Private Hot Tubs on Cruise Ships May Carry Increased Legionnaires Disease Risks: CDC

Two outbreaks of Legionnaires disease on cruise ships have been linked to private hot tubs on the cabin balconies of two different passenger liners, resulting in at least a dozen illnesses in recent years, according to a new report issued by federal health officials.

The U.S. Centers for Disease Control and Prevention (CDC) issued an update in the most recent edition of it’s Morbidity and Mortality Weekly Report, outlining the results of investigations into the outbreaks, which occurred between November 2022 and June 2024, suggesting that  private hot tubs on some cruise ships may not be disinfected as thoroughly as public ones.

Legionnaires Disease Risks

Legionnaires disease is a serious form of pneumonia caused by Legionella bacteria, which thrives in water between 77 and 113 degrees. Hot tubs with low levels of chlorine can allow bacteria to flourish and multiply rapidly. The bacteria can also cause biofilms, or thin layers of bacteria, on the surfaces of the hot tub, making it hard to get rid of even with disinfection protocols.

When the hot tub releases water sprays and mists during normal operation, the bacteria can be carried into the air and people using the hot tubs can breathe it in. Inhaled water droplets infected with Legionnaires can come not only from hot tubs, but also air humidifiers, or air conditioning.

Symptoms include cough, fever, muscle aches and shortness of breath. The disease can also lead to complications in people with weakened immune systems.

Most individuals infected with Legionnaires disease require hospitalization and are treated with antibiotics and oxygen. Only about 10% of patients will die, but if individuals are not promptly treated, the fatality rate can rise to about 30%, making the infection particularly dangerous for cruise ship passengers.

The CDC began investigating the two outbreaks after 12 cases of Legionnaires disease were reported among passengers on two different cruise ships, eight on the first ship and four on the second. The CDC report does not identify the two specific cruise liners involved in the Legionnaires outbreaks.

Testing determined the Legionnaires exposure came from private balcony hot tubs in passenger rooms. These cruise ship hot tubs are not subject to the same stringent operating requirements as hot tubs located in public areas, according to the investigators.

For example, private hot tubs are not subject to the same testing as public ones, and are not required to have automated continuous disinfectant dosing and monitoring or PH monitoring, which are standard for public hot tubs. Additionally, they only have to be drained and refilled weekly or between occupancies, whichever is first.

According to the CDC report, both cruise lines modified the operation and maintenance of the hot tubs by removing the heating elements and draining water between traveler uses. They also increased the frequency of chlorine treatment and cleaning to minimize Legionella growth. However, the report indicates the operating protocols taken by the companies for the private hot tubs are not sufficient to prevent infections.

CDC officials recommend that cruise ship operators inventory their hot tubs across the entire fleet and determine if some design features may increase the risk for Legionella growth and transmission. Every hot tub should be tested for Legionella, and the same protocols should be applied to both public and private hot tubs, CDC researchers concluded.

Legionnaires Disease Outbreaks Often Due to Infected Hot Tubs

Hot tubs played a role in a massive Legionnaires disease outbreak sickening more than 150 people and killing four in 2019. That outbreak was linked to a hot tub display at the North Carolina State Fair.

People affected by the outbreak didn’t even sit in the water, they simply walked by the hot tub display and inhaled contaminated droplets of water.

A CDC report issued in March 2024 indicated pathogens like Legionella and nontuberculous mycobacteria are the leading causes of waterborne disease outbreaks in the U.S. These bacteria are both commonly found in biofilms.

Waterborne pathogens are responsible for more than 7 million illnesses, 118,000 hospitalizations, and nearly 6,630 deaths every year in the United States.