A foodborne norovirus outbreak associated with six events and a single caterer, Canberra, November 2022

An outbreak of gastrointestinal illness was investigated, affecting six events where attendees consumed food catered by a single catering business, in the Australian Capital Territory (ACT).

Event attendees and the catering business were surveyed using tailored food questionnaires developed in REDCap and administered on-line. Descriptive analyses were conducted for all event attendees and employees of the business, and non-fatal productivity loss estimates calculated. Retrospective cohort studies were conducted for events that occurred on two specific days. A food safety inspection was undertaken of the catering business, and food and environmental samples were collected for microbiological analysis. Faecal specimens were collected from symptomatic event attendees.

A total of 82.2% of event attendees (129/157) completed a survey, of whom 49.6% (64/129) reported gastrointestinal illness resulting in an estimated non-fatal productivity loss of AUD $23,700. Univariate analysis of data collected from events on 16 November identified that illness was significantly associated with consumption of vegetarian rice paper rolls (risk ratio [RR]: 1.6; 95% confidence interval [95% CI]: 1.0-3.0; p = 0.04). Multiple foods were significantly associated with illness from events that occurred on 17 November 2022. On multivariable analysis, vegetarian rice paper rolls were associated with illness on 16 November 2022 (RR: 1.7; 95% CI: 1.01-2.8; p = 0.046); however no individual food categories were significantly associated with illness on 17 November 2022. Seven faecal specimens were positive for norovirus. While no food handlers reported illness prior to the outbreak, one food handler reported that their child had had gastroenteritis in the preceding week. Environmental Health inspection of the catering business identified inadequate handwashing facilities. Microbiological testing of seven food samples produced two marginal results: coagulase positive Staphylococcus in a sandwich egg mix and a high standard plate count in the roast beef.

This gastroenteritis outbreak was determined to be due to norovirus. The infection source was suspected to be an asymptomatic food handler and inadequate food handling controls allowing contamination of certain foods. This study demonstrates the importance of effective hand hygiene and food handling practices at all times, given that asymptomatic individuals can excrete and transmit norovirus and these outbreaks can be large and costly.