Private hot tubs on two cruise ships have been linked to outbreaks of a bacterial lung infection known as legionnaires’ disease, which is a “serious pneumonia,” the U.S. Centers for Disease Control and Prevention (CDC) warns.

On Oct. 24, the CDC warned that two outbreaks of legionnaires’ disease had been linked to private-use outdoor hot tubs on two cruise ships between November 2022 and July 2024, sickening 12 people.

The CDC did not state the names of the cruise ships, only that both included European, Caribbean and Mediterranean itineraries.

“Epidemiologic, environmental, and laboratory evidence suggests that private balcony hot tubs were the likely source of exposure in two outbreaks of Legionnaires disease among cruise ship passengers,” the CDC stated in the report.

“These devices are subject to less stringent operating requirements than are public hot tubs, and operating protocols were insufficient to prevent Legionella growth.”

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Legionnaires’ disease is a serious respiratory illness caused by legionella bacteria, which thrive in water and can lead to severe pneumonia, according to the Public Health Agency of Canada (PHAC).

Named after a 1976 outbreak that killed 29 people at an American Legion convention, legionnaires’ disease remains underreported, with fewer than 100 cases reported annually in Canada, though the true number is likely much higher, PHAC added.

People over the age of 40 and those who are immunocompromised are especially at risk of infection, PHAC added.

From December 2022 to May 2023, the CDC was first notified of five cases of legionnaires’ disease among people who travelled on “cruise ship A” during a 14-day exposure period. All five cases were among passengers travelling on the same ship in November 2022.

Two additional cases linked to separate voyages on cruise ship A were confirmed and reported to the CDC between August and September 2023. In April 2024, another laboratory-confirmed case was identified in a guest who had travelled on the ship the previous month.

Six patients were hospitalized and no patients died.

Following the investigation, the CDC requested the closure and sampling of all 10 private balcony hot tubs on the ship, detecting legionella bacteria in six of them.

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The outbreak on cruise ship A is the largest cruise-associated legionnaires’ disease outbreak investigated by the CDC since 2008.

The second outbreak occurred on “cruise ship B,” with the CDC receiving reports of four confirmed cases of legionnaires’ disease among passengers who travelled on the ship between February and July 2024. Four patients were hospitalized, but none died.

In response to the outbreak, the CDC, “requested immediate closure of all hot tubs on the ship, including those in common areas and private balconies, and sampling of all hot tubs and representative potable water locations.”

The bacteria was detected in all eight private balcony hot tubs on the ship.

Cruise operators of both ships notified guests and crew of the potential for legionella exposure while the investigations were ongoing, the CDC said.

“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. Ship A devices were additionally modified to remove filtration elements. Sampling is ongoing for both vessels,” the CDC said.

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Legionnaires’ disease is not common, and the risk of getting it is generally quite low, PHAC said.

Cases of legionnaires’ disease may be difficult to detect because very few of the people exposed to the bacteria get infected. For example, PHAC said if 100 people are exposed to the bacteria, fewer than five of them will get legionnaires’ disease.

The disease is spread from breathing in legionella bacteria via mist or water vapour from a contaminated water source, such as evaporating cooling systems, hot tubs and decorative water features.

Older adults and immunocompromised people are at greater risk of illness.

In addition to pneumonia, symptoms can include fever, cough, muscle pain and headache. Symptoms start within two to 14 days of infection and can last for several months.

If symptoms persist, PHAC advises consulting your health-care provider, as most cases can be effectively treated with antibiotics.