We report a nosocomial outbreak of Legionella pneumophila serogroup (sg) 1 infection at the general hospital, Värnamo, Sweden. From December 1990 to February 1991, 28 patients and 3 staff fell ill with pneumonia and 3 died. L. pneumophila sg 1 together with several other Legionellae were isolated from the hot water supply to 17 of 20 hospital wards, probably being spread by aerosolization via shower nozzles. Raising the hospital's hot water temperature from 45°C to 65°C, together with heat disinfection of the shower equipment, arrested the outbreak within a week. Keeping the hot water temperature < 60°C without chlorination eliminated L. pneumophila from < 75% of the wards. During a period of 2 years after the outbreak we have diagnosed only 1 case of nosocomial legionellosis at the hospital despite an active surveillance program.

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 (≥ 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.

After a nosocomial outbreak of Legionnaires' disease in a 450-bed district general hospital in 1991, the circulating hot water temperature was kept above 55°C as the sole control measure. From 1991 to 2000, all cases of nosocomial pneumonia were clinically monitored and tested for Legionella pneumophila serogroup 1 by serology or urinary antigen detection. Water samples from peripheral tap sites were cultured for Legionella spp. twice a year. An infection with L. pneumophila serogroup 1 was diagnosed in four out of 366 (1.1%) patients treated for nosocomial pneumonia, representing one case per 26 000 admissions. All patients were cured without complications. L. pneumophila serogroup 1 was isolated in 30 of 251 (12%) cultured hospital water samples during the monitoring period. We conclude that control of nosocomial Legionnaires' disease in a primary referral hospital is possible by keeping the circulating hospital hot water temperature above 55°C, together with careful clinical surveillance. Complete eradication of Legionella spp. from the hot water system does not seem necessary.

The genotypic distribution of Legionella pneumophila serogroup 1 was investigated in the water distribution system of a 450 bed Swedish hospital and the surrounding community. A single genotype identified by amplified fragment length polymorphism (AFLP) analysis, was found in all 34 hospital isolates and in 18/20 (90%) community isolates over a 12-year surveillance period. All enviromnental isolates were either monoclonal antibody subtypes Benidorm or Bellingham. In a geographic comparison, the hospital genotype was also identified in two out of six Swedish hospitals; both located within 100 km of the studied community. In all, 70 isolates from seven Swedish communities clustered in four groups, each also containing one AFLP type as defined by the European Working Group on Legionella Infections (EWGLI). It was concluded that a single Legionella pneumophila serogroup 1 genotype may colonise a large water distribution system over a long period of time, and that certain clones seem to be widely spread in the environment. Results frorn molecular typing of isolates originating from a limited geographical area must, therefore, be interpreted cautiously in epidemiological investigations of Legionnaires' disease.

To evaluate current serological criteria for Legionella pneumophila serogroup 1 (Lp1), the antibody response was prospectively studied in all patients hospitalized for Legionnaires' disease in a Swedish county during 11 y (n = 62). A 4-fold or greater rise in antibody titre to ≥ 128 (the minimum convalescent antibody level for diagnosis, as recommended by the Centers for Disease Control and Prevention), using the indirect immunofluorescence antibody test, was found in 21/52 (40%) of tested patients. By referring to the titre levels in healthy residents from the local population (World Health Organization criteria), 45/52 (87%) cases were confirmed serologically. In 21 patients followed longitudinally for 10 y, the median antibody titre fell from 256 (range 32-1024) to 16 (range 2-128) in 3 y. No booster reactions were observed in any patient. After 10 y, the geometric mean titre of this clinical cohort had reached the same level as observed in the background population 5 y earlier. Titre levels in subjects exposed to Legionella from a municipal water system indicate that only 1 out of 10 of all infections are identified clinically. Indirect immunofluorescent antibody testing with local reference sera is a sensitive method for laboratory confirmation of Lp1 in an unselected pneumonia population.