A large outbreak of Legionnaires' disease due to an inadequate circulating and filtration system for bath water--epidemiologic manifestations.

Objective: To study the epidemiologic manifestations of a large outbreak of Legionnaires' disease due to an inadequate circulating and filtration system for bath water.

Patients: In June 2000 at Ishioka City, Ibaraki Prefecture, a large outbreak of Legionnaire's disease occurred, as a result of an inadequate circulating and filtration system for communal bath water. This outbreak was the worst ever experienced in Japan, involving a total of 34 patients (20 confirmed, 14 probable), 3 of whom died.

Measurements And Results: Legionella pneumophila serogroup 1 was isolated from sputum culture in two patients. Bacteriological culture of the public bath water subsequently yielded large numbers of Legionella species. Cleavage of genomic DNA showed that restriction fragment patterns coming from clinical and environmental isolates of L. pneumophila serogroup 1 were closely related, focusing the inquiry to a public bathhouse where a circulating filtration system was suspected as the source of infection.

Conclusions: It was later concluded that the circulating filtration system adopted for bath water was marred by a serious design flaw that subsequently caused the mass outbreak. Specifically, a line of the bath water was being returned to the bath without undergoing heat exchange or sterilization by chlorine; and the Legionella species that had proliferated in the filter and the bright stone filtration unit were allowed to return to the bath, eventually culminating in a mass infection.