OUTBREAK OF LEGIONNAIRES-DISEASE AMONG CRUISE SHIP PASSENGERS EXPOSEDTO A CONTAMINATED WHIRLPOOL SPA

Background Outbreaks of travel-related Legionnaires' disease present a public-health challenge since rapid, sensitive, and specific diagnost ic tests are not widely used and because detection of clusters of dise ase among travellers is difficult. We report an outbreak of Legionnair es' disease among cruise ship passengers that occurred in April, 1994, but that went unrecognised until July, 1994. Methods After rapid diag nosis of Legionnaires' disease in three passengers by urine antigen te sting, we searched for additional cases of either confirmed (laborator y evidence of infection) or probable Legionnaires' disease (pneumonia of undetermined cause). A case-control study was conducted to compare exposures and activities on the ship and in ports of call between each case-passenger and two or three matched control-passengers. Water sam ples from the ship, from sites on Bermuda, and from the ship's water s ource in New York City were cultured for legionellae and examined with PCR. Findings 50 passengers with Legionnaires' disease (16 confirmed, 34 probable) were identified from nine cruises embarking between Apri l 30 and July 9, 1994. Exposure to whirlpool spas was strongly associa ted with disease (odds ratio 16.2, 95% CI 2.8-351.7); risk of acquirin g Legionnaires' disease increased by 64% (95% CI 12-140) for every hou r spent in the spa water. Passengers spending time around the whirlpoo l spas, but not in the water, were also significantly more likely to h ave acquired infection. Legionella pneumophila serogroup 1 was isolate d only from the sand filter in the ship's whirlpool spa. This isolate matched a clinical isolate from the respiratory secretions of a case-p assenger as judged by monoclonal antibody subtyping and by arbitrarily primed PCR. Interpretation This investigation shows the benefit of ob taining a recent travel history, the usefulness or urine antigen testi ng for rapid diagnosis of legionella infection, and the need for impro ved surveillance for travel-related Legionnaires' disease. New strateg ies for whirlpool spa maintenance and decontamination may help to mini mise transmission of legionellae from these aerosol-producing devices.