Legionnaires’ Disease Outbreak Caused by Endemic Strain of Legionella pneumophila, New York, New York, USA, 2015

During the summer of 2015, New York, New York, USA, had one of the largest and deadliest outbreaks of Legionnaires' disease in the history of the United States.A total of 138 cases and 16 deaths were linked to a single cooling tower in the South Bronx.Analysis of environmental samples and clinical isolates showed that sporadic cases of legionellosis before, during, and after the outbreak could be traced to a slowly evolving, single-ancestor strain.Detection of an ostensibly virulent Legionella strain endemic to the Bronx community suggests potential risk for future cases of legionellosis in the area.The genetic homogeneity of the Legionella population in this area might complicate investigations and interpretations of future outbreaks of Legionnaires' disease.L egionella spp.are ubiquitous in nature, live in soil and water, and frequently inhabit human-made water distribution systems, hot water tanks, decorative fountains, and cooling towers (1,2).Persons with underlying health conditions, such as chronic lung disease, or those with compromised immunity are at increased risk for contracting Legionnaires' disease (LD) (also referred to as legionellosis).Signs and symptoms typically include fever, cough, and chest pain; LD is fatal in ≈5%-10% of cases (3,4).Transmission of Legionella pneumophila is believed to occur mainly through exposure to contaminated aerosols and not from other infected persons; to date, only 1 case of humanto-human transmission has been documented (4,5).LD was initially detected in 1976, when an outbreak of illness occurred during a meeting of the American Legion in Philadelphia, Pennsylvania, USA; 221 cases were identified, and 34 infected persons died (6).The outbreak, which remains the largest community-associated outbreak of LD in United States, was later linked to the cooling system of the hosting hotel, and a bacterium classified as L. pneumophila serogroup 1 was subsequently isolated from 4 persons (7,8).In the summer of 2015, a large community-associated LD outbreak affected persons who resided or traveled through a large area in the South Bronx region of New York, New York, USA.During July 2-August 3, a total of 138 adults with LD were linked to the outbreak; 128 patients required hospitalization, and 16 deaths occurred (Figure 1).A joint laboratory investigation to find the source of this outbreak was performed by the New York City Department of Health and Mental Hygiene and the Public Health Laboratory (NYC PHL), the Wadsworth Center (WC) of the New York State Department of Health (Albany, NY, USA), and the Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA).Pulsed-field gel electrophoresis (PFGE), realtime PCR, sequence-based typing (SBT), and wholegenome sequencing (WGS) were used to characterize human and environmental L. pneumophila isolates from the investigation.Epidemiologic data and water testing by PCR quickly led to identification of a cooling tower located on the roof of a South Bronx hotel as a potential source of this outbreak (9).However, L. pneumophila isolates recovered from a sample taken later during the outbreak from a homeless shelter located in the vicinity of the South Bronx hotel and other facilities within the outbreak zone were found to have PFGE and SBT patterns identical to that of the outbreak strain, raising the possibility that the South Bronx hotel might not have been the only source of an aerosolized Legionnaires' Disease OutbreakCaused by Endemic Strain of Legionella pneumophila, New York, New York, USA, 2015