Clinical and epidemiological characteristics of Legionnaires' disease in Southern Sweden, a population-based study.

Background: The study aimed to describe all diagnosed cases of Legionnaire's disease (LD) in south Sweden, regarding incidence, patient characteristics, diagnostics, outcomes, and infection control investigations.

Methods: This population-based retrospective study was conducted in Skåne, on patients with LD between 2011-2021. Inclusion criteria were positive polymerase chain reaction (PCR) for L. pneumophila/Legionella spp. or a positive urinary antigen test (UAT), combined with a clinical presentation consistent with Legionella infection and radiological evidence of pulmonary infiltrates.

Results: A total of 280 patients met the inclusion criteria, with a mean incidence of 2.00 per 100,000 person-years (95% CI: 1.23-2.78). Empiric treatment covering Legionella was administered in 15% of cases. Mortality rates were 13%, 15% and 23% at 30-, 90- and 365 days, respectively. UAT was performed in 88% of patients, with a 66% positivity rate. Lower respiratory tract PCR was performed in 76% of patients with a 90% positivity rate. Bacterial culture was positive in 43% of cases. Out of these, L. pneumophila serogroup 1 was most common (58%), though a significant proportion were serogroups 2-14 (30%), which are not generally detected by UAT. Genetic matches between environmental and patient strains were established in 5% of cases.

Conclusion: Legionnaires' disease in Skåne is an uncommon but clinically significant condition. Few patients received appropriate empiric antibiotic treatment. Reliance on UAT alone is insufficient for establishing diagnosis, and species-specific PCR, particularly from lower respiratory samples, enhances detection. Environmental investigations frequently identified Legionella in suspected settings; however, a definitive source of infection was rarely established.