Legionella on ice

"Legionnaires' disease" was the name originally given to an illness observed at a 1976 American Legion convention. Today, we call the illness associated with Legionella pneumophila infection, which can range from mild respiratory illness to severe pneumonia, "legionellosis". Legionella bacteria exist naturally in water and moist soil and colonies tend to grow in warm water, pools of which often form in improperly operated or maintained HVAC systems, hot tubs, and hot water systems. Legionella is an important cause of both hospital- and community-acquired pneumonia in both immunocompetent and immunosuppressed patients. Hospital-acquired cases are often associated with potable water systems colonised with Legionella.

A story in the Pittsburgh Tribune-Review recently described an unusual outbreak at a Pittsburgh hospital. Although the epidemiology showed an association with ice chips, investigators were unable to find Legionella in the hospital water system. How could Legionella appear in ice from machines supplied by cold (not hot) water lines free from the bacterium? It was ultimately determined reservoirs within hospital ice machines were warmed by internal compressors, thus allowing Legionella colonies to grow.

Previous outbreaks involving ice makers and Legionella have been described in the literature (see, e.g., Schuetz et al, 2009, Graman et al, 1997, and Stout et al, 1985), but I doubt that many would immediately respond "ice machine" when asked about likely sources of Legionella infection in a hospital. Though anecdotal, this story illustrates how counter-intuitive outbreak investigation can be: One wouldn't necessarily think to look in a freezer for a bug that needs warm water to grow. But there it was, and hospital investigators figured it out when the ice chips were implicated. Bravo! 

(image source: Wikipedia)

Infection prevention knowledge: Permutations on "known" and "unknown"

Pathogens are everywhere, and we are increasingly aware of how widely they can be disseminated in healthcare environments. Researchers have found, for example, that

• Well-child visits are a risk factor for subsequent influenza-like illness visits. Infections are thought to spread in waiting and exam rooms.

• Hospital water taps can be contaminated with bacteria including Legionella spp., Acinetobacter spp. and other Gram-negatives. 

• Hospital surfaces and equipment are sometimes contaminated with nosocomial pathogens including Clostridium difficile, VRE, and MRSA.

• Doctors' ties and white coats are anything but sterile.

• Toilet seats can harbor MRSA and probably other pathogens. 

• Hospital privacy curtains can be contaminated with pathogenic bacteria including MRSA and VRE.

• Stethoscopes can carry MSSA and MRSA, among others. 

The list could go on and on. We know that many pathogens can persist on surfaces for a considerable time, and while it's not clear the extent to which contaminated surfaces play a role in HAI in general, there is reason to believe they are important in many infections. The unfortunate reality is that patients suffer nosocomial infection; contaminated surfaces can only add to the risk of infection.

I never thought I'd invoke Donald Rumsfeld in a discussion of infection control, but he once described a useful construct for thinking about infection prevention (among other things). He's quoted as saying

. . . there are things we know that we know. There are known unknowns; that is to say, there are things that we now know we don't know. But there are also unknown unknowns – there are things we do not know we don't know.

If we add another, obvious, category -- things we don't know that we know -- then a 2x2 table can be written for types of knowledge. Done for infection prevention and control, it might look like the table below.

It would be interesting to organize what we know and don't know into such a table. That's a big thing to do; it requires assessing what specific practices are truly evidence-based ("known knowns", like have been described for central line infections and ventilator-associated pneumonia), identifying best practices that aren't necessarily well studied ("unknown knowns"), and enumerating gaps in our knowledge ("known unknowns", such as the role of contaminated surfaces in HAI). Of course, we can never identify the things we don't realize that we don't know (the "unknown unknowns"), but the hope would be to ultimately understand the other three quadrants well enough so that we are sure that the unknown unknowns aren't important. Obviously, that's hard to do.  

It seems to me that seriously trying to fill in the quadrants is an important step towards a complete theoretical picture of infection. Probably the "known knowns" quadrant is smaller than we would hope, and the "known unknowns" quadrant is significant. I wonder how large the "unknown knows" category -- the things we don't realize we know -- is?

(image source: the E. coli micrograph, Wikipedia; the 2x2 table, David Hartley)