October 31, 2024

CDC Links Cruise Ship Legionnaires’ Outbreaks to Private Hot Tubs

Separate outbreaks of Legionnaires’ disease that affected 12 passengers on board cruise ships have been traced to hot tubs on private balconies, according to a new CDC report. The outbreaks occurred on two vessels starting in November 2022. All 12 passengers recovered, although 10 required hospitalization.

On the vessel that CDC designates Ship A, five cases of Legionnaires’ disease were identified among patients who traveled on the same voyage in November 2022. Three additional cases occurred among passengers on voyages in August and September 2023 and March 2024. Sampling and review of the ship’s cleaning and disinfection procedures found that all CDC’s requirements had been met, but the investigation focused only on the potable water system and the hot tubs in common areas. It wasn’t until the sixth passenger, who traveled in late summer 2023, reported using a private hot tub that CDC zeroed in on what was ultimately determined to be the source of the outbreak. Subsequent sampling found Legionella in six of 10 private hot tubs with concentrations in two tubs exceeding 1,000 colony-forming units per milliliter (CFU/mL).

CDC requirements for cleaning and disinfecting private hot tubs are not as stringent as they are for public facilities, the report notes. Private hot tubs need only “shock chlorination,” and CDC allows refilling to occur weekly or between occupancies, whichever is sooner. Public hot tubs, in contrast, must be continuously dosed with disinfectant and their pH monitored.

When CDC learned of the outbreak on Ship B, which affected passengers on voyages in January, February, and May 2024, the agency requested that the crew close all hot tubs, including both private and public facilities, and conduct sampling. Legionella was found in all eight private hot tubs on board.

According to the CDC report, despite the fact that both ships met current agency guidelines, the private hot tubs “were found to be operating for months in a manner conducive to Legionella growth, which included maintaining a water temperature in the Legionella growth range (77°F–113°F [25°C–45°C]) for multiple days without draining and [. . .] with no residual disinfectant.”

The cruise lines responsible for the ships in question have implemented changes as a result of the outbreaks. For private hot tubs, heating elements have been removed, and the tubs are filled only upon a passenger’s request, emptied after each use, and cleaned and disinfected more often.

Read more about the outbreaks in the Oct. 24 edition of CDC’s Morbidity and Mortality Weekly Report.

Related: Read “Performing Legionella Source Risk Assessments” in The Synergist.