From Dog
Polymyositis in a Hungarian Vizsla showing characteristic muscle atrophy to the head[1]
Dysbasia and limb contracture associated with polymyositis in a Boxer dog[2]

Polymyositis is an immune-mediated disease of skeletal muscle.

This disease can affect all skeletal muscles including the masticatory muscles, limb muscles and esophageal, laryngeal and pharyngeal muscles. In rare cases, secondary myocarditis and right atrial dilatation has also been reported[3].

It can occur alone or in association with other immune-mediated diseases such as systemic lupus erythematosus.

Breed-specific polymyositis has been reported in Hungarian Vizsla dogs[4].

Polymyositis has also been reported subsequent to Vogt-Koyanagi-Harada (VKH)-like syndrome in a dog, an autoimmune disease that targets melanocyte antigens[5].

The initiating immune-mediated process associated with polymyositis presumably involves the initial auto-antibody attack of myosin[6] by auto-expression of MHC class I and CD8+ T-cell infiltration[7], followed by subsequent release of serotonin (5-HT)[8], leading to fibrosis and cicatrization of skeletal muscles.

In dogs, a number of reports have linked polymyositis with lymphoma[9][10], thymoma[11], myeloid leukemia, bronchogenic carcinoma, tonsillary carcinoma[12], anaplastic round cell tumor and plasmacytoma[13]. These diseases may well result in secondary infiltration of muscle fibres with inflammatory byproducts.

Clinically affected dogs are often middle-aged, and present with generalized weakness, dysbasia (stiff stilted gait), variable myalgia, intermittent vomiting and dysphagia, dysphonia[14], weight loss and muscle atrophy and contractures to many skeletal muscles, and facial and tongue atrophy[15].

Hematology and biochemical analysis of blood samples is often unrewarding, although there may be a mild eosinophilia and increases in creatinine kinase and aspartate aminotransferase levels.

Diagnosis is usually confirmed by multiple muscle biopsies, such as the biceps femoris or vastus lateralis.

Unlike masticatory muscle myositis, immunohistochemical testing for 2M antibodies is often negative, even with clinical involvement of the masticatory muscles[16].

A differential diagnosis would include Toxoplasma gondii, Hepatozoon americanum, Sarcocystis neurona[17], Neospora spp[18], Hammondia hammondi, masticatory muscle myositis, tetanus, myasthenia gravis[19], Duchenne muscular dystrophy[20], Golden Retriever Muscular Dystrophy, thymoma[21] and dermatomyositis.

Polymyositis is usually responsive to prednisolone at immunosuppressive doses, and full recovery is not uncommon, as evidenced by histological evidence of muscle regeneration provided the inflammation and fibrosis can be stopped or reduced[22].

Periodic screening is also recommended as 10% of affected dogs develop neoplasia within 12 months of diagnosis, particularly lymphoma in Boxers[14].


  1. Veterinary Neurologist
  2. Uni of California, San Diego
  3. Warman S et al (2008) Dilatation of the right atrium in a dog with polymyositis and myocarditis. J Small Anim Pract 49(6):302-305
  4. Haley AC et al (2011) Breed-specific polymyositis in Hungarian Vizsla dogs. J Vet Intern Med 25(2):393-397
  5. Baiker K et al (2011) Polymyositis following Vogt-Koyanagi-Harada-like syndrome in a Jack Russell terrier. J Comp Pathol 144(4):317-323
  6. Neumann J & Bilzer T (2006) Evidence for MHC I-restricted CD8+ T-cell-mediated immunopathology in canine masticatory muscle myositis and polymyositis. Muscle Nerve 33(2):215-224
  7. Morita T et al (2002) Myofiber expression of class I major histocompatibility complex accompanied by CD8+ T-cell-associated myofiber injury in a case of canine polymyositis. Vet Pathol 39(4):512-515
  8. Pavone LM et al (2012) Role of serotonergic system in the pathogenesis of fibrosis in canine idiopathic inflammatory myopathies. Neuromuscul Disord 22(6):549-557
  9. Bennet SL et al (2005) Lymphoma(s) showing epitheliotropism and diffuse skeletal muscle involvement presenting as polymyopathy in a young dog. Aust Vet J 83:612–615
  10. Neravanda D et al (2009) Lymphoma-associated polymyositis in dogs. J Vet Intern Med 23:1293–1298
  11. Aronsohn MG et al (1984) Clinical and pathologic features of thymoma in 15 dogs. J Am Vet Med Assoc 184:1355–1362
  12. Griffiths IR et al (1973) Neuromuscular disease in dogs: Some aspects of its investigation and diagnosis. J Small Anim Pract 14:533–554
  13. Evans J et al (2004) Canine inflammatory myopathies: A clinicopathologic review of 200 cases. J Vet Intern Med 18:679–691
  14. 14.0 14.1 Evans J et al (2004) Canine inflammatory myopathies: a clinicopathologic review of 200 cases. J Vet Intern Med 18(5):679-691
  15. Toyoda K et al (2010) Inflammatory myopathy with severe tongue atrophy in Pembroke Welsh Corgi dogs. J Vet Diagn Invest 22(6):876-885
  16. Shelton, GD (2009) Treatment of myopathies and neuropathies. In: Kirk's curernt veterinary therapy. XIV. Saunders, Elsevier, Iowa. pp:1113-1114
  17. Vashisht K et al (2005) Naturally occurring Sarcocystis neurona-like infection in a dog with myositis. Vet Parasitol 133(1):19-25
  18. Reichel MP et al (2007) Neosporosis and hammondiosis in dogs. J Small Anim Pract 48(6):308-312
  19. Clooten JK et al (2003) Myasthenia gravis and masticatory muscle myositis in a dog. Can Vet J 44(6):480-483
  20. McDonnell JJ et al (2001) Neurologic conditions causing lameness in companion animals. Vet Clin North Am Small Anim Pract 31(1):17-38
  21. Hackett TB et al (1995) Third degree atrioventricular block and acquired myasthenia gravis in four dogs. J Am Vet Med Assoc 206:1173–1176
  22. Salvadori C et al (2005) Muscle regeneration, inflammation, and connective tissue expansion in canine inflammatory myopathy. Muscle Nerve 31(2):192-198