Last week, an outbreak of Legionnaires’ disease on the Upper East Side of Manhattan more than doubled, prompting health officials to rush to find the source.
According to the Health Department, 60 individuals have been diagnosed with this severe bacterial pneumonia. Over 33 patients have since improved and left the hospital, while 15 are still being treated. Thankfully, no fatalities have been reported.
As of Tuesday, health officials hadn’t pinpointed the outbreak’s source, although they acknowledged a decrease in new infections due to their ongoing efforts.
“We are now seeing significantly fewer new cases,” said Dr. Alister Martin, the city’s health commissioner.
What causes a Legionnaires’ disease outbreak?
Typically, in New York City, the source can be found on the rooftops of larger buildings. The Legionella bacteria, responsible for the disease, thrive in cooling towers—structures filled with warm water that facilitate air conditioning.
Fans circulate air over this warm water, increasing evaporation, which can release vapor containing bacteria into the neighborhood, posing a risk to those who breathe it in.
The first cases of this outbreak were noted on July 2. Since then, health investigators have been assessing cooling towers within three ZIP codes—10028, 10128, and 10075—on the Upper East Side.
How do health authorities identify the source?
On Friday, officials revealed they had sampled water from over 180 cooling towers in the area, which marked a more systematic approach compared to past outbreaks.
Following a major outbreak in the South Bronx in 2015 that sickened 120 people and resulted in 12 deaths, the city mandated that property owners register their cooling towers. This step has enabled health inspectors to quickly identify towers needing inspection near an outbreak.
“It eliminated the guessing game,” Daniel Kass, an environmental health expert and former deputy commissioner, stated.
The previous year, over 100 individuals fell ill, and seven died during an outbreak in Central Harlem, which was linked to two city-owned locations, including Harlem Hospital.
As of Tuesday, 76 buildings had returned positive tests for Legionella bacteria capable of causing pneumonia. Dr. Martin, the health commissioner, commented that these findings reflect how prevalent Legionella is in the urban environment.
The city provided a list of addresses associated with the positive tests, which included notable cultural landmarks like the Metropolitan Museum of Art and the Guggenheim Museum along Fifth Avenue.
What does it mean when a building tests positive for Legionella?
Health officials clarify that a positive test does not confirm an issue. Initial tests can find both live and dead bacteria, and since Legionella is common, the presence of inactive bacteria isn’t unexpected. As many as half of cooling towers might test positive for Legionella, according to state health data.
When a facility tests positive, owners are required to promptly drain and disinfect their cooling towers. This is part of Mayor Zohran Mamdani’s initiative to enhance the city’s responsiveness to Legionnaires’ outbreaks.
Health officials expressed optimism that one building with a positive result could turn out to be the outbreak’s source.
“When the offending tower that is potentially causing this cluster is taken off the map, we will see the gradual resolution,” Dr. Martin indicated.
It seems the outbreak may have peaked early last week, with at least 10 diagnoses reported per day on July 6 and 7—the only days new cases hit double digits.
Legionnaires’ has an incubation period of over 10 days, but symptoms often surface five or six days post-exposure, noted Dr. Celia Quinn, a key health department official. She added that new cases could still emerge for a few days even after the outbreak’s cause is treated.
The health department reported just one new case on Monday, a sign Dr. Martin considered encouraging.
How will officials identify the outbreak’s source?
Authorities have started to grow Legionella bacteria in lab dishes from samples collected from buildings that initially tested positive. Eventually, these lab-grown samples will be compared with those from hospitalized patients to see if there’s a genetic match, helping to locate the outbreak’s source.
Growing these samples can take up to two weeks, so officials may need more time to pinpoint the outbreak’s origin.
If a building’s cooling tower tests positive, are the people inside at particular risk?
Not necessarily. Evaporating mist from rooftop cooling towers isn’t meant to mix with the air indoors. Once it evaporates, it spreads across the area. Some studies suggest that infection can occur even from distances of thousands of feet away.
“The safety and well-being of our visitors, staff, and volunteers is critically important to us,” the Metropolitan Museum of Art stated. “Along with other local institutions, we’ve been informed by the Department of Health that a trace of Legionella bacteria was detected in our cooling tower system.”
The museum added that it quickly began disinfecting its cooling tower system.
