To investigate the clinical characteristics of community-acquired Legionella pneumonia and the factors associated with disease severity.

A retrospective analysis was performed among 26 patients with community-acquired Legionella pneumophila pneumonia and 1 patient with community-acquired Legionella longbeachae pneumonia diagnosed by metagenomic next-generation sequencing of bronchoalveolar lavage fluid in the Department of Respiratory and Critical Care Medicine, Beijing Anzhen Hospital, Capital Medical University, from April 2024 to January 2026. Clinical data, laboratory results, imaging features and treatment regimens were analyzed. According to the disease severity, the patients were divided into severe group (14 cases) and non-severe group (13 cases). The clinical data between the two groups were compared by independent-samples t test, Mann-Whitney U test and Fisher’s exact test. The factors associated with pneumonia severity were analyzed by Univariate Logistic regression, and the diagnostic efficacy of the corresponding indicators for severe disease stratification was evaluated by the receiver operating characteristic (ROC) curve.

Most enrolled patients occurred fever, accompanied by multilobar lung lesions and multisystem laboratory abnormalities. Omadacycline and quinolones were the main anti-infection agents, with a favorable overall prognosis. Compared with the non-severe cases, severe patients had significantly advanced age and higher levels of systemic inflammatory indicators, accompanied by myocardial injury, hyponatremia, hypochloremia, reduced platelet and eosinophil counts. Among the 27 patients with Legionella pneumonia, 14 cases (51.9%) were classified as severe pneumonia and 13 cases (48.1%) as non-severe pneumonia. Compared with the non-severe group, the patients with severe disease were significantly older (t=2.130, P=0.043), and had higher proportion of fever (Fisher’s exact test: P=0.041), higher CURB-65 scores (Z=－3.469, P=0.001), alveolar-arterial oxygen gradients (t=3.975, P<0.001), procalcitonin (PCT) (Z=－3.240, P=0.001), serum troponin (Z=－2.666, P=0.008) and serum myoglobin (Z=－2.578, P=0.010) levels, and lower platelet counts (t=－2.943, P=0.010), serum sodium (t=－3.326, P=0.003) and chloride levels (t=－2.079, P=0.048), and peripheral blood eosinophil counts (Z=－2.397, P=0.017), with significant differences. Univariate Logistic regression analysis showed that troponin (OR=1.183, 95%CI: 1.026-1.365, P=0.021), alveolar-arterial oxygen gradient (OR=1.137, 95%CI: 1.030-1.256, P=0.011) and CURB-65 score (OR=13.693, 95%CI: 1.760-106.557, P=0.012) were associated with severe Legionella pneumonia. ROC curve analysis showed that the area under the curve (AUC) of the combined indicator based on these three variables for identifying severe Legionella pneumonia was 0.936, with a sensitivity of 83.3% and specificity of 100.0%.

The clinical manifestations of community-acquired Legionella pneumonia are nonspecific. Severe cases are often accompanied by a marked inflammatory response, impaired oxygenation and multisystem involvement. Troponin, alveolar-arterial oxygen gradient and CURB-65 score may be associated with severity stratification, and the combined indicator of these three variables may provide a useful reference for early identification of severe Legionella pneumonia. Further validation in studies with larger sample size is still required.