Legionellosis linked with a hotel car park--how many were infected?

An outbreak of legionellosis associated with a hotel in Sydney, Australia, and the subsequent epidemiological and environmental investigations are described. Four cases of Legionnaires' disease were notified to the Public Health Unit. A cross-sectional study of 184 people who attended a seminar at the hotel was carried out. Serological and questionnaire data were obtained for 152 (83%) of these. Twenty-eight (18%) respondents reported symptoms compatible with legionellosis. Thirty-three subjects (22%) had indirect fluorescent antibody (IFA) titres to Legionella pneumophila serogroup 1 (Lp-1) of 128 or higher. The only site which those with symptoms of legionellosis and IFA titre > or = 128 were more likely to have visited than controls was the hotel car park (adjusted odds ratio [OR] 14.7, 95% confidence interval [CI]: 1.8-123.1). Those with symptoms compatible with legionellosis, but whose IFA titres were < 128 were also more likely to have visited the hotel car park (adjusted OR 4.4, 95% CI: 1.5-12.9). Seroprevalence of Lp-1 antibodies was higher in those who attended the seminar than in a population sample of similar age. Findings suggested that the 4 cases represented a small fraction of all those infected, and highlighted difficulties in defining illness caused by Lp-1 and in interpreting serology.