Health Stream Literature Summary - Issue 56 - December 2009

An outbreak of viral gastroenteritis linked to municipal water supply, Lombardy, Italy, June 2009.
Scarcella, C., Carasi, S., Cadoria, F., Macchi, L., Pavan, A., Salamana, M., Alborali, G.L., Losio, M.M., Boni, P., Lavazza, A. and Seyler, T. (2009) Eurosurveillance, 14(29).

On 9 June 2009, the local health authority of Brescia (Lombardy region, north Italy) was notified of 21 cases of gastroenteritis among guests of a hotel in the municipality of San Felice del Benaco. In the following days there were also reports of cases among local residents. San Felice del Benaco is located near the lake of Garda and has a resident population of 3,360. Many tourists visit the town during the summer months. This outbreak investigation was instigated to identify the source of infection and implement appropriate control measures.

A probable case was defined as a person who had vomiting or diarrhoea after 7 June 2009 and who stayed in San Felice del Benaco prior to disease onset. A confirmed case was a person who met the criteria of a probable case and whose stool sample was laboratory confirmed for at least one of the following viruses: norovirus, rotavirus, enterovirus or astrovirus. An environmental investigation was initiated at the hotel on 9 June 2009 when food samples were taken from the kitchen, interviewing was conducted and stool samples were collected for microbiological testing from 20 probable cases (both guests and hotel staff). The environmental investigation was then extended to the larger community and water samples were collected from the municipal water supply. Water from the nearby lake is treated with chlorine dioxide and hypochlorite and passes through sand filters before it is distributed to the town as drinking water. There were 94 water samples collected from the lake at the location where water is pumped from filters and from public fountains. Water samples were tested for bacterial pathogens (Salmonella sp., Shigella sp., Campylobacter sp., E. coli 0157, Yersinia enterocolitica, Aeromonas sp., Clostridium perfringens toxins), parasites (Cryptosporidium sp.) and viral pathogens (norovirus, rotavirus, enterovirus, astrovirus).

There were 299 persons who fulfilled the outbreak case definition which included 269 probable and 30 confirmed cases. The attack rate for San Felice del Benaco was 8.9% (299/3,360). Age group-specific attack rates ranged from 7% (50/713) in persons aged 65 years and older to 14% (34/242) in those aged 15-24 years. There were four cases hospitalised, all of them were children. Stool samples were collected from 36 probable cases, of these 17 (47.2%) tested positive for norovirus, 19 (52.8%) for rotavirus, 12 (33.3%) for enterovirus and 4 (11.1%) for astrovirus. Eight cases had both norovirus and rotavirus in their stools and two cases tested positive for norovirus, rotavirus and enterovirus. Salmonella sp. was found in two cases and Clostridium perfringens and Campylobacter sp. in one case each. The food samples from the hotel were negative for the presence of pathogens. There were abnormally high levels of Clostridium perfringens (4 UFC/100ml) and Aeromonas hydrophyla (16 UFC/100ml) in water samples from two public fountains. There were 44 water samples tested from the municipal water system (water from fountains and filters) and these showed the presence of norovirus and enterovirus. The examination of the municipal water network revealed that the water company had undertaken work on the collection reservoir which may have limited the effect of chlorination, also two filters were 10 years old (cleaned weekly but not disinfected) and the chlorine concentration in the water before it passed through the filters was 0.4 mg/l and in the filtered water it was only 0.08 mg/l.

On 17 June 2009, inhabitants were told not to use municipal water for drinking and cooking and provided with alternative water supplies via water tankers. A door-to-door information campaign was organised and leaflets were distributed. On 19 June 2009, sand filters were disinfected with peracetic acid and chlorine dioxide disinfection of the water system was carried out. To monitor the efficiency of control measures, regular water sampling and testing were performed. The restriction on use of municipal water was maintained until all water quality tests complied with safety norms. Water samples collected after the first treatment with chlorine dioxide and peracetic acid were all negative for the presence of norovirus.

A much higher attack rate was prevented in this outbreak by timely control measures and good compliance of the population following the information campaign. The hotel where the initial cases of gastroenteritis were identified is located along the lake, near the water reservoir which may explain why the guests and staff were among the first affected. The municipal water is taken from the lake at a place where the water is stagnant. Water samples from the lake have so far tested negative for viruses however contamination of the lake due to over-capacity of the sewage system and/or illegal wastage can not be excluded.

Comment The presence of enteric viruses implicates human waste as the source of the outbreak, and the occurrence of multiple pathogen infections in one third of tested cases suggests high contamination levels. Case finding was limited to people who telephoned a public information hotline and those who presented at the local hospital emergency unit. It is probable that a higher attack rate would have been found if a representative community survey had been carried out rather than relying on ill people to contact health agencies.

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