Two outbreaks of Legionnaires’ Disease have been traced to private use hot tubs aboard cruise ships, according to a recent report from the Centers for Disease Control and Prevention.

In its October 24 Morbidity and Mortality Weekly Report, the CDC announced that between November 2022 and June 2024 it was notified of 12 cases of non-fatal Legionnaires’ Disease among travelers on two separate cruise ships; “eight on cruise ship A and four on cruise ship B.”

The outbreak on cruise ship A was the largest cruise-associated Legionnaires’ Disease outbreak investigated by the CDC since 2008.

After the first outbreak on cruise ship A, which took place between December 2022 and May 2023, the CDC reviewed water testing results from representative points of use, including the cabins of infected patients, heat exchangers, decorative fountains, and public hot tubs. No Legionella was found. Furthermore, the CDC found that the cruise ship’s operation and maintenance records showed that its water parameters met CDC requirements.

Testing of private balcony hot tubs

According to current CDC requirements, common area hot tubs are required to have automated continuous disinfectant dosing and monitoring or pH monitoring, as is standard for public hot tubs.

By Marcelle Dibrell was another story: six of 10 private balcony hot tubs tested positive for the bacteria.

In the second outbreak, which took place between January and June, 2024, on cruise ship B, the CDC requested immediate closure of all hot tubs on the ship, including those in both common areas and private balconies.

Legionella was not found in the common areas, but all eight private balcony hot tubs tested positive.

“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,” the CDC stated in their report. “To meet CDC requirements, private hot tubs must only be shock-chlorinated, drained, and refilled weekly or between occupancies, whichever is sooner.

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.”

The authors of the report recommended adapting public hot tub maintenance and operations protocols for use on private balcony hot tubs.