An outbreak of Legionnaires’ disease in Harlem, New York, last year resulted in 118 confirmed cases and seven fatalities, with investigations pointing to cooling towers as the source of infection.
The New York City Health Department (NYCHD) identified two cooling tower systems on the same block as the origin of the Legionella pneumophila bacteria. Notably, one of these systems was not registered.
The alert system flagged a cluster of L. pneumophila serogroup 1 urine antigen test results on July 25, 2025. All eight initial patients resided within a 1-kilometer radius in Central Harlem, with five cases quickly confirmed. Following a broader clinical advisory, the NYCHD confirmed a total of 118 cases out of 129 reports. The median age of those affected was 65, with a range of 32 to 97 years. Hospitalization was required for 92 patients. Seven deaths were attributed to the outbreak, with an average of 8 days between symptom onset and death.
Sampling of 43 cooling tower systems by the end of July revealed that eight, including one unregistered tower, tested positive for L. pneumophila or L. pneumophila serogroup 1. Owners were mandated to carry out disinfection and thorough cleaning, with the unregistered system also ordered to be registered. Cleaning was completed by mid-August.
Genetic analysis, specifically whole-genome sequencing, indicated a strong link between isolates from two separate cooling tower systems and the clinical isolates from the initial eight patients. One of these linked towers was the unregistered system, which showed significant deficiencies in routine monitoring, lacked water treatment, and had missed mandatory Legionella sampling. The other implicated tower system, located on the same block, had met its routine monitoring and sampling obligations.
Researchers stated that without established thresholds for infectious bacterial levels or a complete understanding of air movement and exposure, it was not possible to definitively identify a single cooling tower as the sole source of infection. However, the investigation highlighted the importance of a cooling tower registry, which facilitated the swift identification and testing of registered systems within the affected area during the outbreak.