Legionnaires Disease From Private Balcony Hot Tubs on Two Cruise Ships

Legionnaires disease is a serious pneumonia caused by Legionella bacteria, and poorly maintained hot tubs are a known source. Between November 2022 and June 2024, CDC was told of 12 cases among travelers on two cruise ships — eight on ship A and four on ship B. The investigations traced both outbreaks to a source not identified before: private hot tubs on cabin balconies.

Balcony hot tubs on the two ships, before they were modified. Each had features that raise the risk of Legionella growth: aerosol-generating jets, water kept between uses, a heater, and recirculation and filtration systems. Credit: CDC Vessel Sanitation Program.

The patients

Ship A: a long search

CDC was told of five cases among ship A travelers between December 2022 and May 2023; five patients had been on the same November 2022 voyage. That makes ship A the largest cruise-associated Legionnaires outbreak CDC has investigated since 2008.

The usual suspects came up clean. Of 150 routine water samples from August 2022 to February 2023, only one had found Legionella, in the drinking water system, and it was gone after local hyperchlorination. Water-quality readings were within limits, and the public hot tubs met CDC's requirements. In March 2023, the cruise line took 260 one-liter samples — from taps, patients' cabins, heat exchangers, water tanks, decorative fountains and public hot tubs — and found none; 76 more samples that spring and summer were also negative.

The break came in August 2023, when a sixth patient reported using a private balcony hot tub. Those tubs had never been tested. CDC had all 10 closed and sampled: Legionella turned up in six, four of them above 100 colony-forming units per milliliter and two above 1,000. The tubs stayed shut until new operating procedures produced clean results. From March 2024 to August 2024, about 300 samples found no Legionella.

Ship B: found fast

CDC learned of four cases on ship B between February and July 2024 — two from the same January voyage, others in February and May. This time CDC immediately asked for every hot tub, public and private, to be closed and sampled. L. pneumophila was in all eight balcony hot tubs (and at one spot in the drinking water system). Two tubs kept testing positive as the line changed its procedures, prompting more cleanup and tighter protocols. Finding the same problem on a second ship strengthened the case that balcony tubs were the source.

Why the tubs grew Legionella

On ship A, the balcony tubs had run for months in conditions that favor Legionella: water held for days at 77°F–113°F (25°C–45°C) without draining, with no residual disinfectant. Some sat one deck above or below shared outdoor areas — and earlier investigations have shown that private hot tubs can spread aerosols to common areas, infecting people who never used them.

The rules were part of the problem. CDC's Vessel Sanitation Program requires public hot tubs to have automatic, continuous disinfectant dosing and monitoring, and pH monitoring. Private hot tubs need only be shock-chlorinated, drained and refilled weekly or between occupants, whichever comes first. Both cruise lines followed those rules — and they weren't enough.

What changed

• Both lines removed the heaters, filled tubs only when a guest asked, drained them between uses, and cleaned and disinfected them more often. Ship A also removed the filters.
• CDC put out two calls for cases through its Epidemic Information Exchange and alerted the European Centre for Disease Prevention and Control, since both ships sailed European itineraries. The lines told guests and crew about the possible exposure.
• CDC warned the cruise industry about balcony hot tubs in December 2023 and June 2024.

Limitations

• No patient samples were available to compare genetically with the bacteria from the tubs.
• Patients' tests pointed to L. pneumophila serogroup 1, while ship A's tubs grew other types — though any Legionella shows conditions that let it grow.
• Several patients had other possible exposures, such as hotels and shore excursions, but the ships were the only exposure they all shared.

The lesson for the industry

Balcony hot tubs are becoming more common as new ships launch and old ones are renovated. Hot-tub-style devices — public tubs, jetted bathtubs, hydrotherapy pools — are everywhere in cruising, hotels and recreation. Features that raise the risk include outdoor use, water kept between uses, and recirculation, filtration or heating systems. The authors urge cruise lines to inventory every such device, include them in their water management programs, test them for Legionella, and run private outdoor hot tubs to the same standards as public ones.

Sources

Based on Lee S, Edens C, Ritter T, et al., "Two Outbreaks of Legionnaires Disease Associated with Outdoor Hot Tubs for Private Use — Two Cruise Ships, November 2022–July 2024," Morbidity and Mortality Weekly Report 73(42), Centers for Disease Control and Prevention; a work of the United States government in the public domain. Original report.