Table 4. Presence of Norwalk-like virus and Clostridium perfringens enterotoxin in stool samples from ill persons, Italy, July 2000 No. examined No. positive for NLV a No. positive for CPE aAbbreviations used: NLV, Norwalk-like virus; CPE, Clostridium perfringens enterotoxin. Some specimens showed evidence of simultaneous infec- tion with NLV and enterotoxigenic C. perfringens. The food item(s) that could have been the source of infection by C. per- fringens remained unknown. However, the presence of C. per- fringens enterotoxin in a small, defined cluster of patients (7 of 28 stool samples) and the concomitant presence of NLV in the 7 positive stools suggests that C. perfringens played only a minor role, if any, in the outbreak. Control measures to limit the spread of the infection had no effect, probably because they did not address the point source and failed to prevent person-to-person transmission. After July 15, 2000, the consumption of tap water was banned, and only bottled mineral water was served in the resort restau- rant and used to wash vegetables. Water from the main tank, however, continued to be used for showers, to make ice for consumption (through July 28), and for irrigation. Further- more, on July 22, the bypass pipe was removed, the water inside the resort tank underwent superchlorination, and the pipe connecting the resort to the public water supply was shut down. However, NLV do survive high levels of chlorination (3,8), and the treatment was performed only once, at a late stage of the outbreak, and at only one point upstream of the resort water system. Finally, although the resort was serviced by a mobile tank truck that provided water from the main pub- lic water supply, the resort main tank was never emptied and cleaned before treatment. Therefore, after July 22, contami- nated residual water could have been gradually diluted by refilling the resort tank with clean water from mobile tanks. In the cohort study of staff members, having showers on the beach was identified as a risk factor, while consuming drinks with ice was only weakly associated with illness. No exposure to other water sources, including drinking tap water (the use of which was forbidden after July 15) was significant. Our analysis found no evidence that contaminated food was the source of infection: no food preference was associated with an increased risk of being ill, and personnel working in the kitchen had the lowest attack rate. In addition to water contamination, person-to-person trans- mission may have played a role in this outbreak. Typically staff members of tourist resorts share the same living quarters 0 1 6 0 0 7 No. positive for NLV + CPE 0 1 6 0 0 7 RESEARCH and have frequent contact with guests during meals, sport training, entertainment, and other activities. Person-to-person transmission may also explain the fact that the time between arrival and onset of symptoms in guests was longer than the incubation period expected for NLV. Person-to-person trans- mission of NLV infection is well documented (8), and second- ary cases may occur. Airborne and fomite transmission also may facilitate the spread of the virus during outbreaks. Such hypothesis is confirmed by the higher attack rate in the clean- ing staff and in staff members working in close contact with guests. This investigation had several limitations. Since the cohort study was carried out after the outbreak had ended, we could inquire only about food preferences and usual activities rather than actual food histories and activities before the outbreak. Recall bias may have occurred, which may have led to nondif- ferential misclassification of exposure and underestimation of the observed relative risks. If NLV had spread through water and person-to-person transmission had occurred, virtually everyone in the resort would have been exposed to the agent and any epidemiologic association would be difficult to find. Finally, no test specific for NLV was performed on water sam- ples, and the hypothesis of water as the actual source of infec- tion cannot be confirmed. In conclusion, this event confirms that large outbreaks due to NLV may be occurring in Italy, but without the use of appropriate diagnostic methods this pathogen may go unrecog- nized. This occurrence highlights the need for a surveillance system of such outbreaks in cooperation with laboratories capable of diagnosing viral gastrointestinal infections. This investigation was partly supported by the European Union, under the 5th Framework, Quality of Life Programme (grant QLK1- CT-1999-00594, Food-borne Viruses in Europe). Dr. Boccia is a microbiologist with the National Public Health Institute in Rome. Her main research interest is the epidemiology of antimicrobial resistance, and she has been frequently involved in out- break investigations caused by viral and bacterial agents. References 1. Kapikian AZ. The discovery of the 27-nm Norwalk virus: an historic per- spective. J Infect Dis 2000;181:S295–302. 2. Monroe SS, Ando T, Glass RI. Introduction: human enteric calicivirus— an emerging pathogen whose time has come. J Infect Dis 2000;181:S249– 51. 3. Glass RI, Noel J, Ando T, Frankauser R, Beillot G, Mounts A, et al. The epidemiology of enteric Calicivirus from humans: a reassessment using new diagnostics. J Infect Dis 2000;181:S254–61. 4. Evans HS, Madden P, Douglas C, Adak GK, O’Brien SJ, Djuretic T, et al. General outbreaks of infectious intestinal disease in England and Wales: 1995 and 1996. Commun Dis Public Health 1998;1:165–71. 5. Hedlund KO, Rubilar-Abreu E, Svensson L. Epidemiology of calicivirus infections in Sweden, 1994–1998. J Infect Dis 2000;181:S275–80. 6. Koopmans M, Vinje J, de Wit M, Leenen I, van der Poel W, van Dujn- hoven Y. Molecular epidemiology of human enteric caliciviruses in the Netherlands. J Infect Dis 2000;181: S262–9. Sampling date 07/18/01 07/20/01 07/21/01 07/27/01 07/28/01 Total 12 1 8 2 5 28 9 1 7 2 3 22 Emerging Infectious Diseases • Vol. 8, No. 6, June 2002 567