The Southern Nevada Health District, which does an excellent job providing timely write-ups of foodborne illness investigations, reports on a norovirus (NoV) outbreak among 203 attendees of a dinner gala at Griffin Mansions on May 9, 2012 in Las Vegas, Nevada.

The source of the norovirus infection was not identified, but the investigation revealed an unlicensed kitchen and a domestic well that might have provided contaminated potable water. We conducted a retrospective cohort study of attendees, surveillance for additional illnesses, employee interviews regarding food preparation, and testing of well groundwater and clinical specimens. Of 108 attendees included in our study, 51 (47%) reported illness consistent with our primary-case definition. Additionally, 12 persons reported illness consistent with our secondary-case definition, including 5 (nonattendee) household contacts of ill attendees, 4 persons who attended the event with a household partner and developed symptoms of illness several days after their partners became ill, and 3 attendees whose illness symptoms began too late to have been considered primary cases but whose contact with primary-case attendees was not determined. Epidemiologic analysis identified statistical associations between consuming several food items and subsequently becoming ill, but no other evidence exists to explain their associations with the outbreak. Seven stool samples collected from ill gala attendees and an asymptomatic staff member were positive for norovirus genotype I. Fecal coliforms were isolated in potable water collected from the onsite well. Norovirus testing of well water was negative. Businesses that cater to large groups must prepare food in licensed settings and adhere to regulations regarding public water systems.