A study in the journal Science, released last week, shows that the Ebola strain spreading across Western Africa has undergone a surprisingly high amount of genetic drift during the current outbreak. Experts say the mutations could eventually make the virus harder to diagnose and perhaps treat with a new therapeutic, should one come along.
In yesterday’s Wall Street Journal, I wrote that in response to the crisis, the Obama administration has stressed that the disease is unlikely to spread inside America. We will certainly see cases diagnosed here, and perhaps even experience some isolated clusters of disease. For now, though, the administration’s assurances are generally correct: Health-care workers in advanced Western nations maintain infection controls that can curtail the spread of non-airborne diseases like Ebola.
But our relative comfort in the U.S. is based on our belief that our public health tools could easily contain a virus spread only through direct contact. That would change radically if Ebola were to alter its mode of spread. We know the virus is mutating. Could it adapt in a way that makes it airborne?
It’s highly unlikely. It would be improbable for a virus to transform in a way that changes its mode of infection. Of the 23 known viruses that cause serious disease in man, none are known to have mutated in ways that changed how they infect humans. Of course, we only know about a small portion of the existing viruses.
A little background is in order.
The ability of Ebola to spread without direct contact with an infected individual, and whether or not it is efficiently spread through air, are different issues.
It’s already possible that Ebola can spread, in rare cases, through direct contact with respiratory secretions. This might occur, for example, when an infected person coughs or sneezes directly on another, uninfected individual. The Centers for Disease Control specifically recommends “droplet protection” be taken in the hospital setting when healthcare workers are treating patients infected with Ebola. This kind of direct spread is sometimes referred to as “droplet contact,” but it’s distinct from airborne spread.
When a viral infection becomes “airborne,” like ordinary influenza, it means that discharged microbes remain suspended in the air for long periods of time. Generally speaking, this is what is meant by “airborne transmission.” In this case, the organisms must be capable of surviving for long periods of time outside the body and must be resistant to drying. Airborne transmission allows organisms to enter the upper and lower respiratory tracts. This sort of transmission is sometimes also referred to as “droplet contact” or “viral droplet nuclei transmission.”
For this article, I am focused on the latter circumstance — whether or not Ebola could mutate in a way that makes it highly contagious through the air, by allowing the individual viral particles to survive for long periods suspended in dry air.
Right now, Ebola is spread through direct contact with the body fluids of actively infected individuals. Indirect transmission is also possible by means of contact with an object (fomite) that has been soiled by the body fluids of an infected individual.
The widespread belief is that the Ebola virus would be very unlikely to change in a way that would allow the individual virus particles to be concentrated, and remain suspended in respiratory secretions — and then infect contacts through inhalation.
The Ebola virus is comprised of ribonucleic acid (RNA). Such a structure makes it prone to undergoing rapid genetic changes. But to become airborne, a lot of unlikely events would need to occur. Ebola’s RNA genome would have to mutate to the point where the coating that surrounds the virus particles (the protein capsid) is no longer susceptible to harsh drying effects of being suspended in air.
To be spread through the air, it also generally helps if the virus is concentrated in the lungs of affected patients. For humans, this is not the case. Ebola generally isn’t an infection of the lungs. The main organ that the virus targets is the liver. That is why patients stricken with Ebola develop very high amounts of the virus in the blood and in the feces, and not in their respiratory secretions.
Could Ebola mutate in a way that confers these qualities on the virus?
Anything is possible. But such a scientific feat would rate as highly unlikely. A lot of the speculation that Ebola could be airborne stems from a set of earlier studies that showed Ebola virus may have been able to spread through the air between infected pigs and monkeys. There are reasons why these studies are not applicable when it comes to questions around human-to-human transmission. In animals, Ebola behaves differently than it does in people, for example concentrating in lung tissue.
Nonetheless, the fact that the Ebola virus is undergoing rapid changes reinforces the urgency of getting this epidemic under control. We need to snuff it out. While the virus is unlikely to be modified in a way that changes its mode of infection, the resulting mutations could nonetheless make it harder to diagnose, or even treat.
Moreover, our ability to prevent an epidemic here in the U.S. doesn’t relinquish our obligations abroad. Even if the epidemic remains confined to Western Africa, the outbreak could rank as one the cruelest natural catastrophes of recent times—if not in human death and suffering, then certainly in the economic and social devastation caused by declining commerce, and the strife resulting from mass cordons. As I note in the Wall Street Journal, “compared with a one-time act of nature, like a storm, that delivers its destruction at once; the swelling nature of a viral epidemic can magnify its impact on economic and civil life.”
For all of these reasons, and most of all for the humanitarian imperative; we need to be very concerned about the epidemic unfolding in Western Africa, even if the U.S. isn’t at direct risk of an outbreak now. We need a vigorous plan for helping that region deal with this evolving catastrophe. http://www.forbes.com/sites/scottgottlieb/2014/09/03/can-ebola-go-airborne/
