|Re: [stem-ebola] WHO seems to acknowledge (rare) possibility of Ebola 'large droplet' transmission|
Hitchhiking on dust particles is probably a bigger threat into the forseeable future. A spatter of contaminated fecal material or body fluid dries on a dusty surface, flakes off into an air current and then?
> Date: Wed, 15 Oct 2014 22:34:50 -0600
> From: alex@xxxxxxxxxxxxxx
> To: stem-ebola@xxxxxxxxxxx
> Subject: [stem-ebola] WHO seems to acknowledge (rare) possibility of Ebola 'large droplet' transmission
> 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: http://www.who.int/mediacentre/news/ebola/06-october-2014/en/
> “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)
> OPERON LABS COMMENTS:
> 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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