Weather-related risk factors for Legionnaires’ Disease and Pontiac Fever
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Background: Legionellosis is a respiratory infection caused by Legionella pneumophila bacteria that are found in water and soil. Infection may cause severe pneumonia (Legionnaires’ Disease) and milder illness (Pontiac Fever) and is a particular concern for the elderly. Legionella can colonize water systems and may result in human exposure by inhalation of aerosolized bacteria. The incubation period ranges from 2-14 days. Previous studies have suggested that precipitation and humidity may be associated with an increased risk of Legionellosis.
Methods: We used records from the Centers for Medicare and Medicaid Services for the years 1999-2018 to identify hospitalizations for Legionnaires’ Disease (ICD 9/10 codes 428.4/A481) and Pontiac Fever (ICD 9 code A482) among the Medicare-eligible population. Antecedent weather (precipitation, temperature, relative humidity) was obtained from the PRISM Climate Group (https://prism.oregonstate.edu/) using the case’s zip code of residence. We used a time-stratified bi-directional case-crossover design stratifying by month and year and matching on day-of-week, with +/- 14 days between case and control days. Lags of up to 20 days were considered for daily precipitation, relative humidity and maximum temperature. Data were analyzed using conditional logistic regression.
Results: Over the study period 37,883 Legionellosis cases were identified. After correction for false-discovery, increases in precipitation at lags 8-12 and 17 and relative humidity at lags 9, 11 and 13 were associated with increases in Legionellosis hospitalization. The strongest association was for precipitation at day 10 lag (OR=1.22, 95% CI 1.18-1.26 per inch).
Conclusion: Wet and humid weather may be favorable to the survival of Legionella. Among the Medicare-eligible population, increases in precipitation and humidity were associated with hospitalization for Legionellosis at lags consistent with the incubation period for infection.