Hot tubs aboard a cruise ship have been linked to outbreaks of a bacterial disease, the U.S. Centers for Disease Control and Prevention (CDC) announced.
Between November 2022 and April 2024, eight cases of Legionnaires' disease were reported on a single cruise ship, "cruise ship A."
The outbreak on cruise ship A is the largest cruise-associated outbreak of Legionnaires' disease since 2008. A CDC investigation found that the hot tubs on the ships were the likely source of the infection.
Four more cases of the disease were reported on a second cruise ship (cruise ship B) between January and June 2024. Hot tubs were also found to be the cause.
"Epidemiologic, environmental, and laboratory evidence suggests that private balcony hot tubs were the likely source of exposure in two outbreaks of Legionnaires disease among cruise ship passengers," the CDC said in the report.
"These devices are subject to less stringent operating requirements than are public hot tubs, and operating protocols were insufficient to prevent Legionella growth."
Legionnaires' disease is a severe form of pneumonia caused by Legionella bacteria, usually the species often Legionella pneumophila. Legionella bacteria thrive in warm water between 77 and 113 degrees Fahrenheit, which makes hot tubs a prime environment for their growth.
"It gets into the human body through inhaled water droplets, usually from hot tubs, air humidifiers, or air conditioning that contain the bacteria. Of note, it cannot be passed from person to person," Conor Meehan, an associate professor of microbial bioinformatics at Nottingham Trent University, told Newsweek.
Legionnaires' disease gets its name from an outbreak in 1976 at an American Legion convention in Philadelphia. Shortly after the event, many attendees began falling ill with a mysterious type of pneumonia, and scientists identified a previously unknown bacterium as the cause. The bacteria were named Legionella pneumophila.
This disease mainly affects the lungs, causing symptoms like cough, fever, muscle aches and shortness of breath. Sometimes, it can lead to other complications, especially in people with weakened immune systems, older adults, smokers or those with chronic respiratory conditions.
"Patients with the infection are often hospitalized and given oxygen and antibiotic therapy," Primrose Freestone, a senior lecturer in clinical microbiology at the University of Leicester, told Newsweek. "In terms of mortality, Legionella causes a serious lung infection, and while most survive Legionnaires disease, even after treatment with antibiotics, up to 10 percent of those infected die. With no treatment, up to 30 percent of patients infected with Legionella pneumoniae can die."
Six infected patients on cruise ship A and four on cruise ship B were hospitalized, but thankfully, none died, according to the CDC report.
Hot tubs with too low chlorine levels may allow bacteria to survive and multiply. Additionally, bacteria can form biofilms (thin layers of microorganisms) on pipes and surfaces, which makes them harder to eliminate, even with disinfection efforts. When the hot tub releases fine sprays or mists, the bacteria can be carried into the air and breathed in by people in the tubs.
"Cruise ship hot tubs are counted as private hot tubs and not public, so they are not subjected to the same rigor of testing by the state. If not properly maintained, cleaned and heated, they can become infected with Legionella pneumophilia. Combining this with the often more at-risk population in cruise ships—this can lead to more cases," Meehan said.
People on cruise ships are often older than the general population and, therefore, more vulnerable to infections like Legionnaire's disease.
"Morbidity and mortality statistics indicate that older people are more susceptible to serious infection and complications often because they have existing co-morbidities (health conditions), which mean they are more likely to be immunocompromised. It's a bit like with COVID," Anne Clayson, a Population Health program director at the University of Manchester, told Newsweek.
These two outbreaks highlight the importance of hot tub cleanliness, particularly private hot tubs.
"According to current CDC requirements, private hot tubs are not required to have automated continuous disinfectant dosing and monitoring or pH monitoring, as is standard for public hot tubs. To meet CDC requirements, private hot tubs must only be shock-chlorinated, drained, and refilled weekly or between occupancies, whichever is sooner," the CDC report said.
"Although the cruise lines adhered to current CDC requirements for operating and maintaining private hot tubs on ships A and B, these measures were insufficient to prevent Legionella growth."
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