Strengths and limitations of molecular subtyping in a community outbreak of Legionnaires' disease

An epidemiological and microbiological investigation of a cluster of eight cases of Legionnaires' disease in Los Angeles County in November 1997 yield ed conflicting results. The epidemiological part of the investigation impli cated one of several mobile cooling towers used by a him studio in the cent re of the outbreak area. However, water sampled from these cooling towers c ontained L. pneumophila serogroup 1 of another subtype than the strain that was recovered from case-patients in the outbreak. Samples from two cooling towers located downwind from all of the case-patients contained a Legionel la strain that was indistinguishable from the outbreak strain by four subty ping techniques (AP-PCR, PFGE, MAb, and MLEE). It is unlikely that these co oling towers were the source of infection for all the case-patients, and th ey were not associated with risk of disease in the case-control study. The outbreak strain also was not distinguishable, by three subtyping techniques (AP-PCR, PFGE, and MAb), from a L. pneumophila strain that had caused an o utbreak in Providence, RI, in 1993. Laboratory cross-contamination was unli kely because the initial subtyping was done in different laboratories. In this investigation, microbiology was helpful for distinguishing the outb reak cluster from unrelated cases of Legionnaires' disease occurring elsewh ere. However, multiple subtyping techniques failed to distinguish environme ntal sources that were probably not associated with the outbreak. Persons i nvestigating Legionnaires' disease outbreaks should be aware that microbiol ogical subtyping does not always identify a source with absolute certainty.