During February-July 2024, 4 confirmed cases of Legionella infections among passengers on Cruise Ship "B" were detected. Two of the cases occurred in passengers traveling on the same voyage in January 2024. One of the passengers traveled on two consecutive voyages. Two additional cases (Cases 5 and 6) traveled in February and May, respectively. Three patients received a positive Legionella urinary antigen test result, and one received a positive culture test result in which L. pneumophila was detected; four patients were hospitalized, and no patients died.

In response to the outbreak, 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. L. pneumophila serogroup 1 (Lp1) and Lp2–14 species were detected in all eight private balcony hot tubs on the ship, and Lp2–14 was detected in a single location in the potable water system. Of the testing performed on the eight private balcony hot tubs, two samples had Lp1 concentrations >10 CFU/mL. All balcony hot tubs remained closed until each had nondetectable Legionella post-remediation sampling results.