Dermatomyositis in Dogs: Skin and Muscle Disease in Collies and Shelties

By Emiel Maddens  ·  Reviewed in consultation with licensed veterinary professionals  ·  Updated June 2026  ·  10 min read

Photo by Karsten Winegeart on Unsplash

Dermatomyositis is a hereditary inflammatory condition unique among canine skin diseases because it targets two distinct tissue types simultaneously: the skin (dermis) and the skeletal muscles. The disease was first described in Collies and Shetland Sheepdogs in the early 1980s and has since been recognized as one of the best-characterized genetic skin diseases in veterinary medicine. Research has identified specific genes responsible for susceptibility, making genetic testing available to breeders for the first time.

For owners of affected breeds, dermatomyositis can be confusing and alarming. Puppies may develop facial lesions that look like mange, ringworm, or puppy impetigo, leading to misdiagnosis and inappropriate treatment. In mild cases, the lesions may wax and wane and eventually resolve with minimal intervention. In severe cases, however, the muscle component can cause difficulty eating, abnormal gait, and significant loss of body condition.

Understanding dermatomyositis, its genetic basis, the spectrum of clinical severity, and the principles of management allows owners to work effectively with their veterinarian and make informed decisions about treatment, prognosis, and breeding.

The Genetic Basis of Dermatomyositis

Dermatomyositis in dogs is a polygenic disease, meaning multiple genes contribute to whether a dog develops the condition and how severely it manifests. Research led by Dr. Leigh Anne Clark at Clemson University identified three key genetic loci: PAN2, MAP3K7CL, and DLA-DRB1. Each locus has risk and non-risk alleles, and the more risk alleles a dog carries across these three genes, the higher the probability of developing clinical disease and the greater the likely severity.

Risk Assessment Through Genetic Testing

A genetic test is now commercially available that evaluates all three loci and assigns a risk score. Dogs with zero to one risk alleles are at low risk; those with two to three risk alleles are at moderate risk; and those with four or more risk alleles are at high risk for developing clinical signs. Importantly, not every high-risk dog develops the disease, and environmental triggers such as ultraviolet light exposure, vaccines, and viral infections may influence whether and when signs appear.

The DLA-DRB1 gene encodes a major histocompatibility complex (MHC) class II molecule, which plays a central role in immune regulation. Certain DLA-DRB1 alleles appear to predispose the immune system to mount an inflammatory response against the dog's own skin and muscle tissue. This connection to the MHC system underscores the autoimmune nature of the disease and explains why immunomodulatory treatments can be effective.

How Dermatomyositis Affects Skin and Muscle

Skin Pathology

The skin lesions of dermatomyositis result from an interface dermatitis, a pattern where inflammatory cells attack the junction between the epidermis and dermis (the basement membrane zone). This targeted destruction causes the overlying epidermis to separate and slough, producing erosions, crusts, and areas of scarring alopecia. The inflammation also damages hair follicles, contributing to permanent hair loss in areas of repeated flare-ups.

Muscle Pathology

The myositis (muscle inflammation) component involves immune cells infiltrating skeletal muscle fibers, causing degeneration and atrophy. The muscles of mastication (chewing), the temporal muscles on the sides of the head, and the muscles of the distal limbs are most commonly affected. In severe cases, esophageal muscle involvement can lead to megaesophagus, a dilation of the esophagus that impairs swallowing and increases the risk of aspiration pneumonia.

The relative severity of skin versus muscle involvement varies widely between individual dogs. Some dogs develop prominent skin lesions with minimal or no detectable muscle disease, while others experience severe muscle wasting with relatively mild skin changes. This variability is thought to reflect differences in the specific combination of risk alleles each dog carries, as well as individual immune response patterns.

Facial lesions on the nose and around the eyes are among the earliest and most recognizable signs of dermatomyositis in Collies.

Photo by Anya Prygunova on Unsplash

Recognizing the Clinical Signs

Age of Onset

Dermatomyositis typically appears in puppies between 7 weeks and 6 months of age, though adult-onset cases have been reported. The disease often flares initially around the time of vaccination or during periods of stress, leading some owners to suspect a vaccine reaction. However, the genetic predisposition, not the vaccine itself, is the underlying cause.

Characteristic Skin Lesion Distribution

The skin lesions of dermatomyositis have a distinctive distribution that should raise immediate suspicion in predisposed breeds. The face is almost always affected first, with erythema, scaling, crusting, and alopecia developing on the bridge of the nose, around the eyes, on the inner ear pinnae, and on the lips. The ear tips frequently develop crusting and depigmentation. Other commonly affected sites include the tail tip, the point of the elbow, the hock, and the digits, essentially any area where the skin is thin and overlies bone.

Muscle Signs

Muscle involvement may present as a subtle reluctance to chew hard food, dropping kibble while eating, temporal muscle atrophy visible as hollowing on the sides of the head, or a stilted gait. In severe cases, generalized muscle wasting, difficulty swallowing, and regurgitation may develop. Dogs with megaesophagus are at serious risk for aspiration pneumonia, which can be life-threatening.

Severity Classification

Veterinary dermatologists generally classify dermatomyositis severity on a scale. Mild cases show only localized skin lesions on the face and extremities without muscle involvement. Moderate cases have more widespread skin lesions and some degree of muscle atrophy. Severe cases exhibit extensive skin disease, significant muscle wasting, difficulty eating, and sometimes megaesophagus.

Diagnosis of Dermatomyositis

Diagnosis begins with clinical suspicion based on breed, age of onset, and the characteristic distribution of skin lesions. However, definitive diagnosis requires supporting evidence because several other conditions can produce similar-looking lesions.

Skin Biopsy

Histopathology of skin biopsies reveals the classic interface dermatitis pattern, with lymphocytes and macrophages disrupting the dermal-epidermal junction. There is often follicular atrophy, perifollicular fibrosis, and reduced or absent sebaceous glands in chronically affected areas. These findings, in the appropriate clinical context, are highly supportive of the diagnosis.

Electromyography and Muscle Biopsy

For dogs with suspected muscle involvement, electromyography (EMG) can detect abnormal electrical activity in affected muscles. Muscle biopsy reveals inflammatory infiltrates within and around muscle fibers, along with fiber degeneration and regeneration. These tests are typically performed at referral centers and are most useful for distinguishing severe dermatomyositis from other causes of myopathy.

Genetic Testing

The commercially available genetic test for the three DMS-associated loci provides valuable supporting evidence for diagnosis and is essential for breeding decisions. A high-risk genotype in a dog with compatible clinical signs strongly supports the diagnosis. The test is performed on a cheek swab or blood sample and results are typically available within a few weeks.

Treatment and Management Strategies

Pentoxifylline

Pentoxifylline is a methylxanthine derivative that improves blood flow to damaged tissues and has anti-inflammatory properties. It is considered a first-line treatment for dermatomyositis because it can reduce the severity of skin lesions with a favorable safety profile. The typical dose is 10 to 15 mg per kilogram given two to three times daily with food. Improvement may take four to eight weeks to become apparent.

Vitamin E and Omega-3 Fatty Acids

High-dose vitamin E (400 to 800 IU daily for a medium to large breed dog) provides antioxidant protection that may help reduce inflammatory tissue damage. Omega-3 fatty acids from fish oil at therapeutic doses (EPA plus DHA at approximately 100 to 150 mg per kilogram body weight daily) have anti-inflammatory effects that complement other treatments. These supplements are safe for long-term use and are often combined with pentoxifylline.

Immunosuppressive Therapy for Severe Cases

Dogs with severe dermatomyositis, particularly those with significant muscle involvement or megaesophagus, may require more aggressive immunosuppression. Cyclosporine, mycophenolate mofetil, or combination therapy with tetracycline and niacinamide may be considered. Corticosteroids such as prednisone can provide rapid improvement but carry significant long-term side effects and are generally reserved for acute flares rather than maintenance therapy.

Sun Avoidance

Ultraviolet light is a known trigger for dermatomyositis flares. Keeping affected dogs out of direct sunlight during peak hours (10 AM to 4 PM), providing shaded outdoor areas, and applying pet-safe sunscreen to the nose and ear tips can reduce the frequency and severity of flare-ups. Indoor dogs should avoid prolonged sun exposure through windows as well.

Wound Care and Infection Prevention

The erosive skin lesions of dermatomyositis are portals for secondary bacterial infection. Gentle daily cleaning of affected areas with a mild antiseptic solution helps prevent colonization. Keeping the lesions clean and dry, avoiding bandaging that might adhere to fragile skin, and monitoring for signs of infection such as increased redness, swelling, or discharge are essential components of daily care.

Prognosis and Breeding Considerations

The prognosis for dermatomyositis depends heavily on the severity classification. Dogs with mild disease often experience spontaneous improvement or complete resolution of skin lesions by 12 to 18 months of age, and many live normal lifespans without significant treatment. Moderate cases typically require ongoing management but respond well to pentoxifylline and supportive care.

Severe cases carry a guarded prognosis, particularly when megaesophagus develops. Aspiration pneumonia is the most serious complication and can be fatal. Dogs with severe muscle wasting may have a diminished quality of life, and humane euthanasia may become a consideration if the disease progresses despite aggressive treatment.

Breeding Decisions

Genetic testing has transformed breeding programs for Collies and Shetland Sheepdogs. Responsible breeders now test all breeding stock for the three DMS-associated loci and select mating pairs to minimize the number of high-risk offspring. The goal is not to eliminate all risk alleles from the gene pool, as this would excessively narrow genetic diversity, but to avoid mating two moderate-risk or high-risk dogs together.

Affected dogs should not be bred, regardless of their clinical severity, because even mildly affected dogs carry and pass on risk alleles. Carriers (dogs with one or two risk alleles but no clinical signs) can be bred to low-risk partners with careful genetic planning.

Dermatomyositis in Dogs FAQ

Q: What breeds get dermatomyositis?

Dermatomyositis is primarily seen in Rough and Smooth Collies, Shetland Sheepdogs (Shelties), and crosses of these breeds. It has also been reported occasionally in Beaucerons, Australian Cattle Dogs, Pembroke Welsh Corgis, and a few other breeds. The genetic test for the three DMS-associated loci is currently validated for Collies and Shelties.

Q: At what age does dermatomyositis appear in dogs?

Most dogs develop their first skin lesions between 7 weeks and 6 months of age. The initial signs are often subtle, such as mild redness or scaling on the face, and may be mistaken for puppy skin irritation. Some dogs are not diagnosed until they are older, especially if the disease is mild or if early lesions were attributed to other causes.

Q: Is dermatomyositis in dogs the same as dermatomyositis in humans?

Canine dermatomyositis shares several features with the human disease, including the combination of skin and muscle inflammation, the interface dermatitis pattern on biopsy, and the autoimmune pathogenesis. However, the genetic basis differs between species, and the treatment protocols are not identical. The canine disease has served as a valuable natural model for studying the human condition.

Q: Can dermatomyositis in dogs be cured?

There is no cure for dermatomyositis because the genetic predisposition cannot be changed. However, many dogs with mild to moderate disease respond well to management with pentoxifylline, vitamin E, omega-3 fatty acids, and sun avoidance. Some mildly affected dogs experience spontaneous improvement and may not require long-term medication. Severe cases require ongoing treatment and monitoring.

Q: Should I get a genetic test for my Collie or Sheltie puppy?

Genetic testing is strongly recommended for all Collies and Shelties, regardless of whether they show clinical signs. Knowing your dog's DMS risk level helps your veterinarian interpret skin changes appropriately and initiate treatment early if needed. For breeders, testing is essential for making responsible mating decisions that reduce the incidence of the disease in future generations.

Sources

Clark LA, Credille KM, Murphy KE, Rees CA. Linkage of dermatomyositis in the Shetland Sheepdog to chromosome 35. Veterinary Dermatology. 2005;16(6):392-394.

Evans JM, Noorai RE, Tsai KL, et al. Beyond the MHC: A canine model of dermatomyositis shows a complex pattern of genetic risk involving novel loci. PLoS Genetics. 2017;13(2):e1006604.

Hargis AM, Haupt KH, Hegreberg GA, et al. Familial canine dermatomyositis: initial characterization of the cutaneous and muscular lesions. American Journal of Pathology. 1984;116(2):234-244.

Morris DO. Ischemic dermatopathy. Veterinary Clinics of North America: Small Animal Practice. 2013;43(1):99-111.

Shelton GD. Dermatomyositis. In: Cote E, ed. Clinical Veterinary Advisor: Dogs and Cats. 4th ed. St. Louis: Elsevier; 2020.

Related Reading

• Sebaceous Adenitis in Dogs, another autoimmune skin disease causing coat loss and scaling
• Vasculitis in Dogs, immune-mediated blood vessel inflammation that affects skin health
• Omega-3 for Dogs, how fatty acid supplementation supports skin barrier function in inflammatory skin diseases
• Chlorhexidine for Dogs, using topical antiseptics to prevent secondary infections in compromised skin

Emiel Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.