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1. Investigation of Mycobacterium chimaera infection associated with cardiopulmonary bypass
Cases of invasive Mycobacterium chimaera infection in patients who had undergone cardiac surgery have recently been reported in Switzerland and the Netherlands. A Swiss investigation has been published suggesting an association with contaminated heater cooler units used during cardiopulmonary bypass . Detailed investigations are now underway in the UK to assess any historic or ongoing risk to patients.
M. chimaera is a recently identified member of the Mycobacterium avium complex, very similar to M. intracellulare, another member of the complex. These are slow-growing, environmental organisms; in a clinical context they are usually described as causing respiratory infections and disseminated infections in immunocompromised patients. M. chimaera would be identified by most UK laboratories as M. intracellulare or M. avium complex.
Following initial identification of 2 cases of invasive M. chimaera infection in patients who had undergone cardiac surgery at the University Hospital of Zurich, a retrospective study was undertaken. This identified a total of 6 cases of M. chimaera prosthetic valve endocarditis or vascular graft infection associated with cardiac surgery involving cardiopulmonary bypass . The cases underwent surgery between 2008 and 2012.
The intervals between surgery and infection were very prolonged, over 3 years in one instance, reflecting the slow-growing nature of the organism. Given the total number of such surgeries performed in this hospital over the 5 year period (approximately 3000) this suggests a low risk of infection. The outcome for patients infected was however poor with 2 of the 6 cases subsequently dying.
Extensive local investigations, including air and water sampling, attributed the infections to mycobacterial contamination of the water circulating within the heater cooler unit of the cardiopulmonary bypass equipment used in surgery, transmitted by aerosol production.
A specific brand of heater cooler machine was implicated in the Swiss report. This brand of machine is widely used in Europe including the UK. It is not yet known whether any risk is limited to one brand of heater cooler.
As a result, Public Health England (PHE) and the Medicines and Healthcare Products Regulatory Agency (MHRA) have launched a series of investigations to assess the risk in the UK. This includes retrospective case finding, sampling from heater cooler units and enhanced microbiological testing by the National Mycobacterial Reference Laboratory to identify M. chimaera from clinical or environmental samples. Whilst it remains possible that other organisms found in water could be transmitted by the same route, initial investigations will focus on M. avium complex in line with findings from Switzerland.
- Sax H, Bloemberg G, Hasse B, Sommerstein R, Kohler P, Achermann Y et al (2015). Prolonged outbreak of Mycobacterium chimaera infection after open-chest heart surgery. Clin Infect Dis (online, March 11).