RESEARCH Waterbor ne Outbreak of Norwalk-Like Virus Gastroenteritis at a Tourist Resor t, Italy Delia Boccia,* Alberto Eugenio Tozzi,* Benvon Cotter,*§ Caterina Rizzo,† Teresa Russo,‡ Gabriele Buttinelli,* Alfredo Caprioli,* Maria Luisa Marziano,* and Franco Maria Ruggeri* In July 2000, an outbreak of gastroenteritis occurred at a tourist resort in the Gulf of Taranto in southern Italy. Illness in 344 people, 69 of whom were staff members, met the case definition. Norwalk-like virus (NLV) was found in 22 of 28 stool specimens tested. The source of illness was likely contaminated drinking water, as environmental inspection identified a breakdown in the resort water system and tap water sam- ples were contaminated with fecal bacteria. Attack rates were increased (51.4%) in staff members involved in water sports. Relative risks were significant only for exposure to beach showers and consuming drinks with ice. Although Italy has no surveillance system for nonbacterial gastroenteritis, no outbreak caused by NLV has been described previously in the country. N virus (NLV) in the Caliciviridae family, which includes a orwalk virus is the prototype of the genus Norwalk-like large number of genetically related strains that together repre- sent the most important cause of gastroenteritis outbreaks worldwide (1–2). NLV accounts for up to 96% of outbreaks of nonbacterial gastroenteritis in the United States (3) and has been implicated in 43% of all foodborne outbreaks in England, 67% in Sweden, and 80% in the Netherlands (4–6). Outbreaks of NLV gastroenteritis more frequently affect adults and children >5 years of age. Because of the low infec- tious dose of the agent (10–100 viral particles can induce symptoms), outbreaks are characterized by a high secondary attack rate (7). In most documented outbreaks, the incubation period has been reported as 24–48 hours; the average duration of symptoms is 12–60 hours. During an outbreak, >50% of infected persons have symptoms of vomiting, most often in combination with diarrhea (8). The main source of infection is usually contaminated food or water (9–13), while the usual mode of transmission is direct person-to-person contact with saliva, vomit, or aerosols. Transmission may also occur through contact with contaminated objects and surfaces such as showers, sinks, mats, and floors (3). In Italy, which has no surveillance system for nonbacterial gastroenteritis, the impact of NLV infection is unknown, and no previous outbreaks of confirmed NLV infection have been reported. We describe a large outbreak of gastroenteritis caused by NLV at a resort in Italy. Methods The outbreak occurred at a tourist resort in the Gulf of Taranto, southern Italy, during July 7–31, 2000 (Figure 1). The *Istituto Superiore di Sanità, Rome, Italy; †University of Bari, Bari, Italy; ‡Local Health Unit, Montalbano Jonico, Matera, Italy; and §European Programme for Intervention Epidemiology Training (EPIET), Rome, Italy resort has an area of 122 hectares with 456 guest rooms in 19 buildings, in addition to staff quarters. The buildings are situ- ated around a central area where a restaurant, a swimming pool, and the resort management office are located. The resort can accommodate 1,000 guests, who usually arrive on a Satur- day and depart 1 or 2 weeks later, resulting in approximately 50% turnover of guests each weekend. The resort’s water tank is supplied via a 1-km pipe con- nected to the main public water supply (Figure 2). On July 13, a break in this water pipe was observed (Figure 2, point 2). Inspection also showed a bypass connecting the tank to an unused irrigation system (Figure 2, points 4 and 5). On July 18, 2000, the local health unit and the Institute of Hygiene of the Faculty of Medicine in Bari were notified about an outbreak of gastroenteritis at the resort. An epidemio- logic investigation was initiated the same day to identify the agent and the mode and vehicle of transmission and to Figure 1. Map of Italy, showing location of tourist resort on Gulf of Taranto. Emerging Infectious Diseases • Vol. 8, No. 6, June 2002 563