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

Abstract

An epidemiological and microbiological investigation of a cluster of eight cases of Legionnaires' disease in Los Angeles County in November 1997 yielded conflicting results. The epidemiological part of the investigation implicated one of several mobile cooling towers used by a film studio in the centre of the outbreak area. However, water sampled from these cooling towers contained 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 Legionella strain that was indistinguishable from the outbreak strain by four subtyping techniques (AP-PCR, PFGE, MAb, and MLEE). It is unlikely that these cooling towers were the source of infection for all the case-patients, and they 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 outbreak in Providence, RI, in 1993. Laboratory cross-contamination was unlikely because the initial subtyping was done in different laboratories. In this investigation, microbiology was helpful for distinguishing the outbreak cluster from unrelated cases of Legionnaires' disease occurring elsewhere. However, multiple subtyping techniques failed to distinguish environmental sources that were probably not associated with the outbreak. Persons investigating Legionnaires' disease outbreaks should be aware that microbiological subtyping does not always identify a source with absolute certainty.

An Outbreak of Legionnaires Disease Caused by Long-Distance Spread from an Industrial Air Scrubber in Sarpsborg, Norway

“…The purpose was to relate the plumes for each source to the patients' residences and not to estimate the actual concentration level. The identification of the same Legionella genotype from patient samples and the air scrubber but from no other potential source was strong evidence of the involvement of company F. However, it has been reported elsewhere that cooling towers located close to each other may harbor the same Legionella genotype [22]. The microbiological results thus need to be interpreted together with results from the epidemiological and environmental investigation.…”

Legionella and Legionnaires' Disease: 25 Years of Investigation

“…Even so, caution must be used when interpreting results of subtyping for the identification of an outbreak reservoir. A recent study determined that strains isolated from non-outbreak-related sources were identical to the clinical isolates by four subtyping procedures (AP-PCR, PFGE, monoclonal antibody, and multilocus enzyme electrophoresis) (171). Amplified fragment length polymorphism (AFLP) represents the current state-of-the-art subtyping procedure (274).…”

Mass Shock-dosing of Cooling Towers in Response to A Legionella pneumophila Outbreak: Did it Work?

“…In Bloomington, U.S.A, for example [10], a dispersed outbreak was contained by shock dosing only one tower. Two outbreaks stopped although no control measures were performed [11,12], and one outbreak stopped before completion of the decontamination of towers [13]. An outbreak of Legionnaires' disease occurred in 1988 in Bolton, England, with 37 cases of pneumonic disease and 23 which were classified as non-pneumonic.…”