Community spread of Legionella pneumophila serogroup 1 in temporal relation to a nosocomial outbreak

To clarify whether a nosocomial outbreak of legionnaires' disease in the Värnamo hospital in Sweden was part of a wider outbreak in the Värnamo community a number of investigations were performed. First, the proportion of cases of legionnaires' disease in a group with nosocomially acquired pneumonia (11%) was compared to the proportion within a group with community-acquired pneumonia (14%) and the difference was found not to be significant (p > 0.05). Second, the proportion of the nursing staff at the Värnamo hospital with an elevated antibody titre (> or = 16) to Legionella pneumophila serogroup (sg) 1 (33%, 84/258) was compared to the proportion in a group of local residents of Värnamo community (26%, 25/96) and found not to be significant; in contrast, comparison with the proportion in a group from the assistant nursing staff at another hospital 60 km away (5%, 4/80) was highly significant (p < 0.001). Furthermore, Legionella species were cultured from samples drawn from the hospital water supply as well from the water supply from municipal buildings. In 1996 a follow-up study was conducted, which showed that < 1% of the assistant nurses and local residents had an elevated titre to L. pneumophila sg 1. These results indicate that there was a temporary spread of L. pneumophila sg 1 in the Värnamo community at the beginning of 1991, both in the local hospital and the surrounding community. This implies that physicians should be aware of community-acquired cases of legionnaires' disease when a nosocomial outbreak is detected.