Mixed community and nosocomial outbreak of Legionella pneumophila in Montréal, Québec, 2019

Objectives: To describe the investigation of a community-based outbreak of Legionella pneumophila serogroup 1, with retirement home and acute care hospital sub-clusters in Montréal, QC, and the key challenges encountered. Methods: There were 14 cases of L. pneumophila serogroup 1 infection with an onset date between June 7 and August 21, 2019. The environmental investigation included sampling of water cooling towers (WCTs) and other potential sources. Sequence-based typing of clinical and environmental isolates was performed. Public health interventions included WCT decontamination orders and communication with clinicians. Results: Eleven (79%) of the 14 cases were immunosuppressed or immunocompromised. Most (13; 93%) were diagnosed using a urinary antigen test, and five (36%) had a culture. Two sub-clusters were identified: three cases in a retirement home and four cases on an acute care hospital floor. Typing results suggested that the same L. pneumophila serogroup 1 may have caused the community outbreak and the two sub-clusters. A matching environmental source was not identified. Conclusion: Whereas typing of clinical isolates suggested a common environmental source, our investigation failed to identify this source. Future outbreak investigations could benefit from more clinical isolates for typing, local registries of water aerosolization sources other than WCTs, and ongoing access to all WCT routine monitoring results and L. pneumophila isolates for typing.