Ebola hysteria dominated popular news in 2014.  However, since the outbreak began to draw attention in March earlier this year, only two people have been infected in the United States.  Did this really warrant non-stop media coverage? Much of this attention may have been attributed in part to the worry that Ebola may “mutate to become airborne,” but is this even something we need to worry about? Join Science in the News as we investigate the origins of Ebola hysteria and discuss how scientists think about viral mutations and what they tell us about the potential for an airborne strain.   

This year, the Ebola virus jumped from the obscurity of African jungles and into the front pages of every media outlet in the United States. The virus has been and continues to be a major issue in West Africa, but it was given the most attention during September and October of 2014 when a man traveling to the U.S. from Liberia became the first case of Ebola diagnosed in the U.S. Later, two nurses who treated him became infected. Despite constant reassurance from many scientists and public health officials, fear of this virus remained exceptionally high. Since there have been no new cases in the U.S., we can look back and say that those reassurances were correct. One of the biggest causes of concern was that the Ebola virus would mutate to become transmissible through the air and cause a global pandemic. Where is this fear coming from, and is there any precedent for this to happen?

Figure 1 Ebola virus is spread through direct contact with bodily fluids, and potentially through short-distance droplet transmission.

What does “airborne” really mean?

When health professionals talk about a virus being airborne, they mean that the virus can remain suspended in the air after an infected person has released it through a cough, a sneeze, or by talking [1]. Thus, an unsuspecting person can become infected with an airborne virus by breathing in air containing virus even when an infected person is no longer present. One virus that can be transmitted this way is the measles virus.

Airborne transmission is often confused with droplet transmission, but these are considered as different modes of transmission. Droplets containing virus are too large to be airborne for long periods of time and thus do not stay suspended in the air. Droplets can be transmitted in short distances when an infected person coughs or sneezes and the droplets are ingested or enter someone else’s body through a cut [2].

The CDC posits that Ebola could be transmitted though droplets, but it has not been studied in detail. One can imagine a scenario in which an Ebola-infected patient has bleeding gums (a common scenario) and coughs onto someone close by. Although coughing is not a common Ebola symptom (19% of cases in one study [3]), if infectious blood was coughed on to someone and got into his eyes, nose, throat, or in a cut, this could start an infection. However, this is still very different from airborne transmission.

A terrifying true story?

When I was in high school, The Hot Zone was one of the books that sparked my interest in microbiology and virology.  Marketed as “a terrifying true story,” I devoured every page of the book due to the horrifying narrative about incidents involving filoviruses such as Ebola virus.

The Hot Zone describes a scenario that is commonly used as a proof of the possible evolution of airborne Ebola virus transmission. In this scenario, an experiment had a room with two groups of monkeys that were separated enough to prohibit direct contact. One group was infected with a strain of Ebola virus while the other was not. The uninfected group ended up dying due to this Ebola virus, indicating that it spread across the room. The book’s author, Richard Preston, takes this to mean that the virus was airborne. While some of the evidence points to this conclusion, the authors of the study indicate that, “the exact mode of transmission to the control group cannot be absolutely determined” [4]. Crucially, there was zero evidence that the monkeys actually expelled Ebola virus from their lungs as would be necessary for airborne transmission.

Scientists have long questioned these and other claims in the book, and even Preston has recently begun to back off on some of the depictions in the book. In a recent interview he states that he wants to update the book to “to make the clinical picture of the virus more clear and accurate” and admitted that a scene described in the book “almost certainly didn’t happen” [5].

An “outbreak” of conjecture

Are there other sources for the myth of airborne Ebola transmission? To look for answers, we first take a look at a popular movie, Outbreak, which was released in 1995 and debuted at #1 [6].  In the movie, a virus is discovered around 1970 in Zaire, Africa.  It causes a hemorrhagic fever with high mortality. Sounds just like Ebola, right?  However, where the plot breaks off from reality is when the movie virus becomes highly transmissible through the air and goes on to be a global pandemic. With its parallels to the actual Ebola virus, even movie reviewers have a hard time determining where to draw the line between fact and fiction [7]. After all, it seems like a completely possible scenario until you look at what would need to occur for a virus like Ebola to become transmissible through the airborne route.

Why isn’t Ebola airborne?

While the term “virus” is great for classifying certain infectious agents, it can lead to the illusion that all viruses are similar. This is not the case – especially when considering modes of transmission. Mosquitoes spread West Nile virus. Rodents spread Hantavirus. HIV is spread through sexual contact or needle use. Measles virus is airborne. While a measles virus infection can be initiated by the inhalation of particles in the air, Ebola virus infection requires infectious body fluids or droplets to come into contact with the eyes, nose, throat, or skin abrasion of an uninfected person

For Ebola to become airborne, it would first have to infect the respiratory system – cells in the throat or lungs [8]. This change alone would likely require a number of mutations. Mutations occur in individuals in a population, and if one individual or a small subset of individuals is better at surviving under some condition than the others, the mutation will become more prevalent. Thus, even if one of the many copies of Ebola in an infected individual were to gain mutations that, in theory, made it airborne, that mutant would have to be at an advantage compared to un-mutated Ebola in order for it to become dominant. Since Ebola virus is spread efficiently as is, the selective pressure for it to become airborne is likely minimal.

One could argue that the virus would benefit from airborne transmission because it would be able to spread more efficiently. However, if this was true, and airborne transmission was the best way for a virus to be spread, it would be expected that we would see other viruses acquire this ability. We could speculate why this doesn’t happen, but it just doesn’t happen in nature. Even HIV, which has infected tens of millions of people and mutates readily, has never been observed to be airborne. In short, it is not as simple as mutation X leads to airborne transmission; there are many barriers to overcome in order for airborne transmission to dominate.

Media and science perceptions

There is no precedent for any virus to drastically change its mode of transmission, and Ebola is unlikely to be the first. However, movies and books can perpetuate the unjustified fear of airborne transmission for millions of people who have been exposed to them. In times of viral outbreak, it is best to listen to the experts who dedicate their lives to pursuing the science behind these issues. We should be mindful of possibilities and should certainly attack Ebola head on, but we should not worry ourselves with highly improbable fears.

Joseph Timpona is a third year graduate student in the Department of Virology at Harvard Medical School.

References

[1] http://microbiology.mtsinai.on.ca/faq/transmission.shtml#four

[2] http://www.cdc.gov/vhf/ebola/pdf/infections-spread-by-air-or-droplets.pdf

[3] http://www.cdc.gov/vhf/ebola/transmission/human-transmission.html

[4] Jaax, N., et al., 1995. Transmission of Ebola virus (Zaire strain) to uninfected control monkeys in a biocontainment laboratory. Lancet 346: 1669-1671.

[5] http://www.nytimes.com/2014/10/20/books/the-hot-zone-author-tracks-ebolas-evolution.html?_r=1

[6] http://articles.latimes.com/1995-03-13/entertainment/ca-42260_1_weekend-box

[7] http://www.rogerebert.com/reviews/outbreak-1995#disqus_thread

[8] http://www.scientificamerican.com/article/fact-or-fiction-the-ebola-virus-will-go-airborne/