Dermatomyositis (DMS), also known as Juvenile Dermatomyositis or Canine Familial Dermatomyositis is an inflammatory disease of the skin and muscles caused by an over reactive immune system1. This disease has consistently plagued Shetland Sheepdogs and Collies of all varieties. What makes this condition truly insidious is that although it mostly affects immature dogs, it can flare up seemingly out of nowhere to create issues in dogs of any age2. Although testing for the genetic mutations that pre-dispose certain dogs for this disease has been around for some time, interpreting the results can be complicated often leaving owners confused about the results of this test. Misunderstanding genetic results may lead to poor breeding decisions. Let us look closer at DMS, how the disease presents, the complexity of genetic testing, and how to best utilize the results of this test.
What is Dermatomyositis?
DMS is an inherited disease that causes dramatic inflammation of the skin, blood vessels and muscles in affected dogs. Lesions often originate and are limited to the skin of the face with the lips with the area around the eyes particularly affected3. Although the mechanisms of the disease are not completely understood, DMS stems from an over-reaction of the immune system to a stimulus; leading some individuals to inappropriately attribute the disease to a reaction to a vaccine or medication. However, there has been no causal relationship identified between the disease and vaccinations, infections, toxins, drugs or cancers3.
What should you be looking for?
Signs of the condition typically start with dogs less than a year of age. However, some dogs develop clinical signs later in life, seemingly out of nowhere with no pertinent history2. This is part of the reason why DMS has a reputation of spontaneous presentation. An owner will notice what looks like a small wound or sore on their dog’s face that can rapidly progress to significant ulcers and crusty hairless regions around the mouth and eyes. Affected areas can also include the footpads and any bony prominences like the points of the elbow or the tops of the paws3. The lesions tend not to be painful unless a secondary infection develops. Lesions are not limited to superficial skin issues; the muscles may become inflamed (myositis). The condition often affects the muscles of the face responsible for chewing leading to muscle atrophy3. Dogs with myositis have difficulty chewing and in severe cases, swallowing. The degree of affected muscles is consistent with the severity of the overall disease presentation. Dogs with mild presentation may show no muscular involvement, more severe cases may result in dogs that have difficulty walking, widespread atrophy, and stunted growth. Aspiration pneumonia is concerning for any dog that has swallowing difficulties. Mildly affected dogs may have few noticeable lesions which heal on their own with little to no scarring. Some dogs may develop muscle issues with no visible skin problems. For most affected dogs, the full extent of the disease peaks around one year of age or when the dog fully matures.
How is it treated?
Treatment is focused on management of the presenting skin lesions along with regulating the immune system to reduce the presentation.3 Treating the skin lesions with Essential Fatty Acids and Vitamin E may be sufficient for mild cases. Severe presentation may require the use of a drug called pentoxifylline and other immune-modulating medications.3 Antibiotics are needed for secondary infections. Although most dogs will respond to treatment, relapses are common. Severe skin lesions may result in scarring and long-term complications are possible for dogs with chronic DMS lesions.
Can it be prevented?
Because DMS is an inherited condition associated with specific genetic mutations, prevention falls to selective breeding. This condition has been confirmed autosomal dominant in the collie2. However, not every offspring from an affected parent will develop the disease and conversely, DMS can spontaneously appear in breeding lines with no prior history of the disease2. Genetic testing can determine the likelihood of DMS presenting in a dog and can facilitate breeding decisions.
What are the genetic mutations associated with this disease?
Genetic testing is not straight-forward. DMS is polygenic, or resulting from the combination of multiple genes2. Research into Collies and Shetland Sheepdogs with DMS identified variations in the genetic code, or mutations, in three regions of the genome associated with DMS risk. The three regions, or loci, of the genome are: PAN2 (Locus A), MAP3K7CL (Locus B), and DLA-DRB1 (Locus C). Specific combinations of these three loci are associated with an increased risk of developing DMS (see table below). For all three loci, the normal “wild type” versions or “low risk” alleles of the gene are represented by lower case letters, “a, b, or c” whereas the mutant “risk” alleles are represented by capital letters, “A, B or C”. The genetic test results for DMS produce a genotype that falls into one of four risk groups; low, moderate, high, and unknown risk as delineated in the following table.
How do you interpret and use your DMS results?
The genotypic code produced by this test details the risk of the dog tested for developing the condition. When determining appropriate breeding pairs, a recommended strategy is to avoid producing puppies with high-risk genotypes. One method would be to breed any “aa” genotype dog to any “bb” genotype dog because they will not produce puppies with moderate or high-risk genotypes. Likewise, breeding an “aabb” dog will not produce moderate or high-risk puppies. It is possible, by choosing appropriate breeding pairs, to continue to breed a dog with a high-risk genotype. Care must be taken when using the DMS test results, so dogs are not removed from the gene pool unnecessarily. Breeders must endeavor to maintain the desirable traits within their program while selectively breeding away from the high-risk genotypes. If all the dogs with moderate or high-risk genotypes were removed from breeding, the loss of genetic diversity within the breed could create alternative deleterious genetic effects.
Even getting back a result of unknown risk can be useful. If the appropriate mate is selected so moderate to high-risk offspring are avoided, then the positive traits of these dogs may be conserved. The American Shetland Sheepdog Association has an excellent DMS Genotype “Calculator” (link) that can be useful in determining potential outcomes from prospective breeding pairs based on the parental risk genotypes.
Why does this test cost more?
DMS testing does not identify a single mutation like other genetic disease tests. We are, in effect, looking at multiple sections of DNA from different parts of the canine genome for three specific alleles to identify the genotype that will determine a dog’s overall risk for DMS. Paw Print Genetics (PPG) follows the published standards and guidelines for canine genetic testing and produces a true “diagnostic” test. This means, when PPG tests for a mutation we perform the testing two independent times using different testing methods. This increases the accuracy of our testing by an order of magnitude. When you get results from PPG, you know that each sample has, in effect, been tested and then validated with a completely alternative method. What this means for testing DMS risk is that we have tested your sample no less than 6 times to produce your overall risk assessment.
DMS has long plagued the Sheltie and Collie breeds. What can be as simple as small lesions around the mouth and eyes, may also develop into a severe inflammatory condition. Most dogs present at an early age, but signs may also develop well into adulthood with no precursors or history. Because of the variable presentation, genetic testing is invaluable to reduce the prevalence of this disease. Understanding the risk category of dogs within a program will allow the breeder to move away from this disease, while conserving the quality traits of their dogs.
1) Berger D: Canine Dermatomyositis. Clinician's Brief 2016 Vol 14 (11) pp. 50-52.
2) Evans JM, Noorai RE, Tsai KL, Starr-Moss AN, Hill CM, Anderson KJ, Famula TR, Clark LA. Beyond the MHC: A canine model of dermatomyositis shows a complex pattern of genetic risk involving novel loci. PLOS Genetics. 2017 Feb 3; 12(2) [PubMed: 28158183]
3) Miller WH, Griffin CE, Campbell KL: Familial canine dermatomyositis. Muller and Kirk’s Small Animal Dermatology Saunders Elsevier 2013 pp. 585-7.
4) Haupt KH, Prieur DJ, Moore MP, et al: Familial canine dermatomyositis: clinical, electrodiagnostic, and genetic studies. Am J Vet Res 1985 Vol 46 (9) pp. 1861-9.