Mycobacterium chimaera and heater-cooler units: a briefing

This post provides a briefing on the current status of the Mycobacterium chimaera issue related to heater-cooler units (HCUs) used in cardiothoracic surgery. You can download some accompanying slides here. Also, I found this recent ICHE review on the subject very helpful to bring me up to speed.

I did a quick lit review to underpin a talk that I gave this week on the M. chimaera and heater-cooler unit (HCU) issue, so thought I’d share the headline findings. It’s a bit bullet pointy, but I hope that this is useful by way of briefing! This came from a simple pubmed search for ‘mycobacterium chimaera’, which returned only around 100 hits or so.

Initial detection

  • Two cases of unusual infection caused by Mycobacterium chimaera following recent prosthetic valve replacement surgery identified in Zurich, Switzerland in 2011.1, 2
    • Isolates linked by genotyping; M. chimaera usually diverse.
    • Similar isolates of M. chimaera later found in water and air from HCUs.

Seeking the source

  • Some evidence suggests a point-source outbreak in the factory of the most common manufacturer of HCUs.
    • HCUs in Denmark, the US, and the UK shared the same genotype of chimaera, and all contaminated HCUs in Western Australia shared the same genotype of M. chimaera.3, 4
    • Environmental investigations at a factory suggest manufacturer-site contamination.5
  • Other evidence suggests a non-HCU related source of M. chimaera infections in CT surgery.4

Confirming the link with HCUs

  • Investigation of the HCU structure in detail has identified a wholly inappropriate design (see what I did there).6
  • Key studies have found M. chimaera, other NTMs, and other organisms of concern contaminating the water in HCUs.6, 7
  • Smoke testing and active air sampling has demonstrated that these organisms readily make their way to the operating table, even in the presence of laminar flow.7, 8
    • You can watch a youtube vid of a smoke test here, showing the smoke creeping rapidly towards the surgical field!8

Clinical presentation1

  • M. chimaera is a slow-growing, nontuberculous mycobacterium (NTM) included in the M. avium complex (MAC).
  • Unusual clinical presentation (usually infection in immunocompromised hosts).
  • Prosthetic valve endocarditis, disseminated infections, or infections of vascular grafts (less commonly wound or surgical space SSIs).
  • Non-specific symptoms and lab abnormalities.
  • Delayed diagnosis (3 months – 5 years).
  • Poor response to therapy; 50% mortality rate.

UK situation

  • See my recent post on this study for more detail, but in summary:
    • 4-year national look-back exercise to detect potential cases.
    • 18 probable cases; 54 possible cases.
    • Very rare outcome from cardiothoracic surgery (around 1 per 10,000 person-years).
    • Widespread problem, with a quarter of cardiothoracic centres in the UK reporting a case.
    • High case-fatality (50%).
    • Increasing risk (from <0.2 to 1.6 per 10,000 person-years between 2010 and 2013).
    • Large to huge time lag between the surgery and the detection of the infection: of the 18 probable cases identified, more than half (61%) presented >1 year after surgery, with one case presenting >5 years after surgery!
    • For every 10,000 CT procedures involving HCUs, there will be 400 cases of endocarditis, 120 SSIs, and just 1 M. chimaera infection.

Mitigating / managing the risk

  • Useful guidance from PHE here (see figure below):
    • Physical location / encasing HCUs
    • Water and air testing
    • Enhanced decon (water change, filters, chemicals, deep clean)9, 10
    • Enhanced surveillance
    • Communication with patients and staff
    • Risk assessment (aka ‘the show must go on’)
    • Traceability of each unit


Figure: Summary of recommended steps to mitigate / manage the risk from contaminated HCUs.

Other challenges

  • Uncertain epidemiology.
  • Chronic under-reporting.
    • Belgian study found that 65% of 149 “M. intracellulare” were actually M. chimaera.11
  • M. chimaera has also been identified in thermoregulatory devices are used for extracorporeal membrane oxygenation (ECMO).12
  • Notification of exposed patients (local patient notification exercises have been done in the US).13
  • Consent for yet-to-be exposed patients.
  • What else is lurking in the HCUs (e.g. Legionella, other bacteria, fungi)?

Summary / key questions

  • Is this the surgical outcome to focus on?
  • Are the HCUs fit for purpose?
  • What do we do about it?
  • Is this the tip of the iceberg?
  • Why is it getting worse?



[1]        Sax H, Bloemberg G, Hasse B et al. Prolonged Outbreak of Mycobacterium chimaera Infection After Open-Chest Heart Surgery. Clin Infect Dis 2015; 61: 67-75.

[2]        Achermann Y, Rossle M, Hoffmann M et al. Prosthetic valve endocarditis and bloodstream infection due to Mycobacterium chimaera. J Clin Microbiol 2013; 51: 1769-73.

[3]        Svensson E, Jensen ET, Rasmussen EM, Folkvardsen DB, Norman A, Lillebaek T. Mycobacterium chimaera in Heater-Cooler Units in Denmark Related to Isolates from the United States and United Kingdom. Emerg Infect Dis 2017; 23.

[4]        Robinson JO, Coombs GW, Speers DJ et al. Mycobacterium chimaera colonisation of heater-cooler units (HCU) in Western Australia, 2015: investigation of possible iatrogenic infection using whole genome sequencing. Euro surveillance 2016; 21.

[5]        Haller S, Holler C, Jacobshagen A et al. Contamination during production of heater-cooler units by Mycobacterium chimaera potential cause for invasive cardiovascular infections: results of an outbreak investigation in Germany, April 2015 to February 2016. Euro surveillance 2016; 21.

[6]        Chand M, Lamagni T, Kranzer K et al. Insidious Risk of Severe Mycobacterium chimaera Infection in Cardiac Surgery Patients. Clin Infect Dis 64: 335-42.

[7]        Gotting T, Klassen S, Jonas D et al. Heater-cooler units: contamination of crucial devices in cardiothoracic surgery. J Hosp Infect 2016; 93: 223-8.

[8]        Sommerstein R, Ruegg C, Kohler P, Bloemberg G, Kuster SP, Sax H. Transmission of Mycobacterium chimaera from Heater-Cooler Units during Cardiac Surgery despite an Ultraclean Air Ventilation System. Emerg Infect Dis 2016; 22: 1008-13.

[9]        Garvey MI, Ashford R, Bradley CW et al. Decontamination of heater-cooler units associated with contamination by atypical mycobacteria. J Hosp Infect 2016; 93: 229-34.

[10]      Schreiber PW, Kuster SP, Hasse B et al. Reemergence of Mycobacterium chimaera in Heater-Cooler Units despite Intensified Cleaning and Disinfection Protocol. Emerg Infect Dis 2016; 22: 1830-3.

[11]      Soetaert K, Vluggen C, Andre E, Vanhoof R, Vanfleteren B, Mathys V. Frequency of Mycobacterium chimaera among Belgian patients, 2015. J Med Microbiol 2016; 65: 1307-10.

[12]      Trudzinski FC, Schlotthauer U, Kamp A et al. Clinical implications of Mycobacterium chimaera detection in thermoregulatory devices used for extracorporeal membrane oxygenation (ECMO), Germany, 2015 to 2016. Euro surveillance 2016; 21.

[13]      Perkins KM, Lawsin A, Hasan NA et al. Notes from the Field: Mycobacterium chimaera Contamination of Heater-Cooler Devices Used in Cardiac Surgery – United States. MMWR 2016; 65: 1117-8.


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