METHODS: We defined a case as laboratory-confirmed LD with illness onset after July 1, 2015, in a person who lived in or visited a 7-ZIP-code area ≤10 days before illness onset. We assessed patient exposures through structured interviews. We located cooling towers in the area and measured residential space-time clustering ≤1 km from towers. Specimens from all towers were screened for Legionella pneumophila serogroup 1 (LP1) DNA by real-time polymerase chain reaction (PCR) and confirmed with culture. Patient isolates and tower isolates were compared using molecular methods.
RESULTS: In total, 138 cases occurred during July 2–August 3; 16 (12%) persons died. No individual buildings were implicated: the investigation subsequently focused on cooling towers. Of 42 towers sampled, PCR detected LP1 DNA in 18 (43%); all 18 were immediately disinfected. LP1 grew in culture in 6 (14%). All 26 patient isolates matched isolates from Towers A and B by pulsed-field gel electrophoresis and sequence-based typing, but matched only isolates from Tower A by whole genome sequencing (WGS). By residential cluster analysis, Tower A was associated with 56 cases and Tower B with 33 cases.
CONCLUSIONS: We associated the largest U.S. community LD outbreak since 1976 with a cooling tower through epidemiology and molecular methods. PCR results prompted rapid disinfection of cooling towers, potentially preventing further infections. PCR and WGS might aid future LD investigations.