Polymyositis may occur in all breeds of dog. There is little veterinary data to monitor its incidence. A study in 2004 said that breed distribution approximated the 2002 American Kennel Club registration statistics – with the notable exception of Boxers and Newfoundlands.
The veterinary profession has published a few cases in other breeds. Apart from the K00ikerhondje the bodies representing those breeds have not undertaken any research to monitor the incidence of the disease (so far as we know)
Here are some case histories
German Wire-Haired Pointers – affected littermates
Newfoundlands – affected dam + 2 other littermates
The UK Vizsla PM Research project asks for DNA contributions from any breed with a histopathological diagnosis of Polymyositis