Legionnaires’ disease cluster investigation in Sydney

We report an extensive investigation into a cluster of five confirmed cases of Legionnaires’ disease in Sydney’s Inner West in May 2016. The patients were aged 62–89 years, four were men, all were ex-smokers, and four had significant comorbidities, such as immunosuppression. Hospitalisations lasted a median of 6 days and one patient died. All patients tested positive for Legionella pneumophila serogroup 1 urinary antigens; however, L. pneumophila was isolated by culture in only one patient.

Patients were interviewed to obtain the histories of where they had been in the 2–10 days before the onset of symptoms (the incubation period), and then, to identify the focus for our environmental investigation, we mapped the locations against water source locations.1 Local councils assisted in the collection of water samples: 86 samples were obtained from cooling towers and 15 from other sites, such as fountains. At the time of sampling, requests were made for immediate cleaning of all visibly unclean air conditioning cooling towers or those with cloudy water. Only one sample tested positive for Legionella: a fountain with a low positive result of Legionella anisa.

If Legionella isolates are genetically indistinguishable, the genomic sequencing of clinical and environmental samples is a useful tool that can link cases to each other and to an environmental source. In this investigation, L. pneumophila was cultured in only one case and in no environmental samples; therefore, we could not link cases to each other or to an environmental source by genotyping, leaving the possibility that cases were unrelated and sporadic.

Despite this investigation, no environmental source for these infections was identified. It is uncommon in an investigation of this size to not detect L. pneumophila in any cooling towers — a routine New South Wales survey found L. pneumophila in 2% of cooling towers in a non-outbreak situation.2 Possible explanations for not detecting Legionella include low sensitivity of sampling at one point in time, incidental and possibly unrelated cleaning of the source cooling tower before sampling, or underestimating the distance that contaminated aerosols may travel, meaning that the source cooling tower was not sampled.3 No further cases with similar exposures were reported in the period immediately after the environmental investigation. This suggests that the extensive public health interventions taken at the time to remedy the problems identified may have been effective, or that incidental cleaning of cooling towers before our sampling may have eliminated the potential source.