Ebolavirus will not become a respiratory pathogen

sneezeAn otherwise balanced review of selected aspects of Ebolavirus transmission falls apart when the authors hypothesize that ‘Ebola viruses have the potential to be respiratory pathogens with primary respiratory spread.’

The idea that Ebolavirus might become transmitted by the respiratory route was suggested last year by Michael Osterholm in a Times OpEd. That idea was widely criticized by many virologists, including this writer.  Now he has recruited 20 other authors, including Ebola virologists, in an attempt to lend legitimacy to his hypothesis. Unfortunately the new article adds no new evidence to support this view.

In the last section of the review article the authors admit that they have no evidence for respiratory transmission of Ebolavirus:

It is very likely that at least some degree of Ebola virus transmission currently occurs via infectious aerosols generated from the gastrointestinal tract, the respiratory tract, or medical procedures, although this has been difficult to definitively demonstrate or rule out, since those exposed to infectious aerosols also are most likely to be in close proximity to and in direct contact with an infected case.

It is possible that some short-distance transmission of Ebolavirus occurs through the air. But claiming that it is ‘very likely’ to be taking place is an overstatement considering the lack of evidence. As might be expected, ‘very likely’ is exactly the phrase picked up by the Washington Post.

I find the lack of critical thinking in the following paragraph even more disturbing:

To date, investigators have not identified respiratory spread (either via large droplets or small-particle aerosols) of Ebola viruses among 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. Despite the lack of supportive epidemiological data, a key additional question to ask is whether primary pulmonary infections and respiratory transmission of Ebola viruses could be a potential scenario for the future.

Why is the possibility of respiratory transmission of Ebolaviruses a ‘key additional question’ when there has been no evidence for it to date? To make matters worse, the authors have now moved from short-range transmission of the virus by droplets, to full-blown respiratory aerosol transmission.

The authors present a list of reasons why they think Ebolavirus could go airborne, including: isolation of Ebolaviruses from saliva; presence of viral particles in pulmonary alveoli on human autopsies; and cough, which can generate aerosols, can be a symptom of Ebolavirus disease. The authors conclude that because of these properties, the virus would not have to change very much to be transmitted by aerosols.

I would conclude the opposite from this list of what Ebolavirus can do: there is clearly a substantial block to respiratory transmission that the virus cannot overcome. Perhaps the virus is not stable enough in respiratory aerosols, or there are not enough infectious viruses in aerosols to transmit infection from human to human. Overcoming these blocks might simply not be biologically possible for Ebolavirus. A thoughtful discussion of these issues is glaringly absent in the review.

The conclusion that Ebolavirus is  ‘close’ to becoming a full-blown respiratory pathogen reveals how little we understand about the genetic requirements for virus transmission. In fact the authors cannot have any idea how ‘close’ Ebolavirus is to spreading long distances through the air.

It is always difficult to predict what viruses will or will not do. Instead, virologists observe what viruses have done in the past, and use that information to guide their thinking. If we ask the simple question, 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. There is no evidence to believe that Ebolavirus is any different.

Viruses are masters of evolution, but apparently one item lacking from their repertoire is the ability to change the way that they are transmitted.

Such unfounded speculation would largely be ignored if the paper were read only by microbiologists. But Ebolavirus is always news and even speculation does not go unnoticed. The Washington Post seems to think that this review article is a big deal. Here is their headline: Limited airborne transmission of Ebola is ‘very likely’ new analysis says.

Gary Kobinger, one of the authors, told the Washington Post that ‘we hope that this review will stimulate interest and motivate more support and more scientists to join in and help address gaps in our knowledge on transmission of Ebola’. Such hope is unrealistic, because few can work on this virus, which requires the highest levels of biological containment, a BSL-4 laboratory.

I wonder if Osterholm endorses Kobinger’s hopes. After all, he opposed studies of influenza virus transmission in ferrets, claiming that they are too dangerous. And the current moratorium on research that would help us understand aerosol transmission of influenza viruses is a direct result of objections by Osterholm and his colleagues about this type of work. The genetic experiments that are clearly needed to understand the limitations of Ebolavirus transmission would never be permitted, at least not with United States research dollars.

The gaps in our understanding of virus transmission are considerable. If virologists are not able to carry out the necessary experiments to fill these gaps, all we will have is rampant and unproductive speculation.

Comments on this entry are closed.

  • Steve Rhode 21 February 2015, 11:42 am

    Well stated – thanks!

  • Katie Kearns 21 February 2015, 1:36 pm

    I couldn’t understand all the crazy Osterholm was spouting last year, but I’m not a virologist. Glad to hear my instincts were “right”. Why is he using his position to panic the heck out of everyone?? I don’t understand why…

  • Jay Cherwa 21 February 2015, 6:09 pm

    In teaching my freshman/sophomore, college level biology courses in rural Alabama, much of the students’ knowledge of Ebola comes from mainstream media sources. As a trained virologist (bacteriophage), and upon recognizing their ignorance on the topic, I assigned them to write research papers on the virus, which helped them not only understand the virus better, but also allowed them to apply the knowledge gained in our virus lectures to current events. Their interest became so great that we organized a fundraiser to assist with the purchase of supplies for the healthcare workers on the front lines in Liberia. It is most satisfying to provide a broader aspect of an education, which I believe is to become empathetic and well informed citizens. Whether or not they learn all of the biology is less of a concern knowing they are being taught to better critically read and think, rather than rely strictly upon the media. Thank you for all of your work!

  • chris 21 February 2015, 8:58 pm

    Excellent. Convincing the government to shut down actual research on viral transmission will allow people to speculate as wildly as they want in the media, confident that no one will be able to actually test anything.

  • Fernando Pereira 22 February 2015, 11:27 pm

    As Brad DeLong is fond of saying with respect to scare-mongering in economics and politics: “grifters gotta grift.”

  • Chris 23 February 2015, 10:55 am

    because he can and because most people are not knowledgeable. very few read books on the topic, while most digest the mainstream media…

  • dcpattie 23 February 2015, 2:42 pm

    I think we need to revisit the definitions for both droplet transmission
    and airborne transmission. Airborne transmission refers to situations
    where droplet nuclei (residue from evaporated droplets) or dust particles containing microorganisms can remain suspended in air for long periods of time.

    I think the WashPost article got the title wrong, it should read Limited Droplet Transmission…

  • Joey Romanus 24 February 2015, 3:22 am

    Vincent Racaniello (author) says, // “If we ask the simple question, 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. There is no evidence to believe that Ebolavirus is any different. Viruses are masters of evolution, but apparently one item lacking from their repertoire is the ability to change the way that they are transmitted.” //

    This argument is flawed because it’s based off of the false premise that Ebolavirus (EBOV) has the same stability and reproductive rate under all conditions.

    Transmission efficiency of EBOV varies depending on conditions. According to Center for Aerobiological Sciences, U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick, Maryland:

    (1) Ebola has an aerosol stability that is comparable to Influenza-A

    (2) Much like flu, airborne Ebola transmissions need winter type conditions to maximize aerosol infection

    Hence, EBOV does NOT need to change its mode of transmission because, according to the U.S. Army, it’s already more easily transmissible in colder temperatures because it has the same stability as flu under the same conditions. If this is true, then we can deduce the following: The reason EBOV hasn’t caused a larger outbreak extending into (colder) regions of the world, which have never been afflicted with EBOV epidemics, is because the virus has never been introduced by a human-to-human or animal-to-human vector during colder temperatures. The limited opportunity for EBOV to spread globally is because the natural reservoir for EBOV is the African fruit bat whose habitat is consistently warmer climates.

    The reproductive rate of EBOV is about the same as the flu. If EBOV does have the same airborne stability as flu in colder temperatures, and the former gets introduced into colder regions of the world, then there will be larger epidemics dispersed throughout various regions of the world, and potentially a global pandemic.

    In other words, EBOV does NOT have to “change” (mutation or recombinant) to become more transmissible via air. It might already posses the phenotype to be vectored in colder conditions. It simply just needs to be introduced by an index case in such colder climate(s).






  • David Sanders 24 February 2015, 10:52 am

    “If we ask the simple question, 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.” This argument is simply wrong, and people should stop citing its fallacious reasoning. First, in its native host, influenza A is a gastrointestinal virus. It acquires the ability to be transmitted by a respiratory route. It mutates from a virus that prefers the terminal sugars of bird glycoproteins to one that prefers the different terminal sugars of mammalian glycoproteins with very few substitutions required. Second, a virus need not “change” its mode of transmission. It could have multiple modes of transmission that differ in efficiency. When HIV was discovered to be able to be transmitted from mother to infant was that a “new” mode of transmission or merely a previously unrecognized mode of transmission? Third, is the argument being made that there is something special about humans and human viruses? Are human viruses a special creation? Actually influenza A is a human virus, and it does change its mode of transmission, although not in humans per se. Finally, since when is precedent a valid scientific argument? Until there were prions there were no infectious proteins. Until there were human retroviruses discovered, there were no human retroviruses. The “no precedent” argument should be retired.

  • dave 28 February 2015, 1:27 pm

    A definition of what type of pathogen the Ebola virus is, it’s
    characteristics and how it is pathogenic (its method of pathogenicity)

  • Jim Etchison 3 April 2015, 4:52 pm

    When pigs fly.