Waterborne outbreak of Mycobacterium abscessus in a UK specialist heart and lung hospital 2019-2026: patients, mitigations and implications.

BACKGROUND: Three months after a major cardiothoracic hospital in England relocated to a new building, two cases of Mycobacterium abscessus occurred among lung transplant recipients, triggering an outbreak investigation. METHODS: Respiratory samples were collected from transplant recipients and patients with Cystic Fibrosis (CF) and non-CF bronchiectasis and tested for Mycobacterium abscessus (other patient groups on clinical request). Environmental sampling was undertaken in between 2019 and 2025. Whole genome sequencing (WGS) was performed on clinical and environmental specimens. An outbreak case was defined as a patient with a positive culture matching the strain from water samples on WGS. Epidemiological investigations included assessment of clinical outcomes; calculation of incidence rates; and a case-control study among transplant recipients. RESULTS: Between May 2019 and June 2026, 40 outbreak cases occurred in lung transplant recipients (n=11), patients with CF (n=11) and non-CF bronchiectasis (n=14), and other patient groups (n=4), of which 19 were diagnosed with non-tuberculous mycobacterial pulmonary disease. The case-control study found no specific risk factors associated with lung transplant surgery or bronchoscopy. Environmental sampling showed that the outbreak strain had persisted and disseminated throughout the hospital water system. Following extensive control measures, including point-of-use filters and biocide treatment, incidence fell from 12.8 to 2.2 cases per year. No further case has been identified since June 2025, coinciding with a review of enhanced flushing. CONCLUSIONS: Control measures reduced the incidence of infection but did not eliminate the outbreak strain from the hospital water. Risk mitigation is needed for water systems where vulnerable patients are accommodated.