While Legionella has long been the bane of infection control practitioners, the diagnostic tools used in identifying clinical disease have evolved over the years, as has its clinical impact.
A recent article in Clinical Infectious Diseases provides a snapshot of 344 episodes of Legionella pneumonia managed through the Mayo Clinic from January 2019 to September 2025. Median age was 66.6 years with 45.1% immunocompromise. Intensive care unit admission occurred in 36.1%, and mechanical ventilation in 22.7%. Thirty-day and 90-day mortality were 11.9% and 16.6%, respectively. Cirrhosis (odds ratio, 10.2; 95% confidence interval, 2.15-48.3) was the strongest risk factor for 30-day mortality; age, immunocompromise and lymphopenia were also independent risk factors. Gastrointestinal symptoms were reported in 27.6% and pleural effusion in 64.1%. Higher incidence was observed in summer/early fall. Levofloxacin was the final antibiotic in 48.6%, followed by azithromycin (in 36.9%).
Legionella was identified by urinary antigen in 51.5%, with PCR testing of respiratory specimens in 52.9% and culture in 25%. Among the 121 patients that were either PCR-positive or culture-positive and had urinary antigen testing, the urinary antigen was positive in only 31 (25.6%). PCR and sputum positivity were high among patients who underwent bronchoscopy with bronchoalveolar lavage: 156/162 (96.3%) and 30/31 (96.7%), respectively.
This article clearly highlights that Legionella pneumonia is often severe and associated with poor outcomes, particularly in patients with cirrhosis. Utility of Legionella urinary antigen (which only detects L. pneumophila serotype 1) appears poor compared to PCR and/or culture from bronchoalveolar lavage. However, a significant proportion of PCR testing in this study was done via a laboratory-developed test that was routinely included as a part of a standardized immunocompromised host panel for bronchoscopy specimens, so sites that do not have similar protocols may have different diagnostic yields.
One notable issue to consider as more hospitals aim for LEED “green” building certification is the incidence of low-flow water states that can encourage Legionella colonization of water systems, so heightened awareness to mitigation strategies and clinical disease diagnosis is likely in order.
(Pulsipher et al. Clin Infect Dis. Published online: Feb. 21, 2026.)
