Detection, relatedness and environmental sources of emerging Legionella longbeachae infections in Finland, 1989-2024

Legionella longbeachae is an emerging cause of Legionnaires' disease in Europe. We analysed data from the National Infectious Disease Register (NIDR) and the Legionella Reference Laboratory database to characterize L. longbeachae infections and diagnostics in Finland. A total of 41 L. longbeachae patient cases, mainly gardeners' pneumonias and hand wounds, were reported during 1989-2024, with a significant increase after 2019. Patient interviews and environmental sampling linked infections to compost, potting soil or outdoor activities. Whole genome sequencing (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 %) and higher proportion of females (44 % vs. 35 %). The genetic relatedness of patient and environmental isolates and high clonality suggest common environmental reservoirs.