Two outbreaks of Legionnaires disease have been linked with outdoor hot tubs for private use on two cruise ships, according to a report by the Centers for Disease Control and Prevention (CDC).
Released on Thursday, the report found that between November 2022 and June 2024, 12 cases of Legionnaires’ disease were reported among travelers on two separate cruise ships—referred to as ships A and B in the report.
The first ship was associated with eight cases, and ship B was linked to four cases.
Health officials working in collaboration with cruise lines found that private hot tubs on certain cabin balconies were most likely to expose passengers to Legionella bacteria.
“Hot tubs offer favorable conditions for Legionella growth and transmission when maintained and operated inadequately, regardless of location,” the CDC report states.
The agency noted that private hot tubs on cruise ships are not subject to the same stringent maintenance requirements as public hot tubs.
“Both cruise lines ultimately modified the operation and maintenance of the private hot tubs so that heating elements were removed; tubs were only filled upon guest request, drained between uses, and cleaned and disinfected more frequently,” the report said. “Ship A devices were additionally modified to remove filtration elements. Sampling is ongoing for both vessels.”
The eight infections reported on cruise ship A is the largest cruise-associated Legionnaires’ disease outbreak investigated by the CDC since 2008, with five of the cases from passengers on the same trip in November 2022. Six of the ship’s 10 hot tubs tested positive for the bacteria, according to the report.
Four confirmed cases of Legionnaires’ disease were reported on cruise ship B between January and June 2024. Environmental testing found the bacteria in the ship’s eight private balcony hot tubs.
The report said the cruise ships did, however, adhere to current federal requirements, but this wasn’t sufficient to prevent the bacteria’s growth.
“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,” officials wrote.
To prevent additional outbreaks, health officials recommend cruise ship owners inventory any hot tub-style devices across their fleets and include them in their water management programs and adopt public hot tub “maintenance and operation protocols” for private hot tubs.
The CDC advised cruise lines and the hospitality industry to be aware of hot tub features that increase the risk of Legionella bacteria. These features include systems related to recirculation, retention of water between uses, filtration, and heating systems.