Community-Acquired Legionella pneumophila Pneumonia: A Case-Control Study in Adult Inpatients from 2019 to 2024

Legionella pneumophila is frequently acquired in the community and generally linked to contaminated domestic hot water systems, hotels, or other environmental sources or of unknown origin. L. pneumophila is a leading cause of pneumonia, especially in high-risk patients such as those over 50 who are immunocompromised or people with pre-existing illnesses. This study examines the factors linked to L. pneumophila acquired in the community in adult patients with hospitalization due to pneumonitis.

This case-control study included 140 hospitalized adult patients admitted to the University Hospital Paolo Giaccone of Palermo between January 2019 and December 2024. Demographic, laboratory, clinical, and microbiological data were collected electronically. Urinary antigen testing and the BioFire FilmArray Pneumonia Panel were used to detect L. pneumophila and were performed within 48 h upon admission.

Of 140 pneumonia patients, 70 were positive (SG) and 70 were negative (CG) for L. pneumophila. Comorbidities were significantly associated with the presence of L. pneumophila (p = 0.0046). The most frequent comorbidity was only heart failure (p < 0.0015) and, similarly, for smoke (p = 0.0487). There was no difference in mortality between the two groups (SG). Levofloxacin was the most frequent therapy used in SG (p < 0.0001). Additionally, SG showed significantly lower blood sodium, phosphate, and platelet levels (all p < 0.0001) compared to the CG. In contrast, blood parameters such as LDH, CRP, AST, and ALT were significantly higher (all p < 0.0001).

Our research highlights the critical need for early detection of L. pneumophila infections, especially in patients with high CRP levels, moderate hypophosphatemia, or heart failure. In these patients with L. pneumophila, early treatment with macrolide and fluoroquinolone is mandatory to reduce mortality.