Comorbidity Burden and Radiological Features Associated with Legionella pneumophila Pneumonia: A Retrospective Case-Control Study.

Background: Legionella pneumophila is a leading cause of community- and hospital-acquired pneumonia (HAP/CAP), although the clinical and radiological features associated with infection remain incompletely defined. This study aimed to identify clinical, comorbidity-related, and chest computed tomography (CT) features associated with L. pneumophila pneumonia in hospitalized adults with community-acquired pneumonia (CAP). Methods: A retrospective case-control study was conducted at a tertiary-care university hospital in Italy between January 2019 and August 2025. Eighty patients positive for L. pneumophila were compared with 79 controls negative for L. pneumophila. Demographic, clinical, microbiological, and radiological data were collected, and factors independently associated with infection were evaluated using multivariable logistic regression analysis. Results: Patients with L. pneumophila infection exhibited a greater comorbidity burden, particularly heart failure, and were more frequently smokers. Bilateral lung involvement and hilar/mediastinal lymphadenopathy were significantly more common among Legionella-positive patients, whereas overall pulmonary complications and bilateral pleural effusion were more frequently observed in controls. Multivariable analysis identified comorbidity burden as an independent positive predictor of L. pneumophila infection, while intensive care unit admission, oxygen therapy, and pulmonary complication score were independently associated with a lower likelihood of infection. Conclusions: The integration of comorbidity assessment with readily available chest CT findings may help raise clinical suspicion of L. pneumophila pneumonia and support consideration of appropriate diagnostic testing.