Vet’s 2025 Guide to Polymyositis & Dermatomyositis in Dogs – Immune Muscle Disorders 🐶

By Dr. Duncan Houston BVSc

💡 What Are These Conditions?

Polymyositis is an inflammatory, immune-mediated condition causing skeletal muscle inflammation and weakness. It may be acute or chronic. Dermatomyositis adds skin signs—red, crusted lesions—typically in Collies, Shelties, and Australian Cattle Dogs.

⚠️ Who Gets Affected?

• Polymyositis: often adult large breeds like Newfoundlands and boxers
• Dermatomyositis: young collies, Shelties, Aussies—often before 6 months old

🧩 Signs & Symptoms

• Muscle weakness in proximal limbs—difficulty rising, walking
• Stiff/stilted gait, muscle pain, exercise intolerance, muscle swelling
• Muscle atrophy over time
• Possible megaesophagus/regurgitation if the esophageal muscle involved 
• In dermatomyositis: facial, footpad, ear-tip lesions—erythema, crusting, hair loss

🔍 Causes & Triggers

• Autoimmune reactions, sometimes infection, or paraneoplastic processes
• Possible genetic predisposition in certain breeds

🧪 How It’s Diagnosed

Diagnosis involves:

• History & physical exam—gait, skin, and swallowing issues 
• Blood tests—elevated muscle enzymes (CK, AST)
• Electromyography—myopathic patterns
• Muscle and/or skin biopsy—definitive for inflammatory changes
• Imaging (X-ray) if swallowing/regurgitation is present 

💊 Treatment Strategies

• Corticosteroids (e.g. prednisone) are first-line for immune suppression
• Additional immunosuppressives: azathioprine, cyclosporine, methotrexate if needed
• Treat any underlying infection or cancer
• Pain relief and physiotherapy to support muscle recovery
• Feeding modifications (elevated bowls, consistency) for megaesophagus
• Skin care in dermatomyositis: avoid sun exposure, use topical steroids or antimicrobials as needed

📈 Prognosis & Monitoring

• Polymyositis: usually good with treatment, though relapses are common 
• Dermatomyositis: skin flare-ups may recur; muscle involvement depends on severity
• Regular rechecks: enzyme levels, muscle strength, skin evaluation

🛡️ Prevention & Owner Action

• Avoid known triggers—vaccines or drugs if previously implicated
• Early veterinary evaluation if weakness, skin lesions, or swallowing trouble appear
• Genetic counseling—avoid breeding affected dogs

📲 Tools from Ask A Vet, Woopf & Purrz

• Ask A Vet – consult immediately for flare-ups or new symptoms 📱

🌟 Case Example

Case: Jasper, a 3‑year‑old Boxer, developed muscle weakness and a stiff gait. Bloodwork showed elevated CK; biopsy confirmed polymyositis. Treated with prednisone and azathioprine with steady improvement over weeks. Episodes monitored and managed with Ask A Vet follow‑ups. 🐾

✅ Key Takeaways

• Polymyositis = muscle inflammation; dermatomyositis = muscle + skin
• Signs: weakness, pain, gait changes, skin lesions, and swallowing issues
• Diagnosis via enzymes, EMG, biopsy
• Immunosuppressive therapy is effective; relapses need monitoring
• Owner vigilance + Ask A Vet access crucial for flare management 💪

📥 Get Support Anytime

If your dog shows signs like muscle weakness, stiffness, or skin lesions, download the Ask A Vet app for live advice and care planning. Visit AskAVet.com now. 🐾🩺