For autosomal dominant disorders, dogs with one or two copies of the disease variant are at risk of developing the condition. Inheriting two copies of the risk variant may make the risk higher or the condition more severe. They may produce puppies affected with the disorder if bred.
At risk dogs may show signs of this disease in their lifetime, although some will not develop the condition due to absence of additional risk factors.
Partner with your veterinarian to make a plan regarding your dog’s well-being, including any insights provided through genetic testing. If your pet is at risk or is showing signs of this disorder, then the first step is to speak with your veterinarian.
Dermatomyositis is an immune-mediated, inflammatory vasculopathy of the skin and muscles which occurs most commonly in Shetland Sheepdogs and Collies. Clinical signs vary in severity between individuals and often wax and wane over time. The age of onset is typically before 6 months and some as early as 7 weeks of age. Although, occasionally an adult onset is noted and flares in adulthood may occur. An environmental factor is often associated with the onset of DMS, such as vaccination, viral infection or stress (e.g. rehoming). Early clinical signs include scaling and crusting of the skin covering the bony prominences of the face, ear pinnae, tail tip, feet and limbs. Over time, alopecia and scarring may occur if skin lesions persist. Additional signs of DMS include myositis, which can vary in severity from mild pain and inflammation to marked muscle atrophy. Difficulties eating, drinking and swallowing may occur if the masticatory muscles or esophagus are affected. Affected dogs may also demonstrate an atypical, high-stepping gait. A skin or muscle biopsy is considered the gold standard for DMS diagnosis. Prognosis is highly variable and dependent upon the severity of clinical signs. Hormone changes associated with estrus, pregnancy and nursing may exacerbate signs.
There is no cure for DMS, but the condition can usually be managed with supportive care, immunosuppression, and/or symptomatic treatment. Additional sources of skin inflammation should be addressed by treating any secondary infections, limiting UV exposure, and avoiding topical irritants (e.g. harsh shampoos). Skin lesions will often respond to topical therapies, and oral immunosuppressants, vitamin E, and pentoxifylline have been reported to be helpful. Additional support may be required for dogs with severe myositis.
There are many responsibilities to consider when breeding dogs. Regardless of test results it is important that your dog is in good general health and that you are in a position to care for the puppies if new responsible owners are not found. For first time or novice breeders, advice can be found at most kennel club websites.
Three genetic risk factors for Dermatomyositis have been identified so far: locus A, locus B and locus C. The risk variant at the C locus is a DLA (Dog Leukocyte Antigen) haplotype common in both Collies and Shetland Sheepdogs. As the role of specific DLA haplotypes is poorly understood, it is important to view them as likely having both unknown benefits and risks depending on the situation. And, in general, DLA diversity is thought to be beneficial for the immune response. Therefore, it is recommended to select mating pairs based on locus A and B genotypes so that high risk genotypes (AaBB, AABb, AABB) are avoided in the offspring.