I posted a blog a couple of years ago (was it really that long!) on a fascinating study suggesting that only 1/5 of S. aureus in hospital patients is hospital-acquired. My key conclusion from that study was that the number of potential sources for S. aureus that the team investigated was inadequate to draw any firm conclusions (they didn’t include staff, surfaces, or visitors). I concluded that ‘the next frontier of transmission mapping must be a more comprehensive evaluation of other potential sources…’. The authors must have been reading, because this study from the same group was published recently in Lancet ID, which is a more comprehensive evaluation of other potential sources.

The team sampled healthcare workers (once per month), patients (on admission, weekly, and at discharge), and the environment (surfaces and air, monthly) regularly, and used WGS to map potential transmission between these sources. The study was run over 14 months in an ICU / HDU in an English hospital, including around 200 healthcare workers, 40 bed spaces, and 1854 patients. 25 of 97 (around 25%) patient S. aureus acquisitions could be attributed to transmission events from known sources: 16 from other patients, 7 from healthcare workers, and 2 from the environment.

 

Figure: Source of S. aureus acquisitions.

Written informed consent was required for healthcare worker screening, and only 5% declined (which is remarkable by the way). However, I wonder whether any of these healthcare workers declined because they suspected themselves as transmission vehicles?? Nurses, doctors, and physios with direct patient contact were eligible for inclusion. But what about doctors just passing through, visitors, porters, tea tray deliverers etc. These are all very plausible sources of S. aureus transmission.

I was fascinated to see that 8 (4%) of healthcare workers carried MRSA at enrolment, and 10 (5%) carried MRSA at some point during the study. This is higher than I would expect, especially when only 2% of patients carried MRSA on admission to the unit! It was a little disturbing that 2/7 staff to patient S. aureus transmissions were MRSA from the same healthcare worker, over the course of a little over a year. Is this ok? Does this argue for regular screening of healthcare workers for MRSA? Pandora’s box, I know, but better the devil you know (and I am not suggesting for a minute that healthcare workers are demonic in any way).

So, only about a quarter of S. aureus patient acquisitions came from obvious sources. Whilst the team did not (and could not, realistically) cover every source, it does suggest something is going on here. Unmasking of carriage that is not detected by the original surveillance specimen seems the most likely explanation – either that the colonisation is below the limit of detection and unmasked by antibiotics or some other hospital-related exposure, or a poor quality surveillance specimen (aka a waft of a swab).

It is fundamentally difficult to be sure of the direction of travel of S. aureus in this study. If a genetically similar isolate was cultured from a surface after being cultured from a patient, the direction of travel was assumed to be from patient to surface. But, it could have been that the S. aureus was there on the surface all along but just not detected, and the direction of travel was from the surface to the patient. However, I can see no practical way of teasing this out!

Great study (with seriously cool figures by the way), interesting conclusions, and certainly moves the discussion forward on whether or not we should screen healthcare workers to investigate S. aureus transmission.