Where the cooling water drifts: an outbreak of Legionella pneumophila serogroup 1 in the central business district of Sydney, December 2023 - January 2024.

BACKGROUND

Legionnaires' disease causes severe pneumonia. Outbreaks are infrequent in Australia, with cooling water systems (CWS) regulated to reduce risk. In summer 2024, a Legionella pneumophila serogroup 1 (Lp1) outbreak was detected in visitors to the Sydney central business district (CBD). We investigated to identify cases and to control the source.

METHODS

Case-patients were detected through routine laboratory notifications and classified as per surveillance case definitions. Case-patients were interviewed to determine symptoms, and environmental exposures 2-10 days prior to symptom onset. We mapped exposures sites and walking routes to identify areas for investigation. CWS in shared exposure areas were inspected and tested for Legionella. Historical results from routine CWS testing were reviewed. Genomic sequencing was performed on environmental and patient isolates. Clinician and public alerts were issued, and CBD building managers were reminded to maintain CWS.

RESULTS

The investigation identified 15 legionellosis case-patients: two had Lp1 positive sputum cultures, 14 were hospitalised, and six required intensive care. All case-patients visited the CBD during the period 12-26 December 2023. Between 3-12 January 2024, testing was performed on 166 CWS across 118 CBD sites, and on three water fountains. Lp1 was cultured from one CWS. Genomic sequencing from five environmental and two clinical isolates showed a probable link. The positive CWS was decontaminated but continued to have Lp1 detected, possibly due to ongoing dust contamination, necessitating additional maintenance. Weeks later, a case-patient diagnosed in Europe, who had visited key exposure locations in the CBD during 21-23 December 2023, was epidemiologically linked to the outbreak; this took the total number of case-patients to 16.

CONCLUSION

Our investigation indicated that a contaminated CWS may have been the source of this outbreak, with contamination potentially precipitated by nearby construction. This emphasises the importance of strengthening Australian CWS regulations to reduce Lp1 outbreak risk, and of timely reporting under International Health Regulations to identify additional outbreak cases.