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City health inspectors in Minneapolis are investigating a summer increase in foodborne illnesses related to norovirus and Vibrio, a bacteria found in raw oysters.

The increases were highlighted in the city’s “food establishment” newsletter, released Thursday.

“The reason for the spike in norovirus outbreaks is not known,” the advisory stated. “The Vibrio outbreaks are due to higher concentrations of bacteria in some oyster beds during the summer.”

Cases of norovirus, a highly contagious bug that causes severe vomiting and diarrhea, are not required to be reported to the Minnesota Department of Health, but the state agency has received reports of a slight uptick that is unusual for this time of year. However, the number of cases appears nowhere near the numbers seen in peak norovirus season in the winter months, said department spokesman Doug Schultz.

“In England they call it the winter vomiting disease,” he said.

Minneapolis health officials reminded restaurant owners and workers to observe proper hand-washing and food preparation protocols, the primary ways to prevent norovirus infections.

Vibrio cases are more common at this time of year, because summer heat increases the temperatures of oyster beds, allowing the waterborne bacteria to persist in oysters that are then eaten raw.

Jeremy Olson • 612-673-7744