Authorities in Norway are stumped by a mysterious disease that has so far killed as many as 20 dogs. According to the Norwegian Food Safety Authority, it has received six further reports of dogs falling ill with similar symptoms: bloody diarrhea and vomiting. Two of these cases have already died.

Agency spokesman Ole-Herman Tronerud told public broadcaster NRK that the disease is “very serious for a dog.” A majority of the reported cases have been concentrated in Oslo, according to the Associated Press. Others have been found in the areas of Bergen, Trondheim and Nordland.

“But we don’t know yet whether this is contagious or just a series of individual cases,” Tronerund said.

The Norwegian Veterinary Institute has found two unspecified strains of bacteria during the autopsies of the deceased dogs. However, they were unable to confirm if these were the cause of the conditions. The NVI has been swamped by concerned owners in Sweden concerned that the illness may cross the border and begin affecting their own animals. So far, the NVI has no evidence to show that the illnesses represent an outbreak that might eventually threaten animals in Sweden.

“To have healthy and great Norwegian dogs die so quickly is naturally serious. It’s a very special situation I haven’t been involved in before,” the institute’s emergency and safety director, Jorun Jarp, said about the situation.

dogs playing Two dogs, a Golden Doodle and a Border Collie mix play in the water on the north shore of Long Island at Coindre Hall on April 21, 2014 in Huntington, New York. Photo: Bruce Bennett/Getty Images