Like most other infectious disease physicians and microbiologists, I have been following the evolving global Mycobacterium chimaera outbreak with interest. I knew nothing about M. chimaera (or that it even existed) until last year. It is an interesting organism – a slow-growing non-tuberculous mycobacteria (NTM – a “catchall classification” that includes all mycobacteria other than those belonging to the Mycobacterium tuberculosis complex) that was originally classified as one serovar of Mycobacterium intracellulaire within the Mycobacterium avium complex (MAC) before being “elevated” to species level in 2004 by researchers from Italy. Presumably, the blogger from SGH Department of Microbiology will describe in depth how to isolate and identify M. chimaera at some point.

Another 4 years elapsed without further relevant publications in the medical literature until in 2008, German researchers showed that the majority of respiratory M. intracellulaire isolates were actually M. chimaera, and only 3.3% were truly associated with clinical infection. This implied that the organism had low pathogenic potential, and also that most clinical microbiology laboratories around this period were unable or unwilling to identify the organism to species level separately from MAC. There followed sporadic case reports, with the exception of an interesting study in Texas that recruited patients with MAC lung infection and then sampled their household taps/showerheads/water sources for MAC. It turned out that the majority of MAC causing infections were M. intracellulaire, whereas the majority of MAC found in their households were M. chimaera.

Then in 2015, investigators from Zurich, Switzerland reported 6 cases of M. chimaera disseminated infection occurring between 2008 and 2012 in patients who had undergone complex heart/vascular surgery. The incubation period between surgery and manifestation of infection ranged from 1.5 to 3.6 years. There were 2 deaths, and M. chimaera were subsequently cultured from 5 heater-cooler units from that hospital. The brand of heater-cooler unit used – Stockert 3T from LivaNova PLC (previously Sorin Group Deutschland GmbH) is widely used globally (even in Singapore), and soon, more reports of M. chimaera infections post-cardiac surgery started to flow in especially from USA and Europe.

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A Stockert 3T heater-cooler unit from LivaNova (obtained from Google Images).

It turns out that it is extremely difficult to disinfect these units from M. chimaera colonisation. Because  the water from heater-cooler units are not directly pumped into patients, the current understanding is that heavy contamination of these units results in aerosolisation of M. chimaera during surgery, with subsequent infection. The current fear, understandably, is that because of the extensive global use of these machines (the majority of units tested appear to be contaminated) and the long lag time to infection, we will be seeing more and more infections appearing over the next few years. Without high clinical suspicion, M. chimaera infection is difficult to diagnose. Who would routinely send off mycobacterial cultures from blood or tissue samples in post-cardiac surgical infections?

Current recommendations for prevention – given the extreme difficulty in decontaminating the heater-cooler units – is to separate them from the airflow of operating theatres. The optimum treatment regimen and duration of treatment is not known, given the paucity of cases diagnosed until 2015, but the isolates have so far been reported as being susceptible to standard anti-NTM drugs including clarithromycin, rifampicin, ethambutol, fluoroquinolones and aminoglycosides. A variety of antibiotic cocktails have been employed with relatively good results.

To my knowledge, there are no reported cases of post-cardiac surgery M. chimaera infections in Singapore as of now.

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