A mystery illness that is killing dogs across Norway could be linked to high numbers of mushrooms growing wild this year, investigators say.

Vets and scientists racing to discover the cause of the disease thought to have proved fatal for 26 pets are examining a range of possible causes including viruses, pollution, hot weather and new parasites.

But a wet summer in Norway has provided conditions for mushrooms to flourish in gardens and forests - one line of inquiry.

Dogs falling ill suffer bloody diarrhoea and/or vomiting, and their general condition rapidly deteriorates, owners say. Some animals have died in less than 24 hours, before they could be rushed to a vet.

Owners are so worried about the disease outbreak that they have inundated authorities with enquiries.

The Norwegian Kennel Club said desperate owners searching for information over the weekend made the website crash, The Guardian reported.

About 120,000 users were logging on each day to read updates, the club said.

All dog shows in Norway were cancelled at the weekend for fear of spreading the disease.

One dog owner tweeted: “There’s a dog disease epidemic in Norway atm and it’s reached the area where my dog lives and I’m terrified.”

Another posted: “I just woke up to my dog being sick – I’m so scared.”

However, it’s not certain the infection is being spread between animals.

The Norwegian Veterinary Institute told The Independent one owner said at the weekend his two dogs – who were “guaranteed” not to have had contact with other animals – had fallen ill. They recovered with treatment.

Asle Haukaas, of the institute, said that lines of investigation included viruses, extremes of weather, new parasites and this year’s higher than usual growth of mushrooms.

“It’s complex because the symptoms are normal in dogs from eating a mouse or mushrooms or bad water. What’s new is how rapid this illness has become,” he said.

Dogs of all breeds, ages and sizes were vulnerable, he said.

Norway's Food Safety Authority says 26 pets have died so far.

According to the Kennel Club, scientists have ruled out salmonella, campylobacter and rat poison.

The disease was first discovered in Oslo, in the south-east, but cases have since cropped up in 14 of the country’s 18 counties, especially the east, and including in the far north.

Swedish newspaper GT said a dog across the border in Sweden was also being treated after appearing at a show in Trondheim, Norway.

Authorities are warning owners to keep their pets on a lead and away from other dogs until the cause is established.

The Norwegian Food Safety Authority, the Norwegian University of Life Sciences and animal hospitals are working with the Veterinary Institute to find the cause of the disease.

The Kennel Club said social media users had accused certain feed producers of being guilty but vets say the dogs affected have eaten a wide range of feed products, and food is just one of many factors being looked at.

Initial test results are due out within 24 hours.

According to The Norway Post, more than 1,000 species of mushroom are found in Norwegian forests. Of those, up to 15 can be deadly to humans.