Characterization of clinical and environmental isolates of Legionella associated with outbreaks and study of the infection sources

BACKGROUND: The study of isolates of Legionella related with six outbreaks of community-acquired legionellosis and seven nosocomial outbreaks is presented. METHODS: The isolates were serogrouped by indirect immunofluorescence testing and those belonging to serogroup (SG) 1 L. pneumophila strains were subtyped using the international panel of monoclonal antibodies. RESULTS: SG 1 L. pneumophila appeared to be the etiologic agent of all the community outbreaks analyzed and the main one of the nosocomial cases. However, in the latter situation, cases produced by SG3, SG4, SG6, SG8, SG8, 10 y SG4,8,10 were also found. Among subgroup 1 Legionella, the Pontiac, Philadelphia 1 or Allentown 1 subtypes were responsible for four out of the six community and three of the seven nosocomial outbreaks. The Pontiac, Knoxville 1 subtype was the cause of a community-outbreak and of another nosocomial outbreak, and the Pontiac, Benidorm 030E caused a community and another hospital outbreak, also being the main agent found in another hospital. The remaining subtypes appeared in small numbers in some hospitals, where cases due to different isolates coexisted. Likewise, nosocomial cases produced by SGs other than SG1 (Sg 4, 6 and 8) were found in one hospital (with SG 8 also being found in environmental samples) and Sg 4, 8, 10 together with Sg 3 in another (both SGs were also found in different hospital facilities). The sources of infection found were air conditioning systems in two cases and the lavatory water system in the remaining cases, with no colonization being found in a series of natural sources (spring water, wells, water tanks, watering systems) also studied in several of the outbreaks. CONCLUSIONS: These results with suggest that although building installations are colonized by a large variety of species, SGs and subtypes of Legionella, only some (mainly SG 1 L. pneumophila, Pontiac subtype) actually produces outbreaks. This is specially relevant in community outbreaks, while in hospital outbreaks other species or SGs may produce infection in isolated cases or outbreaks due to the presence of highly susceptible hosts.