Legionnaires' disease among residents of a long-term care facility: the sentinel event in a community outbreak.

Background: A long-term care facility (LTCF) reported an outbreak of Legionnaires' disease (LD) in September 2004.
Methods: We conducted case finding through enhanced surveillance, medical record review (n = 131), and community surveys (n = 258). We cultured water samples from the LTCF and assayed their outdoor air-intake filters for Legionella DNA. We also investigated a cooling tower, the only nearby outdoor aerosol source.
Results: Among 7 confirmed cases, 2 LTCF residents never exited, and 2 community residents never entered the LTCF during the incubation period. Among 63 water and biofilm samples collected from throughout the LTCF, we found no evidence of Legionella colonization, either in the potable water or air-handling systems. Conversely, we isolated a common outbreak-causing strain of Legionella pneumophila serogroup 1 from an industrial cooling tower located 0.4 km from the LTCF and recovered L pneumophila DNA from the LTCF's outdoor air-intake filters, suggesting that aerosolized Legionella from the cooling tower most likely entered the LTCF through the air-intake system or, possibly, through open windows.
Conclusion: Residents of LTCFs can acquire LD from community sources. A cluster of LD cases among LTCF residents does not necessarily indicate transmission from within the LTCF.