A mysterious illness in Norway has killed nearly two dozen dogs in recent days, with dozens more falling ill across the country as authorities race to find the cause, officials said.

At least 21 dogs have died after exhibiting symptoms of vomiting and bloody diarrhea, Norwegian Veterinary Institute Emergency and Safety Director Jorun Jarp told public broadcaster NRK on Friday. The Norwegian Food and Safety Authority reported that two more dogs died of the same symptoms over the weekend.

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The disease seemed "very serious for a dog. But we don't know yet whether this is contagious or just a series of individual cases," Food and Safety spokesman Ole-Herman Tronerud told NRK.

Veterinarians and pathologists have ruled out salmonella, rat poison, and food contamination, The Guardian reported. Jarp told the outlet they are investigating possible viral, bacterial, fungal and parasitic causes.

One line of the investigation follows the unusual boom in wild mushroom growth from Norway’s wet summer, the Independent reported.

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“It’s complex because the symptoms are normal in dogs from eating a mouse or mushrooms or bad water,” Asle Haukaas of the veterinary institute told the outlet. “What’s new is how rapid this illness has become.”

The illness was reportedly first discovered in and around the capital Oslo, but cases have since appeared in the cities of Bergen and Trondheim, and the northern Nordland municipality. In total, 14 of Norway's 18 counties have been affected.

The illness prompted the cancelation of all dog shows in Norway over the weekend, and a flood of questions from concerned owners in neighboring Sweden about the possible spread of the disease across the border, officials said.

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The veterinary institute said it currently had no information "to show that it is an infectious outbreak or that it would pose a risk to dogs in Sweden."

However, authorities have warned owners to keep their dogs leashed and away from other pups while the root cause of the illness is investigated, according to the Independent.

The Associated Press contributed to this report.