Background In 2021–2022, Queensland, Australia observed an increase in Legionnaire's disease cases, predominantly due to Legionella longbeachae. This study assessed seroprevalence at time points 2016 and 2023, representing before and after the higher incidence and explored if demographic, environmental and geographical factors associated with legionellosis seroprevalence.

Methods A total of 1001 human plasma samples (496 from 2016/505 from 2023) were analysed for the presence of Legionella antibodies (IgG) using indirect immunofluorescence assays. Primary screens detected IgG to L. pneumophila serogroups (SG) 1–6, SG 7–14, or “other” Legionella spp. Samples positive for “other” underwent secondary screening for L. longbeachae SG 1 and 2. A chi-square test assessed associations between seroprevalence and demographics, while a generalized linear model evaluated rainfall, temperature, and land cover associations.

Results While total Legionella seroprevalence remained stable (32.46% vs 32.28%) between 2016 and 2023, we observed a decrease in L. pneumophila (SG 1–6: 19%–13% [P = .0182] and SG 7–14: 24%–18% [P = .0257]) and an increase in L. longbeachae (1%–3% [P = .0355]) seropositivity. L. pneumophila seroprevalence positively associated with higher rainfall and land cover, with croplands and urban areas showing increased prevalence.

Conclusions Between 2016 and 2023, total Legionella seroprevalence remained unchanged. However, rainfall and specific land cover types were positively associated with seropositivity for certain Legionella spp. This study highlights the importance of assessing Legionella exposure risks in high-risk areas, particularly for vulnerable individuals such as the elderly, immunosuppressed, or those with co-morbidities.