Could Ebola Become Airborne?

Yesterday a student came to class freaked out about Ebola.  Again.  He was freaked out this time because he had read an article which quoted Dr. Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota.  In the article, Dr. Osterholm is quoted as saying about an airborne Ebola virus, "It is the single greatest concern I've ever had in my 40-year public health career.  I can't imagine anything in my career - and this includes HIV - that would be more devastating to the world than a respiratory transmissible Ebola virus."

It is important to note that he was not talking about an already existing airborne version of Ebola.  He was talking hypothetically about a possible future airborne Ebola virus.  In fact there has not ever been a case where any virus has shifted its main mode of infection this dramatically.  However this idea - that Ebola could mutate and become airborne - is pants wettingly scary and could result in mass casualties worldwide were it to become a reality.  But how likely is it that this would happen?

The simple answer to this question is "very unlikely, bordering on impossible".  Let's start with an analogy.  You are a quarterback for a football team, and a darned good one.  Last season you were the top passer in the league where you threw over 230 touchdown passes and broke 600 yards of total passing in every game.  It is offensive how good you are.  On top of that you have a natural scent that perfume companies around the world are attempting to copy for their fall scent line.  Lets face it, you are amazing and everyone wants to be you.

You are such an amazing quarterback and all around heckuva guy that your coach decides to throw you in as a defensive lineman for a few plays.  This goes wrong in an almost every possible way.  You are thrown down repeatedly by the larger, more highly trained offensive linemen and manage to get pushed around almost immediately.  You soldier on, however, and on the next play manage to hurt not only yourself but also several of your teammates when your ill advised diving tackle manages to not only miss the running back but takes out 3 of your own players as well.  The overall consensus is that you are amazing on offense with your quick reflexes and the ball in your hand but terrible on defense when size and strength matter more.

Now imagine that you are asked not just to play on a different aspect of a football team, but to play in the French Open tennis tournament.  Sure, it is still a physically demanding activity and you are used to that sort of thing, but it is demanding in an entirely new way that your body is most certainly not trained to handle.  You are used to throwing a ball on a grass field while 11 large men try to kill you.  Tennis requires finesse and years of practice with a racket, not to mention the fact that you are now playing on a slippery clay surface which is difficult even for most pros. With years of time and dedication our star athlete quarterback might be able to make it to the Open, but why would he want to give up his completely successful career to do so?

Ebola is a lot like this quarterback.  It is amazing at what it does which is infecting people and living in their blood.  For it to become airborne it would be the equivalent of our quarterback playing in the French Open.  Sure, with lots of mutations and some significant changes to its structure it could happen, but why would it do so?  It would have to give up its already excellent infection and transmission pathway in favor of one that is almost completely different.  It would have to abandon the blood stream as its primary hiding/replicating place in favor of the lungs or throat.  Then it would have to mutate to become able to survive in the air or on surfaces which, at present, it doesn't seem to be able to do very well (Piercy, et al 2010).

Let's talk a little science here for a bit.  Not too much, but enough to get through some basics.  Viruses don't just magically attach to cells and infect them.  They have to be able to grab on to the cells and, after they have successfully done that, invade the cell so it can reproduce.  Without getting into a cell a virus is unable to make more copies of itself and without being able to attach to a cell it can't begin to do that.  Ebola is an interesting virus in that, like HIV, it grabs onto little handles (glycoproteins and other receptors) in the cell membrane and tricks the cell into allowing it access.  Studies show that its preferred mode of infection is to grab onto cells involved in the creation of connections between tissue layers (like the connections that hold your skin to your muscles) and some immune system cells.  It basically does the same thing that HIV does only way more aggressively.

The types of cells it prefers to attack are relatively specific cell types and, despite the fact that all of the cells in your body have the same DNA they are not, in fact, all identical.  Many have different glycoproteins (handles) and therefore different attachment points.  There is a reason Ebola doesn't infect your sinuses or your throat and that is because it appears to have difficulty binding to the cells in those areas.

So in order for Ebola to become airborne it would require a whole series of mutations which would allow it to:

1. survive in the air and on surfaces at room temperature for long periods of time.
2. stop attaching to its preferred host cells in the blood and connective tissue
3. begin attaching to a new type of host cell in the lungs and throat
4. stop killing patients so darn quickly

Each of these steps would most likely require several mutations which is the equivalent of our football player picking up a tennis racket and learning to serve the ball like Pete Sampras.  Possible?  Sure. Likely?  Not really.

Last, and probably most importantly, is the evolutionary reason for why this isn't going to happen:  there is no selection pressure.  Selection pressure occurs when an organism finds itself in a difficult situation, environment-wise.  If the environment changes significantly then species have to either adapt or go extinct.  Ebola is not facing any sort of selection pressure at this point.  It is wildly good at what it does and is incredibly successful as a blood/fluid born virus.  Would our hypothetical quarterback switch to playing tennis in the middle of his amazing career if the NFL still exists and is willing to pay him millions of dollars?  Neither would Ebola.

I have purposely simplified the explanations in this post to make it easier to read.  If there are any bits that you feel need some expansion or clarification, let me know!



Piercy, T.J., Smither, S.J., Steward, J.A., Eastaugh, L., Lever, M.S. (2010) The survival of filoviruses in liquids, on solid substrates and in a dynamic aerosol. J Appl Microbiol. 109(5): 1531-9.