Relatedness of Legionella longbeachae isolates in Finland, 1989-2024

Legionella longbeachae, primarily linked to potting soil and compost materials, is an emerging cause of Legionnaires' disease (LD) in Europe and increasingly recognized for its significant impact on public health. We analyzed data from the National Infectious Disease Register (NIDR) and the Legionella Reference Laboratory database to characterize L. longbeachae diagnostics and clinical strains in Finland. Whole genome sequencing (WGS) was performed on available patient and related environmental isolates to assess genetic relatedness and potential shared sources of infection. A total of 41 L. longbeachae patient cases were reported during 1989-2024, with a significant increase after 2019. Environmental sampling of 21 cases yielded L. longbeachae in soil in 11 cases. Patient interviews and environmental sampling linked infections to compost, potting soil or outdoor activities. WGS confirmed identical patient and soil isolates in three cases and revealed several clusters among non-related isolates. Compared to infections caused by other Legionella species, L. longbeachae cases had distinct characteristics, including higher median age (64 vs. 56 yrs), higher fatality (15% vs. 8%), more extrapulmonary infections (12% vs. 1%), higher proportion of females (44% vs. 35%). The genetic relatedness of patient and environmental isolates and high clonality suggest common environmental reservoirs.