Sick worker likely caused large norovirus donut outbreak, finds study

The source of a norovirus outbreak that affected more than 200 people in an Australian state was likely an ill food handler, according to researchers.

In November 2021, the Australian Capital Territory Department of Health (ACT Health) was told by the owner of a food outlet about a report of illness among 11 people following the consumption of donuts. The bakery primarily sold donuts made onsite.

Donuts were a novel vehicle for foodborne illness in Australia, according to the study in Communicable Diseases Intelligence.

ACT Health also received two reports of illness on the same day related to the same bakery. Interviews with patients and their contacts found they had all eaten donuts from the shop 24 to 48 hours before becoming unwell. Orders were from walk-in customers, pre-booked requests or third-party food delivery applications, and no public alerts were issued by ACT Health.

True extent of outbreak unknown
A total of 301 people were interviewed or surveyed as part of the outbreak; 215 met the case definition. A further 16 people reported milder illness and did not meet the definition. It was one of the largest foodborne outbreaks ever investigated in the ACT.

The median age of respondents was 33 with a range of 3 to 91 years old and 159 were female. The median incubation period for ill people was 34 hours but this ranged from 9 to 112 hours.

The most common symptoms were diarrhea; vomiting; nausea and abdominal pain. Eight cases reported the presence of blood in stool and two people were hospitalized. All interviewed patrons ate food purchased from the premises between Nov. 20 and 24, 2021.

Eight cases had an incubation period of 96 hours or more. All these patients had at least one other member of their household who was a case and had earlier illness onset. They were likely infected by secondary transmission, rather than from a food source, said scientists.

Eight of 11 specimens collected from ill individuals were positive for norovirus. Food handlers at the shop did not provide stool specimens for testing.

Link to filled donuts but no flavor identified
A patient-control study included a group of 140 people at a catered function. A total of 192 donuts were prepared on the day. Overall, 59 people did the questionnaire, and 27 met the case definition. Eating any kind of filled donut was associated with becoming ill but no single flavor was identified as the source of infection.

An inspection at the bakery in late November found compliance with storage times could not be confirmed, because date labels were missing from some containers.

No pathogens were detected in the six food samples or 17 environmental swabs. However, no viral testing was done on samples as the ACT Government Analytical Laboratory (ACTGAL) is not able to test for viral pathogens.

The proprietor said staff was not ill during the study period and there were no records of vomiting or diarrhea among employees or customers. Order records were disposed of after being completed.

Two food samples showed results above detectable limits for coliforms; both were fillings made at the premises. Filled donuts required close handling of each one after cooking.

The source of contamination was not confirmed but it is suspected that a food handler worked while infectious and contaminated donuts. The scale of illness also suggests there was a lapse in hand hygiene and proper food handling procedures. Findings highlight the importance of excluding food handlers from work while ill-said researchers.

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