CDC Study Finds Legionella DNA in 84 Percent of Cooling Towers Across the U.S.​

Report tested 196 cooling towers across the country, revealing that 84 percent tested positive for legionella DN

Researchers from the Centers for Disease Control and Prevention said they found the Legionella bacteria prevalent in cooling towers throughout the country.

Water from 196 cooling towers representing eight of nine continental U.S. climate regions were screened for Legionella DNA using multiplex real-time polymerase chain reaction. Samples positive for Legionella DNA were cultured. Resulting Legionella isolate species and serogroup were characterized using PCR, antibody testing, and gene sequencing.

The researchers found that 84 percent of the cooling towers from every region of the country tested indicated positive for Legionella DNA – in other words, that the bacteria were either present or had been there at some point.

Overall, they found live Legionella bacteria in 79 cooling towers – half of which had more than one type of Legionella – in most regions of the country.

Overall, 144 unique Legionella isolates were recovered; 76 (53 percent) were Legionella pneumophila  and 51 percent of these were Lp sg 1, the most commonly detected cause of Legionnaires’ disease. Sixty-eight isolates (47 percent) were non-Lp spp., including L. anisa (50 percent), L. feeleii (12 percent), and L. rubrilucens (6 percent), all of which cause the disease.

The rate of reported cases of Legionnaires’ disease increased 286 percent in the U.S. between 2000 and 2014.

Cooling towers are known to be a major source of outbreaks of Legionnaires’ disease. The CDC study is the first to show how widespread Legionella may be in these devices across the country, its authors said.

Dr. Anna Llewellyn presented the study last April at a CDC conference in Atlanta and said it would be published in a peer-reviewed journal this summer.

“Legionella DNA is ubiquitous in U.S. cooling towers,” Llewellyn, a fellow at the pneumonia response and surveillance laboratory at the CDC, told The Wall Street Journal in a story published May 3, who called the findings “surprising.” That “highlights the potential for cooling-tower-related outbreaks to occur anywhere in the U.S.”

Llewellyn, however, added that just because a cooling tower has Legionella doesn’t necessarily mean it is spreading disease. The researchers plan further research to determine what factors contribute to a cooling tower’s risk of becoming a source of an outbreak.

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