RESEARCH Type of work Kitchen staff Office staff Bar staff Shop assistant Cleaning staff Sports trainers and entertainers Waiters aCI, confidence interval. Table 2. Attack rates and relative risks for staff members (n=69) according to type of work, tourist resort, Italy, July 2000 Attack rate (%) Relative risk 4/34 (11.8) 3/14 (21.4) 3/11 (27.3) 7/21 (33.3) 16/36 (44.4) 19/37 (51.4) 17/28 (60.7) Referent 1.8 2.3 2.8 3.8 4.4 5.2 95% CI a 0.5–7.1 0.6–8.8 0.9–8.5 1.4–10.2 1.6–11.5 2.0–13.6 All food samples tested were negative for enteropatho- genic bacteria. Water samples collected on July 13 from fau- cets in the bar, the kitchen, and a guest room (Figure 2, point 6) had high levels of coliforms (up to 130 CFU/mL) and fecal streptococci (up to 22 CFU/mL). The same level of contami- nation was observed in water samples from the pipe connect- ing the resort to the public water supply (Figure 2, point 2); samples collected from the public water supply outside the resort (Figure 2, point 1) were always negative. After July 15, when chlorine was added to the tank, the level of contamina- tion of tap water inside the resort steadily decreased; no con- tamination was detectable after superchlorination on July 22. Discussion Although NLV gastroenteritis epidemics likely occur as frequently in Italy as in the rest of Europe, to our knowledge this is the first outbreak of NLV infection to be confirmed in the country. It affected many guests and employees at a sum- mer vacation resort and involved high attack rates in all age groups. The actual number of cases has likely been underesti- mated since persons with a mild illness may not have sought medical attention. In fact, the retrospective investigation of staff members showed an attack rate three times higher than in guests. This outbreak had an unusual pattern, with three regular peaks occurring at constant intervals for 3 weeks. This pattern, which is compatible with a point-source infection (3), may be explained by the rapid turnover of guests and their periodic replacement with susceptible persons in the presence of a con- stant exposure to infection. Most guests arrived at the resort on a Saturday and stayed 1–2 weeks. Guests who became ill did so a few days after their arrival, suggesting that exposure to a source of infection was relatively constant during the whole period. Moreover, a large proportion of staff members had onset of illness in the first week of the outbreak. The hypothe- sis of a common source of infection is further supported by the identical nucleotide sequence detected in viruses from eight patients during the outbreak. Water was the likely source of this outbreak. Environmen- tal inspection identified a breakdown in the water system of the resort, and tap water samples from different places in the resort showed contamination with fecal bacteria. Although microbiologic testing for NLV could not be performed on drinking or recreational water, the presence of fecal bacteria suggests that the water system may have been the actual source of NLV. Despite the possible passage of the virus through several hosts during the outbreak, the genome seg- ment used for diagnosis showed complete stability, suggesting that a very high number of human passages may be required to produce the known nucleotide variability for NLV, at least in the RNA polymerase region. Table 3. Attack rates and relative risks according to usual behaviors and activities of staff members, tourist resort, Italy, July 2000 No. (n=69) No. exposed Attack rate (%) Relative risk Exposure Shower on the beach Swimming in the pool Drinking tap water Drinks with ice Swimming in the sea Eating at resort restaurant Eating ice cream Eating meat Eating salad Eating fruit Eating pasta Consuming drinks on draught Eating fish Eating seafood aCI, confidence interval. 566 22 45 104 128 72 159 140 151 123 139 142 91 112 85 14 22 47 55 31 64 56 60 48 54 55 35 40 28 63.6 48.9 45.2 43.0 43.0 40.2 40.0 39.7 39.0 38.8 38.7 38.5 35.7 32.9 Emerging Infectious Diseases • Vol. 8, No. 6, June 2002 1.8 1.4 1.4 1.8 1.2 1.5 1.1 1.2 1.0 1.0 1.2 1.0 0.7 0.7 95% CI a 1.2–2.6 0.9–2.0 0.9–2.2 1.0–3.2 0.8–1.7 0.5–3.9 0.6–1.9 0.6–2.4 0.6–1.6 0.6–1.8 0.6–2.1 0.7–1.4 0.5–1.1 0.5–1.1