Merck Manual

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Professional Version

Polymyositis in Dogs


Joseph Harari

, MS, DVM, DACVS, Veterinary Surgical Specialists, Spokane, WA

Reviewed/Revised Nov 2020 | Modified Apr 2023

Polymyositis is a systemic, noninfectious, possibly immune-mediated, inflammatory muscle disorder in adult dogs. It may be acute or chronic and progressive. Clinical signs include depression, lethargy, weakness, weight loss, lameness Overview of Lameness in Small Animals Clinical signs of musculoskeletal disorders include weakness, lameness, limb swelling, and joint dysfunction. Motor or sensory neurologic impairment may develop secondary to neuromuscular lesions... read more , myalgia, and muscle atrophy. CK may be increased, and electromyography reveals abnormal spontaneous muscle activity. Muscle biopsy reveals myonecrosis, lymphocytic-plasmacytic perimuscular infiltration, phagocytosis, and fiber regeneration. Polymyositis may be associated with megaesophagus and immune-mediated disorders (myasthenia gravis, lupus erythematosus, polyarthritis Polyarthritis in Dogs and Cats Polyarthritis involves inflammation of multiple joints and is classified as infectious ( septic arthritis) or noninfectious (erosive or nonerosive [ immune-mediated]). Nonerosive can be idiopathic... read more ). Prednisone or prednisolone (1–2 mg/kg, PO, every 24 hours for 3–4 weeks) is the treatment of choice; other immunosuppressive agents, such as azathioprine or cyclophosphamide, can also be used. Prognosis is favorable, although relapses are not uncommon.

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