Evolution and main causes of the rising legionellosis incidence in the United States

Legionellosis is an acute respiratory infection caused by the environmental bacteria Legionella pneumophila and other Legionella species upon inhalation of aerosolized bacteria. The legionellosis incidence in the United States has risen rapidly in the past two decades. This study examined the ecological evolution and main causes of the rising legionellosis incidence in the US from 1999 to 2021. Publicly available data on the annual and monthly infection rates, temperature and precipitation, motor vehicle miles driven, and solar ultraviolet were used for fitting analyses, linear regressions, seasonal correlations, and state-wise year-to-year comparisons. The annual and monthly mean temperature and precipitation in the US from 1895 to 2021 were examined first to show significant climate changes in the recent 4 decades. During 1999–2019 before the COVID-19 pandemic, the age-adjusted legionellosis incidence rates rose exponentially by 6.6-fold from 0.40/100,000 in 1999 to 2.37/100,000 in 2019 with a peak of 2.69/100,000 in 2018. During COVID-19, however, the rates dropped to 1.67/100,000 in 2020 and 2.21/100,000 in 2021, which paralleled the reduction of motor vehicle miles for COVID containment. In regression, the annual incidence rates during all 23 years correlated strongly with cumulative temperature anomaly, annual precipitation amount, and motor vehicle mileage. Analyses of the monthly incidence rates during 1999–2019 revealed emergence of a seasonal legionellosis incidence plateau in 2018 and 2019 that spanned from June to October. Interactions between monthly precipitation and motor vehicle traffic were most notable during COVID, supporting the notion of road exposure to soil-derived Legionella spp. In state-wise analyses, the above effects were seen consistently across diverse landscapes and climates. Together, four decades of warming temperature have driven the rapidly rising legionellosis incidence in the US. Road exposure to soil-derived bacteria likely constitutes the main exposure route upon interactions of motor vehicle traffic, precipitation, and attenuation or lack of sunlight.