RESEARCH Figure 2. Water supply system in tourist resort, Italy. implement control measures. By July 20, when the local health unit notified the Istituto Superiore di Sanità in Rome, the out- break had already been in progress for approximately 2 weeks and >150 persons were ill. Outbreak Investigation A case was defined as illness in any guest or employee who stayed at the tourist resort during the period July 1–31 and who had diarrhea (three or more loose stools in any 24-hour period) or vomiting (at least one episode) or both, in the same period. Case finding was done by checking records of the resort medical center; after July 20, a door-to-door search was initiated. Demographic data and information on symptoms were collected in face-to-face interviews by the medical staff of the local health unit and the University of Bari. Because of the high number of cases in resort staff mem- bers, a retrospective cohort study was performed to assess risk factors associated with illness in this group. Persons eligible for the study were staff members employed at the resort from July 1 to 31. Standard questionnaires were sent to all 224 staff members in the first week of August. Information requested included name, date of birth, sex, room number, job type, date of onset and type of symptoms, and water and food prefer- ences. A month had elapsed between onset of symptoms and distribution of the questionnaires. We did not inquire about actual food history and activities of staff members during the outbreak but rather about their food preferences and usual activities. Statistical Analysis The questionnaires from guests and staff members were returned to the Istituto Superiore di Sanità, where the data were analyzed by using SPSS Base 10.0 (SPSS Inc., Chicago, IL) and Epi-Info 6.04 (Centers for Disease Control and Pre- vention, Atlanta, GA). Information collected on cases was used to construct the epidemic curve and describe the clinical presentation of the disease. Attack rates, denominator data, personal characteristics, and clinical symptoms of cases were compared between guests and staff members by chi square or Fisher exact test when appropriate; the Mann-Whitney U-test was used for comparisons of age. The room location of ill per- sons was plotted on a map of the resort that included water pipelines in an attempt to identify any clustering of cases along the pipeline. Statistical test for clustering was performed by the cluster k-means method with SPSS Base 10.0 (SPSS Inc.). In the cohort study, the attack rate was calculated for the total staff and also by specific job type. Relative risks and 95% confidence intervals were also calculated for job type, behav- iors and activities, and food preferences. Laboratory Investigations From July 18 to 28, samples (28 fecal and 2 vomit speci- mens) were collected from 30 participants whose illness met the case definition. Part of each specimen was stored at -20°C until examination for viral particles and free fecal cytotoxins, and the rest was refrigerated and processed within 12 hours of collection. Ova and parasites were detected by direct microscopy, and Salmonella, Shigella, Campylobacter, Yersinia enterocolitica, Staphylococcus aureus, and enteropathogenic E. coli were sought by standard methods (14). The presence of Clostridium perfringens enterotoxin (CPE) was determined either by assaying the cytopathic effect on Vero cells or by reverse pas- sive latex agglutination (RPLA) test (Oxoid Italia Spa, Gar- bagnate Milanese, Milan) according to the manufacturer’s instructions (14). Stool and vomit suspensions were examined by NLV-spe- cific reverse transcription/polymerase chain reaction (RT/ PCR) with generic primers JV12–JV13 to a consensus sequence on the RNA polymerase segment of the genome shared by most NLV strains (15). For confirmation of the diag- nosis, gels were further analyzed by Southern blot with a mix- ture of NLV-specific probes (15). The 327-bp amplification product was subjected to sequence analysis with PCR primers, and the sequences obtained were aligned with those in the Euro- pean Molecular Biology Laboratory Nucleotide Data Bank. After July 13, water samples were repeatedly collected from the main public water supply and from various points outside and inside the resort. Samples from food in the kitchen and the refrigerators were collected and sent to the University of Bari on July 18. Water and food samples were subjected to culture tests for enteric bacterial pathogens, according to stan- dard methods. Results Descriptive Epidemiology Of 344 cases identified from July 1 to 31, 69 (20%) were in staff members. Information on personal characteristics and clinical presentation was available for 248 ill persons (Table 1). Diarrhea, vomiting, and abdominal pain were observed in 564 Emerging Infectious Diseases • Vol. 8, No. 6, June 2002