Research and analysis

HPR volume 18 issue 4: news (25 April 2024)

Updated 20 December 2024

M. chimaera infection associated with cardiopulmonary bypass: findings from a retrospective case finding exercise

An exercise to identify any unreported cases of Mycobacterium chimaera infection linked to heater cooler units used in open heart surgery has just been completed. The cases stem from the global outbreak associated with heater cooler units (HCUs) used during cardiopulmonary bypass contaminated at the point of manufacture and distributed worldwide (1). Cases were first reported in Switzerland in 2014 and, with the identification of cases in a second country in 2015, many countries including the UK mounted investigations and implemented control measures to reduce patient risk. To date, at least 13 countries worldwide have reported cases (2).

Since the first cases were identified in the UK in 2015, cases have continued to be reported, reflecting the long latency period associated with these infections and the challenges in implementing control measures without disruption to life-saving surgery. Numbers of cases being diagnosed and reported to UKHSA have tailed off considerably, suggestive of reduction in risk since the implementation of control measures from 2015 onwards (3).

In recognition that waning clinical awareness of the potential link between patients diagnosed with disseminated M. chimaera infection and past cardiac surgery may have affected identification and reporting of cases, a retrospective study was launched in September 2023. Assessing ongoing risk is critical to our understanding of the effectiveness of control measures. Utilising reference and routine laboratory reports linked to Hospital Episode Statistics to identify past cardiothoracic surgery, alongside genomic analysis of isolates to identify the outbreak clade, potential cases were identified and followed up with local diagnostic laboratories.

A total of 8 new cases were identified as part of the retrospective case finding to date in the UK, increasing the total number of cases in the UK to 58, all of whom had surgery before 2017. Cases had undergone procedures previously associated with these infections, namely heart valve or coronary artery bypass surgery. One case was linked to surgery at a hospital not previously associated with M. chimaera cases. Cardiac centres where the surgery took place have been contacted and reminded of Duty of Candour obligations.

Of the 58 cases diagnosed to date in the UK, 55 underwent surgery in England, 2 in Wales and 1 in Northern Ireland. Cases were linked to surgery in 24 different cardiac centres (NHS and independent sector) with the majority of cases (50) associated with cardiac valve repair or replacement surgery. The number of cases diagnosed each year has fallen since 2017, with the most recent case diagnosed in 2023. The earliest implicated surgery was performed in 2006 and the most recent in 2017. The median interval between surgery and diagnosis has increased slightly from 24 to 26 months (IQR 18-40 months) with 4 cases diagnosed eight or more years after surgery, 1 of whom developed symptoms over 12 years after surgery. Forty-four of the 58 cases are known to have died.

Monitoring of ongoing risk has focused on patients undergoing heart valve surgery given the higher risk for these patients (4). Risk estimates have been recalculated in light of the additional 8 cases identified. Of cases undergoing such procedures in the NHS in England between 2007 and 2022, the highest risk remains for surgery in 2014, with 0.8 cases arising per 1,000 patients. Between 2007 and 2017, approximately 167,000 patients underwent valve repair or replacement surgery in the NHS in England, according to Hospital Episode Statistics; the estimated risk during this period has increased slightly from of 0.23 to 0.26 cases of M. chimaera infection arising per 1,000 patients, or 1 in 4,000 patients. The risk to patients substantially fell after 2014 with the lowest rate reported in 2017, the last year of implicated surgery at this time, at 0.06 per 1,000 patients.

Given the long latency period, it is not possible to ascertain at which point the risk is no longer present as further patients who underwent surgery in recent years may yet be diagnosed. However, to date, no infection in patients who underwent surgery after 2017 has been reported. Infection and cardiothoracic specialists should maintain suspicion of possible M. chimaera infection in patients who have had exposure to heater cooler units and who present with a compatible clinical syndrome, and undertake mycobacterial culture accordingly (4, 5).

Specialist advice can be sought from the nearest reference service as follows:

• National Mycobacterial Reference Service (London): nmrs-south@nhs.net
• National Mycobacterial Reference Service (Birmingham): uhb-tr.nmrs@nhs.net
• Wales Centre for Mycobacteria: matthijs.backx2@wales.nhs.uk or Jason.Evans@wales.nhs.uk
• Northern Ireland Mycobacterium Reference Laboratory: NIPHL
• Scottish Mycobacteria Reference Laboratory: Loth.Smrl@nhslothian.scot.nhs.uk

All isolates of mycobacteria from suspected or definite cases should be submitted to the respective national mycobacterial reference services.

Non-tuberculous mycobacterial infections in patients who have had cardiothoracic surgery or ECMO, or which are strongly linked to HCUs, should continue to be reported to their local health protection team (HPT). The HPT will request information using the Atypical mycobacterial infection diagnosed following cardiopulmonary bypass: surveillance form, which will be submitted to the Healthcare-Associated Infection and Antimicrobial Resistance department at UKHSA Colindale.

References

1. Chand M, Lamagni T, Kranzer K, and others (2017). ‘Insidious risk of severe Mycobacterium chimaera infection in cardiac surgery patients’. Clinical Infectious Diseases: volume 64, number 3, pages 1,033 to 1,041.

2. PHE/MHRA/NHSE (2021). ‘M. chimaera infection associated with cardiopulmonary bypass: an update’. Health Protection Report: volume 15, number 16.

3. PHE (2017). ‘Infections associated with heater cooler units used in cardiopulmonary bypass and ECMO: Information for healthcare providers in the UK’.

4. Hasse B, Hannan M, Keller PM, and others (2020). ‘International Society of Cardiovascular Infectious Diseases: Guidelines for the diagnosis, treatment and prevention of disseminated Mycobacterium chimaera infection following cardiac surgery with cardiopulmonary bypass’. Journal of Hospital Infection: volume 104, number 2, pages 214 to 235.

5. PHE (2017). ‘Mycobacterium chimaera infections: guidance for secondary care’.

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