Today in Morbidity and Mortality Weekly Report (MMWR), researchers from the US Centers for Disease Control and Prevention (CDC) describe two cruise-ship outbreaks of Legionnaires' disease tied to private balcony hot tubs.
From November 2022 to June 2024, 12 cases of Legionnaires' disease were reported to the CDC among travelers on two cruise ships, with eight on ship A and four on ship B. The ship A outbreak was the largest cruise Legionnaires' disease outbreak that the CDC had investigated since 2008. Legionnaires' is a serious type of pneumonia caused by Legionella bacteria.
Private hot tubs subject to less-stringent requirements
CDC investigators interviewed the ill travelers and analyzed environmental samples, concluding that private balcony hot tubs were the most likely source of the bacteria.
Hot tubs offer favorable conditions for Legionella growth and transmission when maintained and operated inadequately, regardless of location.
Hot tubs on ship A had been operating for months under conditions conducive to Legionella growth, which included maintaining a water temperature of 77°F to 113°F for multiple days without draining or residual disinfectant.
Some tubs on this ship were located on decks only one floor above or below common outdoor amenities. The CDC noted that previous investigations have shown that hot tubs in private areas can spread pathogens via aerosols to common areas and expose people in those areas—even those who don't use the tubs themselves.
"Hot tubs offer favorable conditions for Legionella growth and transmission when maintained and operated inadequately, regardless of location," the researchers wrote. "Private hot tubs on cruise ships are not subject to the same maintenance requirements as are public hot tubs in common areas."
The investigators recommend that cruise-ship water-management staff inventory and evaluate private balcony hot tubs and adapt maintenance and operations procedures used for public hot tubs for use in private outdoor hot tubs. Both cruise lines modified the operation and maintenance of the private hot tubs by removing the heating elements, draining water between uses, and cleaning and hyperchlorinating them more often.
Ship A also removed filtration elements from the tubs. Sampling is ongoing on both ships.