Legionnaires’ disease largest outbreaks are caused by cooling towers.
Source: Fitzhenry R, Weiss D, Cimini D, Balter S, Boyd C, Alleyne L, et al. Legionnaires’ Disease Outbreaks and Cooling Towers, New York City, New York, USA. Emerg Infect Dis. 2017;23(11):1776. https://wwwnc.cdc.gov/eid/article/23/11/16-1584_article.
“The incidence of Legionnaires’ disease in the United States has been increasing since 2000. Outbreaks and clusters are associated with decorative, recreational, domestic, and industrial water systems, with the largest outbreaks being caused by cooling towers. Since 2006, 6 community-associated Legionnaires’ disease outbreaks have occurred in New York City, resulting in 213 cases and 18 deaths. Three outbreaks occurred in 2015, including the largest on record (138 cases). Three outbreaks were linked to cooling towers by molecular comparison of human and environmental Legionella isolates, and the sources for the other 3 outbreaks were undetermined. The evolution of investigation methods and lessons learned from these outbreaks prompted enactment of a new comprehensive law governing the operation and maintenance of New York City cooling towers. Ongoing surveillance and program evaluation will determine if enforcement of the new cooling tower law reduces Legionnaires’ disease incidence in New York City.”
In a densely populated area, cross-contamination between towers is a real possibility.
Source: Pascal Lapierre, Elizabeth Nazarian, Yan Zhu, Danielle Wroblewski, Amy Saylors, Teresa Passaretti, Scott Hughes, Anthony Tran, Ying Lin, John Kornblum, Shatavia S. Morrison, Jeffrey W. Mercante, Robert Fitzhenry, Don Weiss, Brian H. Raphael, Jay K. Varma, Howard A. Zucker, Jennifer L. Rakeman, Kimberlee A. Musser. Legionnaires’ Disease Outbreak Caused by Endemic Strain of Legionella pneumophila, New York, New York, USA, 2015. Emerg Infect Dis, 2017 ,23(11):1784.
“Conventionally, cooling towers are believed to be seeded by municipal water distribution networks, and although this factor might be true, in a densely populated area such as New York, New York, cross-contamination between towers is a real possibility. This contamination can potentially lead to reestablishment of L. pneumophila in cooling towers after decontamination and cause long-term persistence of endemic strains in communities.”