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

Thanks Ira... The best I can find on the common definition of 'airborne' transmission is via comparison to influenza. Influenza would be characterized by very small droplets that can stay suspended in air. Airborne transmission seems to consist of particles are generally 5 microns or smaller.

In one Influenza study, 89% of the infectious aerosols were under 5 microns.

I think 'Droplet' transmission (which I think WHO is correct that it is not the same as 'airborne') involves particles which are much larger in size , do not travel as far, and do not stay suspended in air. A 100 to 1000 micron particle would be droplet transmission.

With this new information, I think it's fair to say there is no evidence that Ebola is 'airborne' like the flu, but that Ebola can spread short distances via droplets of infected fluid (per WHO).

The critical question is what fluids contain viral PFU and when into the illness do they contain the virus? Another important criteria would be to check viral titers in the all fluids of deceased victims.

Thanks ,

On 2014-10-16 05:03, Ira Schwartz wrote:
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

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:

source: [1]

“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

Droplet Size and Penetration of Respiratory Passages

source: [7]

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Dr. Ira B. Schwartz
Head, Nonlinear Dynamical Systems Section
Code 6792
Naval Research Laboratory
Washington, DC 20375


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