An Outbreak of Pontiac Fever due to Legionella pneumophila Serogroup 7: II. Epidemiological Aspects

From August 20 to 22, 1994, an outbreak of acute febrile illness occurred in a Training Center building of a company in Shibuya-ku., Tokyo. All 43 trainees attended in two groups and 2 Center staffs were attacked. Illness was self-limiting, generally lasting three days. Though strains of legionellae, isolated from the water of the cooling tower located at the top of the building, were identified as Legionella pneumophila by microplate DNA-DNA hybridization, they failed to agglutinate with antisera against L. pneumophila serogroups 1 through 6. Two strains were sent to the Centers for Disease Control, Atlanta, Georgia, USA, and determined as serogroup 7 of the species. Since the clinical courses agreed with the definition of Pontiac fever by Glick et al. and seroconversion in a patient against the cooling tower strain (EY3698) from 1: 16 to 1: 256 was determined by indirect fluorescent antibody technique, the epidemic of acute febrile illness was concluded as an outbreak of Pontiac fever due to L. pneumophila serogroup 7. The cooling tower was a cylindrical open style, with volumetric flow rate of 130 liter/min, and was used for air-conditioning exclusively to the third floor of the building. The building equipped no air-inlet, and indoor-air of the training room exchanged at every break time through windows of 168 cm in height and 72 cm in width. The cooling tower was not operated for five days before the Group A trainees checked in the Center on 18 August followed by Group B trainees on 19 August. It was speculated that high atomospheric temperature and stagnation of cooling water during this period would lead L. pneumophila to overly multiply, which could be a source of infection by flowing in through opened windows to the training rooms.