Asymptomatic infection of Legionella pneumophila in four cases with pulmonary diseases

In view of the wide-spread existence of legionellae in cooling-tower and other environmental water, asymptomatic infection of this organism could occur. In order to verify the possibility of colonization of legionellae at lower respiratory tract of patients with various pulmonary diseases, a total of 22,036 sputum samples from in- and out-patients at National Sanyoso Hospital were examined during a five-year period from September, 1984 to August, 1989. Four (0.073%) out of 5,502 cases were culture-positive for L. pneumophila. L. pneumophila strains were isolated from expectorated, subsequently washed sputum samples of two male and two female patients with respiratory tract diseases. The identification of the isolates was genetically confirmed by the fluorometric microplate DNA-DNA hybridization method. The serogroup (SG) and viable counts of L. pneumophila per ml of sputum of each patient were as follows: 73 y/o female K.H., SG-6, 10(3) CFU; 75 y/o male H.J., SG-5, 10(4) CFU; 61 y/o female M.S., SG-5, 10(5) CFU; and 77 y/o male M.G., not-agglutinable against SG-1-6 antisera, 10(4) CFU. None of the four patients was clinically suspected of legionellosis and antibody titer of paired sera remained 1:64 or lower than 1:32. From these findings, we concluded that L. pneumophila can cause, though quite rarely, asymptomatic infection in human respiratory tract. None of the environmental samples obtained from in- and out-side of the Hospital was culture-positive for legionellae. Thus, the source of infection has remained unknown.