Three people have died and more than 60 have been sickened in a fast-growing outbreak of Legionnaires’ disease in Central Harlem that has health officials still searching for the source more than a week after people began turning up ill.
Legionnaires’ disease is a form of pneumonia caused by the Legionella bacterium, which thrives in warm, stagnant water. In New York City, many of the outbreaks are caused by water vapor spewed from rooftop cooling towers. The bacteria can float along on water vapor for sometimes thousands of feet before someone inhales the pathogen and is sickened, researchers believe.
Most healthy people don’t become sick after being exposed to the bacteria. But many people are vulnerable, including older adults, smokers and people with chronic diseases and compromised immune systems.
On average, about 200 to 500 people annually are diagnosed with Legionnaires’ disease in New York, and they generally require hospitalization. Just over a dozen die each year.
The disease got its distinctive name from a 1976 convention in Philadelphia held by the American Legion, the veterans’ organization, that resulted in a huge outbreak of mysterious pneumonia cases. Months later, scientists at the Centers for Disease Control and Prevention revealed the cause: a never-before-identified bacterium, now called Legionella. Scientists quickly realized that it was responsible for tens of thousands of cases of pneumonia each year. The number has only grown.
New York has an especially high rate of cases.
Many cases trace back to mist emanating from cooling towers atop buildings, which play a critical role in cooling systems. Warm water is piped to the rooftop cooling tower, where fans cool the water through evaporation, sending the mist into the environment. If not properly maintained, the cooling towers can become incubators for the Legionella bacteria, which thrive in warm, standing water.
Cooling towers are a common source of outbreaks, but the bacteria can spread other ways, such as through poorly maintained plumbing systems. In such instances, health officials advise residents to take baths rather than showers to reduce the amount of mist in the bathroom.
The outbreak in Central Harlem was detected in late July through routine surveillance. The city’s health department is alerted to every positive lab test for Legionella.
“When we see a concerning signal, we spring into action,” said Dr. Shama Ahuja, the assistant commissioner in the department’s communicable disease bureau. By July 25, the city had identified a cluster of five cases, which had grown to 67 by Wednesday.
Health department officials suspect a cooling tower is a culprit because the cases aren’t confined to a single building, suggesting wider dispersal of Legionella, officials said.
In recent days, inspectors have detected Legionella bacteria in 11 cooling towers, but it remains unclear which — if any — might be the source of the outbreak.
In recent years, the highest rates of Legionnaires’ disease have been found in parts of the Bronx and Harlem, especially Central Harlem. In neighborhoods with the highest burden, the average rate of being sickened with a confirmed case of Legionnaire’s disease is about 10 to 20 per 100,000 people. Relatively high rates have also been identified in a swath of the city from Union Square to Chelsea and up to Hell’s Kitchen.
The higher rates in the Bronx and Harlem reflect a range of factors, experts say. Those neighborhoods have higher rates of chronic diseases, such as diabetes.
“There is the population vulnerability component,” said Daniel Kass, an expert on environmental health and former deputy commissioner in the health department.
“Then, there is the environmental and social factor,” said Mr. Kass, who played a key role in the response to the city’s largest Legionnaire’s outbreak, in 2015.
In lower-income neighborhoods, buildings are more likely to have plumbing systems and cooling towers that aren’t maintained as well, leading to a higher incidence of Legionella bacteria in those systems, experts said. Mr. Kass noted that nursing homes and other larger institutional settings were especially vulnerable. Five residents of an Upper Manhattan nursing home died of Legionnaire’s in 2022, the deadliest cluster since the 2015 outbreak, which killed 16 and was linked to a cooling tower atop a Bronx hotel.
Case counts have risen about ninefold across the United States during the past 20 years. Expanded testing is responsible for some of the increase. Other hypotheses abound, including an aging population and an increase in the number of people who are immunocompromised because of cancer or autoimmune disorders or from taking certain medications.
Some researchers speculate that the rise is a byproduct of cleaner air. Scientists at the State University of New York at Albany published a study theorizing that lower levels of sulfur dioxide, an air pollutant, mean that the Legionella bacteria can survive longer in the air.
One of the authors of the 2024 paper, Professor Fangqun Yu of the Atmospheric Sciences Research Center at SUNY Albany, said, “These bacteria can survive longer if we have less acidity in the air.”
Joseph Goldstein covers health care in New York for The Times, following years of criminal justice and police reporting.
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