ISSN 2398-2969      

Idiopathic polymyositis


Synonym(s): Immune-mediated polymyositis


  • Diffuse non-suppurative inflammatory process dominated by lymphocyte infiltration of skeletal muscles presumed to have immune-mediated basis.
  • Large breed adult dogs most commonly affected. Breed-associated in Hungarian Vizslas, Newfoundlands, German Shepherds, Doberman Pinchers, Labradors and Boxers.
Print off the owner factsheet on Myositis Myositis to give to your client.



  • Cause unknown, although responsiveness of disease to immunosuppressive therapy suggests immune-mediated pathogenesis.
  • Can be paraneoplastic syndrome Neoplasia: paraneoplastic disease secondary to tumor such as extramuscular multicentric lymphoma and primary cutaneous round cell tumor. Neoplasia tends to develop with 12 months of the diagnosis of polymyositis.

Predisposing factors

  • None identified.


  • Polymyositis suspected to be autoimmune in origin.
  • Individual muscle fibers targeted by immune system leading to necrosis, vasculitis Peripheral vascular disease and fibrosis.
  • Immunohistochemical studies have provided strong evidence for cellular immune effector mechanisms in pathogenesis; HLA-restricted, antigen specific, T-cell mediated myocytoxicity is predominant component of the immune response.
  • Likely that cytotoxic CD8+ T cell recognizes antigen(s)-presented class I molecules on the sarcolemma.
  • Inflammatory exudate is predominantly endomysial, selectively enriched with CD8+ T cells; concentration of B cells is negligible at endomysium. However strong macrophage component to the inflammation is present.
  • CD8+ natural killer (NK) cells virtually absent in pathology of this disease.
  • Polymyositis reported in dogs with various autoimmune diseases.
  • Multifocal necrosis and phagocytosis of type I and type II myofibers is accompanied by vasculitis Peripheral vascular disease due to perivascular accumulation of lymphocytic and plasmocytic cells.


  • Timecourse of disease extremely variable depending on predominant clinical signs.
  • Dysphagia noted quickly by owner leads to rapid presentation for diagnosis.
  • Intermittent weakness however, can be present for several weeks before progression can cause significant exercise intolerance to warrant presentation for veterinary investigation. Onset may be acute in cases that are accompanied by significant muscle pain.


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login


This article is available in full to registered subscribers

Sign up now to start a free trial to access all Vetlexicon articles, images, sounds and videos, or Login

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Tauro A, Addicott D, Foale R D et al (2015) Clinical features of idiopathic inflammatory polymyopathy in the Hungarian Vizsla. BMC Vet Res 11, 97 PubMed.
  • Massey J, Rothwell S, Rusbridge C et al (2013) Association of an MHC class II haplotype with increased risk of polymyositis in Hungaria Vizsla dogs. PLoS One (2), e56490 PubMed.
  • Gianella P, Avallone G, Bellino C et al (2012) Primary cutaneous undifferentiated round cell tumour with concurrent polymyositis in a dog. Can Vet J 53 (5), 549-553 PubMed.
  • Haley A C, Platt S R, Kent M et al (2011) Breed-specific polymyositis in Hungarian Vizsla dogs. J Vet Intern Med 25 (2), 393-397 PubMed.
  • Toyoda K, Uchida K, Matsuki N (2010) Inflammatory myopathy with severe tongue atrophy in Pembroke Welsh Corgi dogs. J Vet Diagn Invest 22 (6), 876-885 PubMed.
  • Neravanda D, Kent M, Platt S R et al (2009) Lymphoma-associated polymyositis in dogs. J Vet Intern Med 23 (6), 1293-1298 PubMed.
  • Shelton G D (2007) From dog to man: The broad spectrum of inflammatory myopathies. Neuromusc Disord 17 (9-10), 663-670 PubMed.
  • Platt S R, McConnell J F, Garosi L S et al (2006) Magnetic resonance imaging in the diagnosis of canine inflammatory myopathies in three dogs. Vet Radiol Ultrasound 47 (6), 532-537 PubMed.
  • Evans J, Levesque D, Shelton G D (2004) Canine inflammatory myopathies: A clinicopathologic review of 200 cases. J Vet Intern Med 18 (5), 679-691 PubMed.
  • Podell M (2002) Inflammatory myopathies. Vet Clin North Am: Sm Anim Pract 32 (1), 147-167 PubMed.
  • Taylor S M (2000) Selected disorders of muscle and the neuromuscular junction.Vet Clin North Am: Sm Anim Pract 30 (1) 59-75 PubMed.
  • Morozumi M, Oyama Y, Kurosu Y, Nakayama H, Goto N, Yasuda K, Sasaki N & Tokuriki M (1991) Immune-mediated polymyositis in a dog. J Vet Med Sci 53 (3), 511-512 PubMed.
  • Presthus J & Lindboe C F (1988) Polymyositis in two German wirehaired pointer littermates. J Sm Anim Pract 29 (4), 239-248 VetMedResource.

Other sources of information

  • Braund K G (2003) Myopathic disorders. In: Clinical Neurology in Small Animals - Localization, Diagnosis, and Treatment. International Veterinary Information Service, Ithaca NY (

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field