This week’s blog is 6th in a series by Dr Evonne T Curran (@EvonneTCurran).
The pre-pandemic Infection Prevention and Control (IPC) model for respiratory transmission (droplets or aerosols) with droplets being the dominant mode is un-evidenced. Although this has yet to be admitted by some guidance writers – the evidence for airborne infection is overwhelming. Much of what I have been trying to do is persuade people that most respiratory infections including SARS-CoV-2 are airborne. I naively thought that if the new paradigm was accepted (everything ≤100μ being inhalable), this would naturally lead to the correction of control measures and the introduction of indoor air quality and respirators. This is yet to happen.
However, the further I investigated the existing 3 Modes of Nosocomial Transmission (MoNT), and the 3 corresponding precautions (airborne, droplet, contact), the more I realised that these modes of transmission omit another MoNT altogether – Airborne Dissemination. Having been ignored when demonstrating that the droplet or airborne paradigm needs amending, perhaps this argument – you have omitted a 4th MoNT altogether – might yield better results.
When I started nursing, emphasis was placed on avoiding scattering pathogens on used linen into the care environment. This is because the earliest guidelines recognised from experiments what was called ‘dust-borne’ bacteria (Harries et al 1944). Pathogens sit on skin scales like a genie on a magic carpet and scatter their payload like dandelion seeds in the wind. Thus, nurses were instructed not to shake the linen, to place it directly into linen skips and not to use brooms to sweep the floors. These ubiquitous airborne disseminated bacteria generally do not cause lung disease – so current airborne infection precautions won’t stop transmission. The skin scales are not in fluid, so droplet precautions won’t stop transmission. Finally, contact precautions (hand hygiene and environmental hygiene) can only prevent transmission via touch. Therefore, there are no current IPC precautions which can stop these skin scales travelling via the air and landing on people. The first question is whether this dissemination of pathogens via the air is significant. Well all humans shed about a million skin scales per day on about 10% of which sit pathogens like staphylococci. Not everyone sheds staphylococci; but, when there are infected wounds patients can shed vast viable amounts into the air. Although there are no studies (I can find) that show the relative importance any MoNT, there is sufficient evidence presented here to confirm airborne dissemination is an important issue. Thus, the case for a 4th MoNT begins…
Let us start with a (proposed) definition:
Airborne dissemination is the dissemination of pathogens from human and environmental sources via the air, before further human contact, for which hand/environmental hygiene MAY NOT prevent transmission.
This definition must however be evidenced. To be clear airborne dissemination was an accepted phenomenon, Williams et al 1960 state:
“And all the time, in most wards, staphylococci are present in the air, perhaps in sufficient numbers to establish themselves in noses, wounds, lungs or other sites.”
Sources of surgical site infections were often identified as pathogens disseminated from people in the theatre (Williams et al 1960 p73-83).
One recent study examining dry microbial airborne levels over 24 hours in ICU, found a direct correlation between microbial load and ward activity. Yet it’s more than staphylococci on skin scales. Investigations of scarlet fever have shown plausible airborne dissemination. [During one outbreak, I placed an agar plate on a clean towel on a laundry shelf which yielded S. pyogenes – nothing other than airborne dissemination could have put it there]. It is also plausible that transmission to 24 HCWs, in one nosocomial ICU Streptococcus pyogenes outbreak, resulted from something other than a 100% failure of hand hygiene or contaminated instruments. The term ‘cloud healthcare workers’ was used to describe how some, uninfected but colonised health care workers (HCWs), disseminated vast quantities of pathogen via the air resulting in outbreaks.
Vomiting due to norovirus can be spread 3m from the source. The virus has been found in the air and airborne transmission is now considered an important route. Of note, airborne disseminated pathogens must not just land on people but find an entry point to infect. Environmental sources of pathogens which are disseminated via the air include dust carrying pathogens from construction sites and fungi liberated from moldy walls and ceiling tiles, that present risks especially to immune compromised patients.
In recent years wet sites, e.g., underused taps, drains in showers and sinks have been recognised as a problem (and are elegantly reported here). One newly accepted control measure is the removal of water sources. What is important to note here is this: the hand hygiene stations deployed to reduce infection risk – presented infection risks via splash dispersal through the air. Also, it is now evidenced that flushing toilets disseminates faecal aerosols which remain airborne. More recently investigators demonstrated that aerosols rapidly migrated between the various ward sub-compartments and air cleaning rapidly reduced them. It is possible that others may find a solution before IPC detects the problem.
To abridge the issue simply: if it sprays, splashes or floats, it can disseminate pathogens into the air. And, when we do what we do on the wards, we are literally shaking them all about.
The schematic below illustrates the issue.
So, what now?
If we don’t recognise all sources of nosocomial transmission, we will never identify nor implement what is needed to prevent them. This is an IPC alarm call. The 3 existing MoNT fail to include that which can be seen in every shaft of sunlight, airborne particles carrying a potentially pathogenic payload travelling through the air. This blog has not considered the cause of this omission. However, the focus on the as yet evidence-lacking hypothesis of hand contact being the primary transmission mode to prevent infection should now be reassessed. Our current IPC research gaps on transmission are clearly a chasm wide.
What is urgently needed is re-recognition of Airborne Dissemination as the 4th MoNT and studies on the relative importance of each in the dissemination of pathogens in the care setting environment to prevent both transmission and infection. The goal for IPC is to keep people safe – when the environment itself maybe unsafe. Our current MoNTs leave us unable to identify all risks and therefore unable to prevent transmission via airborne dissemination.
So please, can we consider the IPCMs again; and this time include aerosol scientists and acknowledge airborne transmission and airborne dissemination as a MoNT in care environments.
Cross-transmission is the transfer of a pathogen from one person to another via a variety of possible routes. Infection is by no means ubiquitous after cross-transmission. However, in clinical setting with vulnerable people, with invasive devices and with wounds, the opportunity for cross-transmission to become cross-infection (or healthcare acquired infection) is vast.
Harries EHR, Allison VD, Bradley DM, Crosbie WE, Cruickshank R, Dobbs RH, Frazer WM, McLaggan JDPBA, Taylor D, FE Vine, Wright J, Chalmers DKM. (1944) The control of cross infection in hospitals. Medical Research Council War Memorandum No. 11. His Majesty’s Stationery Office: London.
Williams, RFO, Blowers R, Garrod LP & Shooter RA. Hospital Infection causes and prevention. Lloyd–Luke. London. 1960.
One thought on “The 4th Mode of Nosocomial Transmission – Airborne Dissemination”
As always, a fascinating read.
The way Evonne describes it, it seems so obvious.
Yet so often the ‘obvious’ is overlooked (hidden in plain sight).
It takes an insightful person to make us aware of it.