid changes are required to allow airborne transmission of H5N1 virus among ferrets. These experiments were met with a storm of protest from individuals – among them Michael Osterholm – who thought they were too dangerous. Do you want us to think about airborne transmission, and do experiments to understand it – or not?

The other important message from the Fouchier-Kawaoka ferret experiments is that the H5N1 virus that could transmit through the air had lost its ability to kill. The message is clear: gain of function (airborne transmission) is accompanied by loss of function (virulence).

When it comes to viruses, it is always difficult to predict what they can or cannot do. It is instructive, however, to see what viruses have done in the past, and use that information to guide our thinking. Therefore we can ask: has any human virus ever changed its mode of transmission?

The answer is no. We have been studying viruses for over 100 years, and we’ve never seen a human virus change the way it is transmitted.

HIV-1 has infected millions of humans since the early 1900s. It is still transmitted among humans by introduction of the virus into the body by sex, contaminated needles, or during childbirth.

Hepatitis C virus has infected millions of humans since its discovery in the 1980s. It is still transmitted among humans by introduction of the virus into the body by contaminated needles, blood, and during birth.

There is no reason to believe that Ebola virus is any different from any of the viruses that infect humans and have not changed the way that they are spread.

I am fully aware that we can never rule out what a virus might or might not do. But the likelihood that Ebola virus will go airborne is so remote that we should not use it to frighten people. We need to focus on stopping the epidemic, which in itself is a huge job.