Objectives: Outbreaks of acute gastroenteritis associated with 'Norwalk-like viruses' (NLVs) cause significant health problems in hospitals. Hospital outbreaks in the Stockholm area in 1996 were investigated, in order to identify the magnitude of the problem, the mode of transmission, the effect of control measures and the genetic variability of outbreak strains. Determining the epidemiological and clinical significance involves a broad range of possibilities.

Methods: Ten hospitals, representing 66% of the hospitals in the Stockholm area, participated in the study, which included 211 wards. Of these, 18 were selected as control. A standardized protocol that included personal contacts was administered. Outbreak wards were visited between 5 and 10 times. Wards that had reported outbreaks in 1996 were prospectively followed through 1999 by personal contacts, and the available data from 1991 on outbreak reports were collected. A total of 253 stool samples from outbreaks in 1996 were analyzed by electron microscopy (EM) for the presence of NLVs. Positive samples were confirmed by the reverse transcriptase-polymerase chain reaction (RT-PCR).

Results: In total, 4 326 patients and 1 119 staff were exposed on the 43 wards that reported 54 outbreaks. The mean attack rate was 13% for patients and 21% for staff. The number of outbreaks in 1996 outnumbered the reported outbreaks in the preceding years (4-70%) and later years (35-40%). Admission to 24 (56%) of the outbreak wards was stopped. The mean duration of illness for patients was 35 hours and for staff, 30 hours. The main symptoms were diarrhoea (80%) and vomiting (68%). Genotyping revealed that the majority of the hospital outbreaks in the Stockholm area in 1996 were caused by a single NLV strain.

Conclusions: The study confirmed that outbreaks of NLV are an increasing public-health problem in hospitals. The risk of being affected by an outbreak was significantly greater on wards that had reported outbreaks in the previous year. It was not obvious which measures had helped to shorten the outbreaks to any appreciable extent. Different managements must therefore be carefully interpreted and adapted to the prevailing circumstances. Genotyping of strains is an important tool of getting a better insight into transmission routes and the mechanism behind the appearance of epidemic strains.