BACKGROUND: Norovirus causes approximately 60% of foodborne outbreaks identified in Minnesota. Nearly all are detected through the Minnesota Department of Health (MDH) Foodborne Illness Hotline, a centralized statewide complaint system. During August 2016, several complaints were received, and investigations were initiated at: a restaurant; two Ice Cream X retail locations; and a private gathering.
METHODS: Sanitarians from the City of Minneapolis Health Department, Hennepin County Human Services and Public Health Department (HSPHD), and the Minnesota Department of Agriculture (MDA) visited the food service facilities to implement norovirus interventions and assess food preparation procedures. MDH and HSPHD staff interviewed patrons and conducted a case-control study for the private gathering. A case was defined as an individual from a complainant group who developed vomiting or diarrhea (≥3 loose stools in a 24-hour period). Stool samples were tested at MDH, and ice cream and frozen raspberries were submitted to the FDA Center for Food Safety and Applied Nutrition (CFSAN) for norovirus testing.
RESULTS: Fifteen cases from four venues were identified. Meal dates occurred during August 4-14. Stool samples from patrons of three venues yielded the outbreak strain of norovirus GII.17 Kawasaki. All cases reported eating raspberry chocolate chip flavor Ice Cream X. Consumption of raspberry chocolate chip ice cream (5/5 cases vs. 1/8 controls; odds ratio, undefined; p=0.005) was significantly associated with illness at the private gathering. Ice Cream X is manufactured at a central commissary in Minnesota. Imported frozen raspberries were the only ingredient used exclusively in the raspberry chocolate chip flavor, and were not subjected to a kill step. A large shipment of raspberries sourced from Serbia, China, and Chile was received on July 22. No commissary workers reported recent illness. The manufacturer initiated a product withdrawal. Results from food samples sent to CFSAN in September are pending.
CONCLUSIONS: This was an outbreak of norovirus gastroenteritis associated with consumption of ice cream at multiple venues. The ice cream was produced and distributed by a Minnesota manufacturer. Imported frozen raspberries used in the implicated flavor were the suspected vehicle. Though norovirus outbreaks have been linked to frozen raspberries in Europe, this is the first norovirus outbreak identified in Minnesota due to a non-shellfish commercially distributed food. A centralized complaint hotline, rapid follow-up and investigation of all suspected norovirus outbreaks, and good agency collaboration were all key to the successful identification of a norovirus outbreak caused by a distributed food item.