Polymyositis is an immune-mediated, inflammatory myopathy of the appendicular muscles. It primarily affects adult dogs; however, it has been known to occur in cats.
Polymyositis may be acute or chronic and progressive. Breed-associated polymyositis occurs in Newfoundlands, Boxers, and Vizslas. Any dog breed can be affected; however, large-breed dogs, Newfoundlands, and Boxers appear to be overrepresented.
Clinical signs of polymyositis include the following:
lethargy
fever
generalized weakness that often worsens with exercise
weight loss
shifting leg lameness
myalgia
muscle swelling
regurgitation (due to megaesophagus)
voice change
muscular atrophy (in the chronic phase; see and )
If pharyngeal or laryngeal muscles are involved, voice change, dysphagia, or dyspnea is possible (see ).
The Pembroke Welsh Corgi exhibits dysphagia with severe tongue atrophy (1), as well as facial muscular atrophy and occasional difficulty walking.
Courtesy of Shinichi Kanazono, DVM, DACVIM (Neurology).
Courtesy of Shinichi Kanazono, DVM, DACVIM (Neurology).
Diagnosis is usually based on clinical signs. Serum CK and AST activity may be increased. Electromyographic evaluation reveals abnormal spontaneous muscle activity. Muscle biopsy is ideal for confirming polymyositis and reveals myonecrosis, lymphoplasmacytic perimuscular infiltration, phagocytosis, and fiber regeneration.
Polymyositis may be associated with immune-mediated disorders (myasthenia gravis, systemic lupus erythematosus, polyarthritis) and the use of trimethoprim-sulfa antimicrobials.A subset of Boxers developed round-cell tumors months after developing autoimmune polymyositis (2), suggesting that the latter may represent a preneoplastic syndrome in this breed.
Glucocorticoids, such as prednisone (2 mg/kg, PO, every 24 hours), are the treatment of choice for polymyositis (3, 4). They should be administered at immunosuppressive doses until clinical remission is achieved, then tapered. Other immunosuppressive agents (eg, azathioprine, cyclosporine, or cyclophosphamide) can be administered in combination with steroids in severe or relapsing cases or to maintain control of disease after steroids have been tapered.
The prognosis for dogs and cats with polymyositis is favorable; however, relapses can occur.
For other inflammatory myopathies in dogs and cats, see the Overview for this chapter.
For More Information
Haley AC, Platt SR, Kent M, et al. Breed-specific polymyositis in Hungarian Vizsla dogs. J Vet Intern Med. 2011;25(2):393-397.
Hong HP, Thomovsky SA, Lewis MJ, Bentley RT, Shelton GD. Clinical characteristics of non-infectious inflammatory myopathy in the boxer dog. J Small Anim Pract. 2021;62(9):765-774.
Neravanda D, Kent M, Platt SR, Gruenenfelder FI, Shelton GD, Schatzberg SJ. Lymphoma-associated polymyositis in dogs. J Vet Intern Med. 2009;23(6):1293-1298.
Also see pet owner content regarding polymyositis in dogs.
References
Ito D, Okada M, Jeffery ND, Kitagawa M, Uchida K, Watari T. Symptomatic tongue atrophy due to atypical polymyositis in a Pembroke Welsh Corgi. J Vet Med Sci. 2009;71(8):1063-1067. doi:10.1292/jvms.71.1063
Evans J, Levesque D, Shelton GD. Canine inflammatory myopathies: a clinicopathologic review of 200 cases. J Vet Intern Med. 2004;18(5):679-691. doi:10.1892/0891-6640(2004)18<679:cimacr>2.0.co;2
Kornegay JN, Gorgacz EJ, Dawe DL, Bowen JM, White NA, DeBuysscher EV. Polymyositis in dogs. J Am Vet Med Assoc. 1980;176(5):431-438.
Han J, Jang K, Cho SB, Kim S, Oh S, Kim HJ. Successful diagnosis and treatment of canine polymyositis: utilizing MRI and immunohistochemistry for accurate detection. BMC Vet Res. 2024;20(1):560. doi:10.1186/s12917-024-04356-6
