BACKGROUND: Microsporidia are rarely reported to cause outbreaks of diarrhea. We describe a foodborne outbreak of microsporidiosis from a workplace canteen in November 2020 in Denmark.

METHODS: A probable case was defined as any person using the canteen between November 4 and December 13, 2020, reporting at least one gastrointestinal symptom, whereas a confirmed case also had an Enterocytozoon bieneusi positive stool sample. A web-based questionnaire was used to collect clinical, epidemiological, and food exposure data. We performed a retrospective cohort study and tested stool samples from affected individuals for bacterial, viral, and parasitic pathogens, including E. bieneusi.

RESULTS: Altogether, 195 individuals completed the questionnaire. We identified 52 cases (65% male; median age 45 years [range 25 - 65]). Diarrhea (90%), fatigue (83%), and abdominal pain (79%) were the most commonly reported symptoms. Eight cases were laboratory-confirmed, and had E. bieneusi genotype C. The incubation period was between 5 and 12 days, and PCR-detectable spore shedding occurred up to 43 days after symptom onset. Disease was associated with consuming food from the workplace canteen on November 4, 2020 (RR, 2.8 [95% confidence interval [CI] 1.4 - 5.4]) and lunchboxes containing open sandwiches (RR, 3.2 [95% CI 1.4 - 7.2]) served that day.

CONCLUSIONS: This is the second documented foodborne outbreak of E. bieneusi genotype C-associated diarrhea worldwide. Epidemiological findings advocated an open sandwiches lunchbox from November 4, 2020, as a likely source. E. bieneusi may be an under-reported cause of outbreaks of diarrhea, and testing for it might be useful in foodborne outbreak investigations.