Veterinarians in the Oslo area began reporting sick dogs to the Norwegian Food Safety Authority this month. Since then, dogs in Bergen on the west coast, in the northern municipality of Nordland and a dozen other towns have fallen sick.

In a statement on Tuesday, the food safety authority said that “a mixed culture” of two types of bacteria, Providencia alcalifaciens and Clostridium perfringens, was found in some dogs’ guts. The bacteria are sometimes associated with diarrhea in people as well as dogs.

“But we can’t conclude that these are the cause of the symptoms we see,” the statement said.

At one point investigators hypothesized that the disease might be connected to an unusually large number of wild mushrooms growing this year. This theory remained under consideration on Tuesday, along with bacteria, viruses, and parasites, according to the food safety agency.

Salmonella and rat poison have been ruled out, according to the Norwegian Veterinary Institute.

It’s not yet clear how the illness is spreading — if it even is contagious — or whether the cases are related, according to the authorities.

But the Norwegian Food Safety Authority urged pet owners to avoid allowing their dogs near “dressage courses, exhibitions, hunting trials and the like” until more answers had emerged.