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Response to kestrel91316 (Reply #60)

Thu Oct 16, 2014, 06:57 PM

73. Your model & terminology are overly simplistic & out of date.

Here's the "better background" you requested.

http://www.cidrap.umn.edu/news-perspective/2014/09/commentary-health-workers-need-optimal-respiratory-protection-ebola
How are infectious diseases transmitted via aerosols?

Medical and infection control professionals have relied for years on a paradigm for aerosol transmission of infectious diseases based on very outmoded research and an overly simplistic interpretation of the data. In the 1940s and 50s, William F. Wells and other "aerobiologists" employed now significantly out-of-date sampling methods (eg, settling plates) and very blunt analytic approaches (eg, cell culturing) to understand the movement of bacterial aerosols in healthcare and other settings. Their work, though groundbreaking at the time, provides a very incomplete picture.

Early aerobiologists were not able to measure small particles near an infectious person and thus assumed such particles existed only far from the source. They concluded that organisms capable of aerosol transmission (termed "airborne" can only do so at around 3 feet or more from the source. Because they thought that only larger particles would be present near the source, they believed people would be exposed only via large "droplets" on their face, eyes, or nose.

Modern research, using more sensitive instruments and analytic methods, has shown that aerosols emitted from the respiratory tract contain a wide distribution of particle sizes—including many that are small enough to be inhaled.5,6 Thus, both small and large particles will be present near an infectious person.

The chance of large droplets reaching the facial mucous membranes is quite small, as the nasal openings are small and shielded by their external and internal structure. Although close contact may permit large-droplet exposure, it also maximizes the possibility of aerosol inhalation.

As noted by early aerobiologists, liquid in a spray aerosol, such as that generated during coughing or sneezing, will quickly evaporate, which increases the concentration of small particles in the aerosol. Because evaporation occurs in milliseconds, many of these particles are likely to be found near the infectious person.

The current paradigm also assumes that only "small" particles (less than 5 micrometers [mcm]) can be inhaled and deposited in the respiratory tract. This is not true. Particles as large as 100 mcm (and perhaps even larger) can be inhaled into the mouth and nose. Larger particles are deposited in the nasal passages, pharynx, and upper regions of the lungs, while smaller particles are more likely to deposit in the lower, alveolar regions. And for many pathogens, infection is possible regardless of the particle size or deposition site.

It's time to abandon the old paradigm of three mutually exclusive transmission routes for a new one that considers the full range of particle sizes both near and far from a source. In addition, we need to factor in other important features of infectivity, such as the ability of a pathogen to remain viable in air at room temperature and humidity and the likelihood that systemic disease can result from deposition of infectious particles in the respiratory system or their transfer to the gastrointestinal tract.

]We recommend using "aerosol transmissible" rather than the outmoded terms "droplet" or "airborne" to describe pathogens that can transmit disease via infectious particles suspended in air.

Altogether, these epidemiologic and experimental data offer enough evidence to suggest that Ebola and other filoviruses may be opportunistic with respect to aerosol transmission.28 That is, other routes of entry may be more important and probable, but, given the right conditions, it is possible that transmission could also occur via aerosols.

Guidance from the CDC and WHO recommends the use of facemasks for healthcare workers providing routine care to patients with Ebola virus disease and respirators when aerosol-generating procedures are performed. (Interestingly, the 1998 WHO and CDC infection-control guidance for viral hemorrhagic fevers in Africa, still available on the CDC Web site, recommends the use of respirators.)

Facemasks, however, do not offer protection against inhalation of small infectious aerosols, because they lack adequate filters and do not fit tightly against the face.1 Therefore, a higher level of protection is necessary.

To summarize, for the following reasons we believe that Ebola could be an opportunistic aerosol-transmissible disease requiring adequate respiratory protection:

Patients and procedures generate aerosols, and Ebola virus remains viable in aerosols for up to 90 minutes.
All sizes of aerosol particles are easily inhaled both near to and far from the patient.
Crowding, limited air exchange, and close interactions with patients all contribute to the probability that healthcare workers will be exposed to high concentrations of very toxic infectious aerosols.
Ebola targets immune response cells found in all epithelial tissues, including in the respiratory and gastrointestinal system.
Experimental data support aerosols as a mode of disease transmission in non-human primates.

Risk level and working conditions suggest that a PAPR will be more protective, cost-effective, and comfortable than an N95 filtering facepiece respirator.

Acknowledgements

We thank Kathleen Harriman, PhD, MPH, RN, Chief, Vaccine Preventable Diseases Epidemiology Section, Immunization Branch, California Department of Public Health, and Nicole Vars McCullough, PhD, CIH, Manager, Global Technical Services, Personal Safety Division, 3M Company, for their input and review.

References

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ECDC. Outbreak of Ebola virus disease in West Africa: third update, 1 August 2014. Stockholm: ECDC 2014 Aug 1 [Full text]
Martin-Moreno JM, Llinas G, Hernandez JM. Is respiratory protection appropriate in the Ebola response? Lancet 2014 Sep 6;384(9946):856 [Full text]
Papineni RS, Rosenthal FS. The size distribution of droplets in the exhaled breath of healthy human subjects. J Aerosol Med 1997;10(2):105-16 [Abstract]
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Francesconi P, Yoti Z, Declich S, et al. Ebola hemorrhagic fever transmission and risk factors of contacts, Uganda. Emerg Infect Dis 2003 Nov;9(11):1430-7 [Full text]
Dowell SF, Mukunu R, Ksiazek TG, et al. Transmission of Ebola hemorrhagic fever: a study of risk factors in family members, Kikwit, Democratic Republic of Congo, 1995. J Infect Dis 1999 Feb;179:S87-91 [Full text]
Roels TH, Bloom AS, Buffington J, et al. Ebola hemorrhagic fever, Kikwit, Democratic Republic of the Congo, 1995: risk factors for patients without a reported exposure. J Infect Dis 1999 Feb;179:S92-7 [Full text]
Kuhl A, Hoffmann M, Muller MA, et al. Comparative analysis of Ebola virus glycoprotein interactions with human and bat cells. J Infect Dis 2011 Nov;204:S840-9 [Full text]
Hunt CL, Lennemann NJ, Maury W. Filovirus entry: a novelty in the viral fusion world. Viruses 2012 Feb;4(2):258-75 [Full text]
Bray M, Geisbert TW. Ebola virus: the role of macrophages and dendritic cells in the pathogenesis of Ebola hemorrhagic fever. Int J Biochem Cell Biol 2005 Aug;37(8):1560-6 [Full text]
Mohamadzadeh M, Chen L, Schmaljohn AL. How Ebola and Marburg viruses battle the immune system. Nat Rev Immunol 2007 Jul;7(7):556-67 [Abstract]
Lindsley WG, Blachere FM, Thewlis RE, et al. Measurements of airborne influenza virus in aerosol particles from human coughs. PLoS One 2010 Nov 30;5(11):e15100 [Full text]
Caul EO. Small round structured viruses: airborne transmission and hospital control. Lancet 1994 May 21;343(8908):1240-2 [Full text]
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Best EL, Snadoe JA, Wilcox MH. Potential for aerosolization of Clostridium difficile after flushing toilets: the role of toilet lids in reducing environmental contamination. J Hosp Infect 2012 Jan;80(1):1-5 [Full text]
Gerba CP, Wallis C, Melnick JL. Microbiological hazards of household toilets: droplet production and the fate of residual organisms. Appl Microbiol 1975 Aug;30(2):229-37 [Full text]
Barker J, Jones MV. The potential spread of infection caused by aerosol contamination of surfaces after flushing a domestic toilet. J Appl Microbiol 2005;99(2):339-47 [Full text]
Piercy TJ, Smither SJ, Steward JA, et al. The survival of filoviruses in liquids, on solid substrates and in a dynamic aerosol. J Appl Microbiol 2010 Nov;109(5):1531-9 [Full text]
Jaax N, Jahrling P, Geisbert T, et al. Transmission of Ebola virus (Zaire strain) to uninfected control monkeys in a biocontainment laboratory. Lancet 1995 Dec 23-30;346(8991-2):1669-71 [Abstract]
Kobinger GP, Leung A, Neufeld J, et al. Replication, pathogenicity, shedding and transmission of Zaire ebolavirus in pigs. J Infect Dis 2011 Jul 15;204(2):200-8 [Full text]
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