Epidemiological investigation
The following case definition was used: A probable case was defined as any resident or employee of the nursing home or their household contact with acute onset of diarrhoea (>3 loose stools/day), vomiting, fever (>37°C) or abdominal pain from 1 July. To identify cases, a questionnaire was distributed on 27 July 2007 among ill residents and ill staff of the nursing home. The questionnaire included information about gastrointestinal symptoms and/or fever experienced from 1 July, date of onset and duration of symptoms, food consumed during three days before beginning of symptoms, therapy, hospitalisation. The response to the questionnaire was 100%.
Additional questions and inquiries were addressed to the head of the nursing home.

Among the 117 residents of the nursing home, 25 cases were identified (six male, 19 female) – 15 in residents of the assisted living department (requiring basic assistance) and 10 in residents of the nursing care department (bedridden patients). Further five cases (all female) were identified among the 23 members of staff – four in employees of the assisted living department and one in the personnel of the nursing care department.

The attack rates were: 19% in residents of the assisted living department, 26% in residents of the nursing care department, 33% in staff working in the assisted living department and 9% in the nursing care department staff.

No cases were identified among the household contacts of the staff or among visitors of the residents. The most frequent symptoms were diarrhoea (73%), vomiting (40%), abdominal pain (33%), malaise (30%) and fever (10%). The duration of symptoms ranged from 1 to 2 days, except in the first case in whom symptoms lasted more than two weeks. One resident (of the nursing care department) was admitted to hospital because of dehydration, with good outcome.

The epidemic curve (Figure) showed several peaks – on 18, 23, 25 and 31 July. The earliest reported date of onset of symptoms was 1 July. More cases started to appear only after 15 July, the latest case was registered on 31 July. The outbreak initially affected only the residents and staff of the assisted living department. The first two cases in the nursing care department were reported as late as 23 July.

Laboratory analysis
Stool samples from seven patients (with onset of symptoms after the notification date of 27 July) were taken for culture. Only two samples were taken during the acute phase of the disease, the rest was taken during reconvalescence. The samples were tested for Salmonella, Shigella, Campylobacter, enteropatogenic, enterotoxigenic and enterohaemorrhagic Escherichia coli. Samples were tested also for adenoviruses, rotaviruses and noroviruses. Six samples were negative for all tested pathogens, in one sample norovirus was confirmed with the ELISA test. No other laboratory tests for norovirus have been performed.

Environmental investigation
A sanitary and hygienic inspection of the kitchen and the dining room was performed, and six swabs were collected to assess surface cleanliness, as well as two foodstuff samples (a sample of Brussels sprouts salad and a sample of espresso coffee from a coffee vending machine), and a sample of drinking water from communal tap water. Drinking water was tested for Escherichia coli and other coliform bacteria, Clostridium perfringens, number CFU at 22°C and number CFU at 37°C.

All environmental samples were within acceptable microbiological ranges, except the drinking water sample which did not comply with the regulations because of the presence of Enterococcus spp. in the amount of 1 CFU/100 ml. However, further water samples taken from the same source showed compliance with regulations therefore the possibility that water was the cause of the outbreak was ruled out.

Control measures
During the outbreak, enhanced hand hygiene practices were implemented, and disinfection was recommended besides the usual cleaning of dishes and tableware, toilets and doorknobs. Special recomendations for people handling food were issued.

Discussion and conclusion
Viruses from the Caliciviridae family belong to the most frequent causes of epidemics in closed communities, such as nursing facilities, hotels and cruise ships [1]. Epidemic outbreaks caused by caliciviruses can affect a great number of people who get infected with contaminated food and water. Viruses can spread among people also by direct contact.

The clinical picture of the calicivirus infection is similar to that of the rotavirus infection but it is less severe. The incubation period is 10 to 48 hours, less than half of all patients experience slight fever at the beginning, they have diarrhoea and abdominal pain, and they vomit in less than 50%. The infection lasts from 2 to 4 days. This has also been observed in the outbreak described in this paper.

The epidemiological investigation revealed that the first residents and staff who fell ill were from the assisted living department. The infection in the nursing care department which is situated in another building and where bedridden patients reside did not appear until 23 July and peaked on 31 July. Personnel working in both departments come into contact several times a day. The infected employees do not eat the food which is prepared for residents in the home’s kitchen; this is why the suspicion that the infected residents and staff ingested contaminated food was ruled out at the beginning of the outbreak investigation. Water is supplied through the water distribution system. The sample taken for inspection did not comply with regulations because of the presence of Enterococcus spp. in the amount of 1 CFU/100 ml. Other samples from the same source, however, complied with regulations. Hence, water was ruled out as the cause of infection outbreak, also due to the dynamics of the appearance of infection.

Considering the epidemiological data, clinical picture and laboratory results, the cause of the outbreak was thought to be norovirus spread by contact transmission. Late notification of the outbreak and delayed fecal sampling could explain the fact that norovirus was confirmed in only one patient.