Study identifies likely source of infection in heart surgery patients

Whole genome sequencing study affirms heater-cooler units contaminated with
M. chimaera during production as a main cause, while a few samples also showed M. chimaera contamination from the hospital environments

A genome sequencing study has identified likely sources of M. chimaera infection during surgery

A study1 using whole genome sequencing has identified contaminated heater-cooler units (HCUs) produced by a specific factory in Germany as the likely source of Mycobacterium chimaera (M. chimaera) infection in 21 open-heart surgery patients, according to The Lancet's Infectious Diseases journal.

The authors of the study said the similarity of samples from almost all patients with M. chimaera infection following open-heart surgery point to a common source of infection.

However, they have warned against prematurely closing the outbreak investigation because hospital water systems and another brand of HCUs were also found to be contaminated during production and use. Therefore, the infectious risk might still occur despite controlling contamination at the identified German production line.

Infection with M. chimaera bacteria during open-heart surgery can cause infection of the inner lining of the heart and spread to the rest of the body. Since 2013, over 100 cases of M. chimaera infection have been reported in the EU, US and Australia.

Previous studies have suggested a link to contaminated HCUs used during open-heart surgery, but until now, there has been no firm evidence linking the global outbreak to a source, whether at a production site or local hospitals.

The new study analysed the DNA from 250 M. chimaera samples. This included samples from 21 cardiac surgery-related patients in Switzerland, Germany, the Netherlands and UK, as well as samples provided by manufacturers of HCUs, and other samples collected from water-containing medical devices, tap water and drinking water dispensers in hospitals.

The study found a high degrees of similarity between isolates of M. chimaera in these patients' samples, and those from the HCUs from a specific manufacturer and its production site.

As a result, the authors said that HCU contamination with M chimaera at one factory seems to be “a likely source for cardiothoracic surgery-related severe M chimaera infections diagnosed in Switzerland, Germany, the Netherlands, the UK, the USA and Australia.”

The researchers also compared their findings to publicly available datasets containing samples from open-heart surgery patients from the US and Australia, matching 12 additional patients to a specific production site named in the report.

Professor Stefan Niemann, National Centre for Mycobacteria, Forschungszentrum Borstel, Germany, a co-author of the study, said: “Molecular epidemiological investigation by applying whole genome sequencing is the most powerful tool for tracing pathogen transmission. Our study closes the missing gap and provides evidence that the international healthcare related M. chimaera outbreak can most likely be attributed to a point source.”

Co-author, Professor Dr Hugo Sax, University Hospital Zurich, Switzerland, added: “Local contamination of heater-cooler units with M. chimaera also occurred and at least one patient could have been infected through this route. Operating rooms and other hospital settings with patients at increased risk of infection should be devoid of such uncontrolled water sources.”

The authors were not able to link individual patients to individual HCUs. Also out of a large number of samples, too few were taken from tap water, heater-cooler water and air samples to reliably pinpoint exact transmission events.

1. Hugo Sax et al., Global outbreak of severe Mycobacterium chimaera disease after cardiac surgery: a molecular epidemiological study. The Lancet Infectious Diseases, 13 July 2017; DOI: 10.1016/S1473-3099(17)30324-9