Clinical Insights From Legionnaires’ Disease: A Retrospective Analysis of Severe Cases on Account of a Cluster

Background Legionnaires' disease is a form of severe pneumonia most commonly caused by the Gram-negative bacterium <i>Legionella pneumophila</i>. During a sudden increase in Legionnaires' disease cases, a regional cluster on the northern coast of Portugal led to multiple cases of severe illness requiring admission to a local intensive medicine department. This epidemiological event provided the opportunity to retrospectively review and analyze all patients with severe Legionnaires' disease admitted to this department (intensive care units (ICU) and intermediate care units (IMCU)) in order to report the epidemiology, clinical features, treatment strategies, and outcomes of these patients. Materials and methods This was a retrospective, observational, single-center study conducted at the ICU and IMCU of a peripheral hospital. All adult patients (aged ≥18 years), admitted to these units during a four-year period from January 1, 2020, to December 31, 2023, with a microbiologically confirmed diagnosis of Legionnaires' disease, were included. Diagnosis was confirmed by positive urinary antigen test and/or positive polymerase chain reaction analyses. Results During the study period, 10 patients with severe Legionnaires' disease were admitted, most of whom were associated with the 2023 outbreak. Among 17 patients who tested positive at our hospital during that cluster, four (23.5%) required IMCU admission and three (17.6%) ICU admission - a total admission rate of 41.2%. The remaining cases occurred in 2020 (n=1) and 2021 (n=2). The mean age of patients was 63 years (standard deviation (SD): 19.4), and 80% (n=8) were male. Diagnosis was made in all cases by urinary antigen testing and by PCR in two cases (20%). Radiologic lung abnormalities were present in all patients at admission. The average length of stay was 11 days (SD: 5.0) in the ICU and 6.9 days (SD: 6.3) in the IMCU. At least one classical risk factor (e.g., smoking, pulmonary disease, diabetes, or immunosuppression) was present in 60% (n=6). The main cause of admission was respiratory failure (n=9, 90%), but 60% (n=6) of patients also presented multi-organ failure, and 50% (n=5) presented acute kidney injury. No patients developed septic shock. While a smaller subset required invasive mechanical ventilation (n=2, 20%, mean duration 11 days, SD: 4.3), the majority were managed with non-invasive ventilatory strategies or high-flow nasal cannula (n=7, 70%). Regarding antibiotic choice, azithromycin was used in 60% (n=6) of patients. A C-reactive protein decrease by half or more was noted 48 hours after antibiotic treatment initiation in seven (70%) patients. All patients survived to hospital discharge. Conclusion This study provides a detailed description of the epidemiology, clinical presentation, management strategies, and outcomes of patients with severe Legionnaires' disease admitted to a peripheral hospital's intensive medicine department. Although such cases are infrequent, regional outbreaks can place a significant strain on healthcare resources. Our findings highlight that timely diagnosis and appropriate treatment management, even in the presence of severe illness and underlying comorbidities, can lead to favorable clinical outcomes.