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Update: Outbreak of Legionnaires' Disease Associated with a Cruise Ship, 1994

On July 15, 1994, CDC was notified by the New Jersey State Department of Health of six persons with pneumonia who had recently traveled to Bermuda on the cruise ship Horizon (1). In conjunction with local and state health departments, an investigation was initiated; as of August 10, a total of 14 passengers had Legionnaires' disease (LD) confirmed by either sputum culture (one patient), detection of antigens of Legionella pneumophila serogroup 1 (Lp1) in urine by radioimmunoassay (seven patients) (2), or fourfold rise in titer of antibodies to Lp1 between acute- and convalescent-phase serum specimens (six patients). Possible cases in 28 other passengers with pneumonia that occurred within 2 weeks after sailing aboard the Horizon are under investigation. Cases have occurred from nine separate week-long cruises during April 30- July 9, 1994.

To identify the source of Legionella sp., a case-control study was conducted, and environmental sampling of the ship's water system was performed. Exposure to the whirlpool baths was strongly associated with illness (odds ratio=16.4; 95% confidence interval=3.7-72.3). Cultures taken from a sand filter used for recirculation of whirlpool water yielded an isolate of Lp1; this isolate and the clinical isolate had matching monoclonal antibody subtyping patterns (3).

A variety of interventions were completed, including hyperchlorination of the ship's potable water supply, removal of the whirlpool filters, and discontinuation of the whirlpool baths. Following completion of these interventions, on July 30 the Horizon resumed its weekly sailing schedule from New York City to Bermuda.

Reported by: I Guerrero, MD, Community Medical Center, Toms River; C Genese, MJ Hung, S Paul, MD, H Ragazzoni, DVM, J Brook, MD, L Finelli, PhD, KC Spitalny, MD, State Epidemiologist, New Jersey State Dept of Health. BA Mojica, MD, KJ Mahoney, MSW, RT Heffernan, MPH, Div of Disease Intervention, New York City Dept of Health; SF Kondracki, DL Morse, MD, State Epidemiologist, New York State Dept of Health. ML Cartter, MD, J Hadler, MD, State Epidemiologist, Connecticut Dept of Public Health and Addiction Svcs. JT Rankin, Jr, DVM, State Epidemiologist, Pennsylvania Dept of Health. C Groves, MS, Maryland State Dept of Health and Mental Hygiene. Div of Quarantine, National Center for Prevention Svcs; Office of the Director, National Center for Environmental Health; Div of Field Epidemiology, Epidemiology Program Office; Childhood and Respiratory Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This outbreak represents the first documented instance of LD aboard a cruise ship docking in U.S. ports. Whirlpool spas previously have been associated with transmission of Legionella (4,5); hyperchlorination of water systems and replacement of filter devices have successfully terminated outbreaks of LD linked to whirlpool spas. CDC recommends post-intervention environmental sampling of whirlpool circulation systems in conjunction with ongoing surveillance for cases of pneumonia to ensure the efficacy of these interventions. Suspected cases of LD among Horizon passengers should be reported to CDC through state and local health departments.

Additional recommendations to reduce the transmission of Legionella sp. from whirlpool baths and aboard cruise ships will be the subject of a special meeting of public health officials, LD experts, and members of the whirlpool and cruise line industries; the meeting is tentatively scheduled for the fall of 1994 in Atlanta. Additional information about the meeting is available from CDC's Office of the Director, National Center for Environmental Health, telephone (404) 488-7093.


  1. CDC. Outbreak of pneumonia associated with a cruise ship, 1994. MMWR 1994;43:521.

  2. Kohler RB, Zimmerman SE, Wilson E, et al. Rapid radioimmunoassay diagnosis of Legionnaires' disease: detection and partial characterization of urinary antigen. Ann Intern Med 1981;94:601-5.

  3. Joly JR, McKinney RM, Tobin JO, Bibb WF, Watkins ID, Ramsay D. Development of a standardized subgrouping scheme for Legionella pneumophila serogroup 1 using monoclonal antibodies. J Clin Microbiol 1986;23:768-71.

  4. Vogt RL , Hudson PJ, Orciari L, Heun EM, Woods TC. Legionnaires' disease and a whirlpool spa {Letter}. Ann Intern Med 1987;107:596.

  5. Spitalny KC, Vogt RL, Orciari LA, Witherell LE, Etkind P, Novick LF. Pontiac fever associated with a whirlpool spa. Am J Epidemiol 1984;120:809-16.

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