Symptoms of the disease – bloody diarrhoea, vomiting and fatigue – has showed in at least 173 dogs since August 1st, according to Norwegian food safety authority Mattilsynet.
A total of 43 have died, including one dog from Sweden, said the authority.
The case was reported to the authority by a Swedish veterinarian, who said that the dog had experienced bloody diarrhoea after returning from a visit to Norway and had later died.
However, it has not been confirmed that the dog was infected in Norway. Importantly, nor has it been confirmed that all cases, including the Swedish dog, are connected to the same outbreak – bloody diarrhoea is not uncommon for dogs and is not always serious, although owners are advised to consult a veterinarian.
Around 90 different dog breeds – breeds as diverse as Bichon Frise and German Shepherds – have been affected and there appears to be no clear link between them. Many of them have recovered after treatment.
Autopsies have so far been carried out on 15 dogs. The bacterium Providencia alcalifaciens has been detected in 12 of them, and the bacterium Clostridium perfringens in several of them, but it has not been confirmed they are causing the symptoms.
Norway's Veterinary Institute has ruled out a number of possible causes, including salmonella, parvovirus, Campylobacteria, tularemia and the accidental consummation of rat poisoning.
Tick-borne diseases are also believed to be unlikely.
Meanwhile, health authorities are warning owners to keep their dogs on a lead, and as a result of the mysterious outbreak, many owners have stopped taking their pets to public areas and a number of dog shows and gatherings have been cancelled.
At the first sign of symptoms, dog owners are urged to bring their pets to a veterinarian. While the illness has appeared mostly in greater Oslo, there have been a number of cases elsewhere in Norway.
There is nothing to indicate at this stage that the illness can be transmitted to humans. Apart from the one dogs who lived in Sweden, there have been no reports of similar cases in neighbouring countries.
Initial assessments indicate that in most cases where dogs live in the same household, only one of them has become ill, which the food safety authority writes suggests that the disease is not particularly contagious.