A pseudo community outbreak of legionnaires' disease on Merseyside; implications for investigation of suspected clusters.

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 multidisciplinary 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.