Microbiological Aspects Of An Outbreak Of Legionnaires' Disease In South Western Sydney

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In April 1992 an outbreak of Legionnaires' disease occurred in South Western Sydney. Twenty four patients were diagnosed as having Legionnaires' disease either on the basis of a positive culture of Legionella pneumophila serogroup I from a respiratory specimen (14 patients) or a four-fold or greater rise in antibody titre or isolated convalescent titre > 1024 with IgM antibodies present (10 patients). This is the largest outbreak in New South Wales since 44 cases of Legionnaires' disease were reported in Wollongong in 1987.

Culture and direct immunofluorescent staining (DFA) were performed for early laboratory diagnosis. The DFA test and a set of defined clinical criteria were then used to classify patients as probable cases of Legionnaires' disease for subsequent Public Health investigations. The DFA test had a predictive value for a positive culture of 45.8% and for a negative culture of 97.1%. Its positive predictive value for a definite diagnosis of Legionnaires' disease (either positive culture or seroconversion), however, was 78.3% with a negative predictive valve of 95.1%.

During a 27 day period DFA and culture for Legionella species were performed on 198 specimens from 127 patients. Cultures became positive after a mean of 5 days incubation (range 1–11 days). The use of both selective and non-selective media is recommended for optimal recovery of L. pneumophila since some isolates were obtained on only one culture plate. After an initial evaluation of heat decontamination of specimens this process was abandoned, since in 8/63 specimens so treated, L. pneumophila was recovered prior to but not following heat decontamination and was not grown only in heated specimens.

The microbiological investigations of this outbreak caused profound disruption to the routine laboratory. As a result of our experience we recommend the Microbiology laboratories formulate contingency plans in anticipation of involvement in an outbreak. Laboratory equipment may need to be augmented, channels of communication must be developed to convey frequent updates of laboratory results, a centralized media release team — organized by the Area Public Health Unit — should co-ordinate liaison with the media, and due recognition should be given to the inevitable cost of laboratory reagents and staff overtime.

The provision of an area microbiology diagnostic and consultative service to most hospitals in South Western Sydney from our Microbiology Department facilitated the initial recognition of the outbreak, the definition of probable cases and the Public Health investigation of possible sources of the outbreak.

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