Outbreak News Today reports that officials with the Norwegian Institute of Public Health report seeing an increase in listeriosis cases in December, prompting a warning for high-risk groups.

According to an official notice Friday (computer translated), six cases were reported this month when the country typically sees 1-2 cases a month.

Four of the six patients reported in December are from Hedmark and Oppland.

Health officials are working to identify if their is a common food source linked to the increase in cases.

Listeria is usually transmitted through food, especially long-life foods that are refrigerated and eaten without further heat treatment. Many of these food products are popular as Christmas foods and can be found on many Christmas parties.