Dermatomyositis (DMS) is an autoimmune disease of the skin and muscle caused by a combination of environmental and genetic factors. DMS typically develops following an environmental trigger, such as vaccination or viral infection, and is exacerbated by subsequent stressors like exposure to UV light. Consistent with an environmental trigger, age at onset is variable with many cases occurring between seven weeks and six months of age, but others not developing until well into adulthood. The earliest clinical signs of DMS are crusting and scaling on the face, ears, tail tip, and/or across the bony prominences of the limbs and feet. Alopecia and more extensive skin lesions may develop over time. Lesions persist for weeks to months, and may or may not chronically recur.

Recently three genes that are associated with DMS were identified. A 3-gene DNA test helps determine the likelihood of an individual dog developing DMS. Certain combinations of alleles at these three genes are associated with an increased risk for development of DMS, while other combinations are rarely observed in affected dogs. Since the appearance of DMS in a dog is due to a combination of genetic and environmental factors, some dogs with a high genetic risk may not be exposed to environmental triggers and never develop the disease.

It is recommended that breeding pairs are selected according to their genotypes at 3 tested genes. Ideally, matings that could produce puppies with high-risk genotypes should be avoided. The American Shetland Sheepdog Association published more information about the disease and the use of DNA test results on their website. They also provided DMS Genotype Calculator and Punnett Squares for breeders to use the test results to determine possible genotypes that might result from a particular breeding.