As reported in PACE earlier this year, in March and May 2016, 15 people who had visited the Sydney CBD during their incubation period were diagnosed with Legionnaires’ disease (Legionella pneumophila serogroup 1). This was among some 35 other cases that were diagnosed during the first half of the year. Sydney saw two deaths due to the disease, with one being linked to the CBD outbreak.
NSW Health has released its investigation into the outbreaks, which details the results of all tests conducted on cooling towers and other possible sources of Legionnaires’ disease.
According to the report, of the 15 Legionnaires’ cases examined, two were excluded from the outbreak following genetic sequencing of available clinical and environmental isolates. This left 13 cases linked to the outbreak.
L. pneumophila SG1 was isolated from several cooling towers, including two cooling towers with excessively high colony counts of greater than 1000cfu/mL.
The use of whole genome sequencing provided strong evidence that at least five of the 13 cases were linked to one common source, or separate sources contaminated by the same microorganism. Two cooling towers were found to be contaminated with L. pneumophila SG1 that was genetically identical to the five patient samples, which suggests that either or both of the towers could have been responsible for the infection. Epidemiological investigations suggested that the remaining eight cases were also linked to the outbreak.
“While we cannot pinpoint the ultimate source of the outbreak, we were able to confine the origin of the bacteria to buildings in downtown Sydney west of Hyde Park between Park and Margaret Streets,” NSW chief health officer Dr Kerry Chant told Fairfax Media.
“Further regulatory action is being considered in respect of the regulated system at the Clarence Street site 2 premises, [and] the York Street site 4 tower was followed up by the City of Sydney with a warning letter as this count was less than 100cfu/mL,” according to the report.
The report notes that an unknown source contaminated with the same miroorganism could have been the culprit of the infections and the contamination of the two towers, but was either not sampled or cleaned prior to sampling.
It also notes the possibility that considering the six-week period between March and May without any reported cases, contaminated towers may have been cleaned and disinfected due to the initial March outbreak, but the contamination was not completely removed, therefore enabling the microorganism to grow back to unsafe levels.
The report continues on to list a range of limitations in detecting the source of the outbreaks. Some of these include uncertainties around Sydney CBD wind tunnel effects and wind pattern variability, uncertainty about patient movements, infected towers not being tested and failure of the microorganism to grow on the sampling culture.
NSW Health has assured that “while the actual source of the human cases cannot be proven”, NSW Health, City of Sydney and building owners have controlled the risk by identifying and cleaning any (known) contaminated water cooling systems.