• legionnaires' disease

Summary

During the winter of 1995, a space-time cluster of seven suspected cases of legionnaires' disease occurred on Merseyside, involving three neighbouring health districts. A multi-disciplinary investigation team was established but failed to obtain convincing evidence of an outbreak. Investigation of the seven cases confirmed two as sporadic cases; other infections were found to explain the remaining cases that had been initially confirmed presumptive cases on the basis of false positive Legionella pneumophila type 1 urine ELISA antigen test result. The epidemiological and clinical features of the unconfirmed cases were also not characteristic of legionnaires' disease. The investigation team concluded that this was a pseudo outbreak on the basis of detailed epidemiological investigation. This incident raised the importance of a strong index of suspicion when investigating suspected outbreaks of legionnaires' disease in the community wherever the results of one test have not been supported by others.

Legionella outbreaks first became matters of particular concern where these were related to common sources within hospital and residential settings because of the combination of high attack rates in susceptibles and high fatality rate.1-4 More recently community outbreaks of legionellosis have also been recognised and have often been found to be associated with wet cooling tower systems.5-7 However, a definitive identification of a source of infection is not always possible because of the ubiquitous nature of legionella organisms in the environment and a low attack rate in the community.7-9 We describe an investigation of a suspected community outbreak of legionellosis that illustrates some of the practical difficulties encountered when investigating apparent clusters.