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Re: [stem-ebola] WHO seems to acknowledge (rare) possibility of Ebola 'large droplet' transmission

Hi Alex:
Thanks for that one. If it is spread through aerosol, it would probably have a much higher rate of transmission. As you note, evidence is still out on that one. But it is definitely worth watching for.

On Thu, Oct 16, 2014 at 12:34 AM, <alex@xxxxxxxxxxxxxx> wrote:
I understand the possibility of Ebola 'aerosol' transmission is a controversial topic.  Ebola aerosol transmission is not well-studied, and existing experimental evidence is contradictory (some studies show it is possible in animals and primates, wile other studies show it is not possible).

I remain unconvinced either way, at the moment.  I think primarily Ebola is currently spreading via direct contact right now.  But I don't want to make the classic logical fallacy... "Absence of evidence is not evidence of absence"   So I'm going to keep an open mind on this one.

In a WHO advisory email sent Monday 10/06 regarding Ebola, the following (curious) quotes were included:


“Theoretically, wet and bigger droplets from a heavily infected individual, who has respiratory symptoms caused by other conditions or who vomits violently, could transmit the virus – over a short distance – to another nearby person” -WHO

[ED NOTE:  So here WHO admits that 'large droplets' could spread Ebola, but do not give particle sizes.  To me, this suggests that the droplets are of size 100 microns to 1000 microns in diameter. If this is the droplet size, these would be rapidly deposited on the mouth , face, or into the nasopharynx of the second individual.  Particles of this size would be unlikely to remain suspended in air for any substantial period of time.  If this WHO claim is true, Ebola would not be 'airborne' in a traditional understanding, but could transfer without 'direct contact' overt short distances, potentially through coughing, sneezing, or vomiting.  This area needs immediate further research.]

“[Transmission of Ebola] could happen when virus-laden heavy droplets are directly propelled, by coughing or sneezing (which does not mean airborne transmission) onto the mucus membranes [ED NOTE: MOUTH & NASOPHARYNX] or skin with cuts or abrasions of another person.” -WHO

[ED NOTE: This statement is a bit contradictory.  What is the definition for 'airborne' that we will all agree on?  Is it a particle size range that WHO refers to?  Does a particle have to land in the alveoli to become 'airborne'? Also, what is the source for expelled droplet material in this WHO example?  Are we talking about Sputum? Saliva? Mucus?  Do we know human levels of PFU/mL in these tissues during the phases of disease progression?]

“However, the studies implicating these additional bodily fluids were extremely limited in sample size and the science is inconclusive. In studies of saliva, the virus was found most frequently in patients at a severe stage of illness.” -WHO

[ED NOTE: So a low sample size... This means we need further research to either confirm or disconfirm this information.  What about Sputum and Mucus during earlier stages of illness?  When do these fluids start to have viral PFUs present within them in regards to the course of the disease?  What about tissue tropism in the lung in the 2014 outbreak?  Do we see viral titers in human lung?  If so, what levels, in Ebola PFU/g of wet lung tissue?]

“Epidemiological data emerging from the outbreak are not consistent with the pattern of spread seen with airborne viruses, like those that cause measles and chickenpox, or the airborne bacterium that causes tuberculosis” -WHO

[ED Note: Agreed.  We can model this quite well via direct contact.  But this particular issue has way more questions than answers, particularly in regards to 'long-term' strategies over the next 8 to 36 months, as well as what are suitable levels of PPE for healthcare workers.]


USAMRID reference regarding VHF and Filoviruses:

"All of the VHF agents (except for dengue virus) are laboratory infectious hazards by aerosol, even though dengue virus has been nosocomially transmitted by blood splash.  There aerosol infectivity for many VHF agents has been studied and measured in experimental animal models.  VHF agents cause severe disease, and many have extremely high fatality rates."

"In several instances, secondary transmission among contacts and medical personnel without direct body fluid contact exposure has been documented.  These instances prompted concern of a rare phenomenon of aerosol transmission of infection. [...] A negative pressure isolation room is ideal."

-USAMRIID, Medical Management of Biological Casualties Handbook, Seventh Edition (September 2011)


The controversy over 'airborne' transmission is unlikely to end anytime soon.  Part of the problem is we do not have a universally agreed upon transmission of 'airborne'.  (What aerosol particle sizes are we talking about?  What are the levels of virus (PFU/mL) in these droplets?  What biological materials are the droplets comprised of? etc).

The size of an expelled particle determines whether it will be deposited in the nasopharynx, trachae, alveoli, or other surface.  Many studies on aerosol deposition fraction involve drug-delivery rather than infectious viral particles.

First, let's look at the epidemiology of aerosol particle sizes... Take for example sneezing...


Characterizations of particle size distribution of the droplets exhaled by sneeze

Measured data and fitting curves of two sample sneezes:

"For the two peaks of the bimodal distribution, the geometric mean (the geometric standard deviation) is 386.2 µm (1.8) for peak 1 and 72.0 µm (1.5) for peak 2. "  (sneezing)

>From the same study, everyday 'speech' results in the expulsion of aerosol particles of sizes ranges from 10 microns to 1000 microns, with an average of about 100 microns in diameter.

We can thus expect sneezing to result in particle deposition mainly to the upper respiratory tract of another host (mouth, face, eyes, & nasopharynx) during coughing, talking, and sneezing.  The question is not whether infected hosts expel PFU-laden aerosol particles.  *The question is whether these particles contain any viral PFU (Plaque Forming Units)*.  We simply don't know that critical fact here in 2014.

At the present time, airborne transmission doesn't seem to be a significant mode of transmission.  But we should definitely stay aware of this possibility, since it was referenced recently by the WHO.

Droplet Size and Penetration of Respiratory Passages


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Dr. Ira B. Schwartz
Head, Nonlinear Dynamical Systems Section
Code 6792
Naval Research Laboratory
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