Increased Incidence of Legionellosis after Improved Diagnostic Methods, New Zealand, 2000-2020.

Legionellosis, notably Legionnaires' disease, is recognized globally and in New Zealand (Aotearoa) as a major cause of community-acquired pneumonia. We analyzed the temporal, geographic, and demographic epidemiology and microbiology of Legionnaires' disease in New Zealand by using notification and laboratory-based surveillance data for 2000‒2020. We used Poisson regression models to estimate incidence rate ratios and 95% CIs to compare demographic and organism trends over 2 time periods (2000-2009 and 2010-2020). The mean annual incidence rate increased from 1.6 cases/100,000 population for 2000-2009 to 3.9 cases/100,000 population for 2010-2020. This increase corresponded with a change in diagnostic testing from predominantly serology with some culture to almost entirely molecular methods using PCR. There was also a marked shift in the identified dominant causative organism, from Legionella pneumophila to L. longbeachae. Surveillance for legionellosis could be further enhanced by greater use of molecular typing of isolates.