Inflammatory myopathies, a group of disorders characterized by muscle inflammation and degeneration, are some of the most common muscle diseases in dogs (2,13). The most prevalent inflammatory myopathies caused by autoimmunity are idiopathic polymyositis and masticatory myositis (5,13). The latter is a focal myositis affecting the muscles of mastication and leading to characteristic clinical manifestations such as jaw pain, trismus, and muscle atrophy (1,9,13,15). Masticatory muscles contain type 2M myofibers that differ histopathologically, immunologically, and biochemically from fibers in the limb musculature. More than 80% dogs with masticatory myopathy show positive findings in tests for autoantibodies against type 2M myofibers, which show high sensitivity (85-90%) and specificity (100%) (9,15,18).

In contrast, idiopathic polymyositis is a more generalized inflammatory myopathy that can affect multiple muscle groups, often presenting with nonspecific clinical signs such as fever, depression, and weight loss (2,5,8,13). These symptoms can overlap with those of masticatory myositis and other neuromuscular diseases. Therefore, a thorough clinical examination, including diagnostic imaging and muscle biopsies, is required for an accurate differential diagnosis (5,8,13).

Here we describe the clinical course and diagnostic test findings for a dog with idiopathic polymyositis showing nonspecific clinical signs, including lethargy, reluctance to walk, and intermittent fever, that was initially suspected to be masticatory myositis.

An 11-year-old spayed female mixed-breed dog weighing 7.6 kg presented with lethargy and reluctance to walk. The veterinarian at the local animal hospital suspected orthopedic disease and prescribed meloxicam (0.1 mg/kg, q24h, Metacam^® tab; Boehringer Ingelheim, Ingelheim, Germany). However, the clinical signs waxed and waned, and the dog was referred to the Western Referral Animal Medical Center 3 weeks after symptom onset.

Physical examination revealed a decrease in body weight from 7.6 kg before symptom onset to 6.8 kg, with a body condition score of 3/9, mild muscle loss indicated by the muscle condition score, and fever (body temperature of 40°C). In addition, mild cervical stiffness and hindlimb weakness were observed. No abnormalities or pain reaction were seen in the neurological examination, including cranial nerve assessment, postural reaction tests, tests for spinal reflexes, and palpation. Complete blood count revealed non-regenerative anemia (red blood cells: 3.48 × 10¹²/L; reference interval [RI], 5.65-8.87 × 10¹²/L; hematocrit: 28.9%; RI, 37.3-61.7%; and hemoglobin: 9.6 g/dL; RI, 13.1-20.5 g/dL), leukocytosis with neutrophilia (neutrophils: 30,170/L; RI, 2,950-11,640/L), and thrombocytopenia (platelets: 110,000/μL; RI, 148,000-484,000/μL). Blood-gas analysis revealed hypokalemia (potassium: 2.7 mmol/L; RI, 3.5-5.8 mmol/L). The serum biochemical profile revealed markedly elevated levels of creatine kinase (CK: 3,667 U/L; RI, 10-200 U/L) and C-reactive protein (CRP: 9.9 mg/dL; RI, 0.1-1.0 mg/dL). Hypoalbuminemia (albumin: 2.0 g/dL; RI, 2.2-3.9 g/dL) and increased alanine aminotransferase (272 U/L; RI, 10-125 U/L) and alkaline phosphatase (1,129 U/L; RI, 23-212 U/L) levels were also observed. Coagulation parameters included an elevated D-dimer level (9,033.36 ng/mL; RI, 50-250 ng/mL), and thromboelastography showed hypercoagulability (a decreased K time and increased angle, maximum amplitude, and clot strength). A fever of unknown origin polymerase chain reaction panel (IDEXX Laboratories, Inc., Seoul, Korea) using the whole blood and urine showed negative results for DNA amplification of Babesia spp., Anaplasma spp., Ehrlichia spp., Rickettsia spp., Hepatozoon spp., Leishmania spp., Bartonella spp., Brucella canis, Trypanosoma cruzi, Blastomyces dermatitidis, Coccidioides spp., Cryptococcus spp., Histoplasma capsulatum, Neospora spp., Leptospira spp., and Toxoplasma gondii.

Thoracic and abdominal radiographs revealed no abnormal features. Abdominal ultrasonography revealed gallbladder sludge, hepatomegaly with hyperechoic parenchyma, and left adrenomegaly (cranial and caudal pole thicknesses of 16.1 mm and 14.7 mm, respectively) with hyperechoic nodules in the cranial pole. In relation to these observations, an adrenocorticotropic hormone (ACTH)-stimulation test was performed, which revealed increased pre- and post-stimulation cortisol levels (9.9 and 29.9 μg/dL; RI, 1.0-6.0 and 6.0-18.0 μg/dL, respectively). Although the total T4 level was low (0.8 μg/dL; RI, 1.0-4.0 μg/dL), the free T4 (0.7 ng/dL; RI, 0.6-3.7 ng/dL) and thyroid-stimulating hormone (0.41 ng/mL; RI, 0.05-0.42 ng/mL) levels were marginally within the normal range.

According to the above results, a neurological disease, including intervertebral-disc disease or steroid-responsive meningitis-arteritis, was suspected. While the owners were reluctant to proceed with further diagnostic tests, the dog’s appetite gradually decreased, and trismus newly developed 5 days after visit to our hospital. Therefore, serology for circulating autoantibodies against type 2M myofibers and magnetic resonance imaging (MRI) were performed. Owing to trismus, the maximum mouth opening was approximately 1.5 cm during intubation for inhalation anesthesia; this was observed to be the same even under full anesthesia with no palpebral reflex (Fig. 1A, B). MRI revealed high-signal intensities on T2-weighted images of the muscles in general, including the epaxial and hypaxial muscles around the spine, although the hyperintensities were more prominent in the masticatory muscles, including the masseter and temporalis muscles (Fig. 2). Masticatory myositis was suspected on the basis of the MRI findings and the presence of trismus. While the results of the serology test were awaited, oclacitinib (1 mg/kg, q12h, Apoquel^®; Zoetis, Florham Park, NJ, USA) was administered after reference to a recent case report (4); this improved the dog's mouth opening to 2 cm and slightly restored its vitality and appetite, with gradual weight gain. However, fever and elevated CRP levels persisted, and the result of the type 2M-myofiber antibody test was negative. Ultimately, a biopsy of the temporalis muscle was performed. A 1-cm skin incision was placed, and the subcutaneous tissue was bluntly dissected to expose the underlying muscle. Two biopsy samples were obtained from the temporalis muscle, extending from the fascia to the periosteum, using 4-mm punch biopsy. Histopathological examination performed by Antech Diagnostics (Irvine, CA, USA) revealed severe multifocal, polyphasic, lymphoplasmacytic, and histiocytic myositis with fibroplasia, along with degeneration, necrosis, and loss of myofibers and skeletal muscle regeneration (Fig. 3). On the basis of the overall clinical course and diagnostic test findings, idiopathic polymyositis was established as the primary diagnosis. With the administration of immunosuppressive treatment including prednisolone (1 mg/kg, q12h, Solondo^® tab; Yuhan, Seoul, Korea) and mycophenolate mofetil (10 mg/kg, q12h, My-Rept Cap; CKD Pharm, Seoul, Korea), the dog’s vitality, gait, and mouth opening returned to pre-illness levels (Fig. 1C). Moreover, both the body temperature and CRP levels decreased to within the normal range. Despite prednisolone tapering, the dog had been doing well for 3 months without recurrence of clinical signs.

Figure 1. Maximum mouth opening before and after treatment in a dog with idiopathic polymyositis. (A, B) During intubation for inhalation anesthesia, the maximum mouth opening is 1.5 cm. (C) After diagnosis of idiopathic polymyositis and treatment with prednisolone and mycophenolate mofetil, the mouth can be opened up to 7 cm.

Figure 2. Magnetic resonance imaging findings for a dog with idiopathic polymyositis. (A) Marked T2-weighted hyperintensity in the masseter (red arrow) and temporalis (yellow arrow) muscles. (B) Marked T2-weighted hyperintensity in the digastric muscles (yellow arrow). (C) Marked T2-weighted hyperintensity in the interspinal muscle and atrophy of the left epaxial and hypaxial muscles at T13-L1 (yellow arrow). (D) Marked T2-weighted hyperintensity in the left epaxial muscle at L3-4 (yellow arrow).

Figure 3. Histopathological examination of a temporalis muscle specimen from a dog with idiopathic polymyositis. Muscle fibers are separated by numerous foci of inflammatory cells, including lymphocytes, plasma cells, and histiocytes, and fewer eosinophils (arrows). Degenerative fibers exhibit a loss of cross-striations and sarcoplasmic vacuolation. Regeneration is indicated by basophilia, variable cell sizes, and centralized nuclei and nucleoli. The inflammatory infiltrate is supported and separated by reactive fibroblasts. Stained with hematoxylin and eosin. (A) 10×, scale bar = 0.5 mm; (B) 40×, scale bar = 125 μm.

Polymyositis is characterized by multifocal or diffuse infiltration of skeletal muscle tissue by lymphocytes. A diagnosis of idiopathic polymyositis is based on characteristic clinical signs, elevated CK levels, abnormalities in electrophysiological testing consistent with myositis, exclusion of infectious causes of myositis by serology, and specific observations in muscle biopsy (11,13). In the present case, infectious causes of myositis were ruled out through molecular diagnostic tests for identifying the origin of fever. Suspicion of an infectious cause is increased when eosinophils are observed in histopathological examination of biopsied muscle tissue from dogs with polymyositis (11). The chief complaints and physical examination findings in this case, such as lethargy, reluctance to walk, cervical stiffness, and hindlimb weakness, were considered neurological problems resulting from intervertebral disc disease, steroid-responsive meningitis-arteritis, or other causes (7,17). Decreased appetite and inability to open the mouth began to appear 3 weeks after the onset of lethargy. Elevated cortisol may have delayed the onset of anorexia. Although masticatory myositis was initially suspected, the final primary diagnosis was idiopathic polymyositis, considering the clinical course, elevated CK level, absence of type 2M myofiber antibodies, and generalized inflammatory changes in the muscles observed on MRI. CK is more commonly increased in polymyositis than in masticatory myositis (5,11). A limitation of this case was that biopsy specimens were only obtained from the temporalis muscle, which exhibited the most marked inflammatory changes on MRI. A more comprehensive biopsy involving multiple muscle groups would have provided additional diagnostic clarity. Moreover, while masticatory myositis is characterized by a higher number of CD4+ T cells than CD8+ T cells, along with multifocal clusters of B lymphocytes, CD8+ T cells are dominant in idiopathic polymyositis, with no B cells visible on immunostaining (12,14). However, the owner did not consent to additional histopathological examination.

The treatment for idiopathic polymyositis, similar to that for other immune-mediated diseases, comprises immunosuppressive doses of corticosteroids and additional immunosuppressive drugs (5,11,13). In the present case, one of the reasons for corticosteroid administration only after definitive diagnosis was left adrenomegaly with elevated cortisol levels. Oclacitinib, a selective Janus kinase inhibitor, is a United States Food and Drug Administration-approved drug for the control of pruritus associated with canine allergic dermatitis (3,6). Recently, successful clinical outcomes were reported for three dogs with masticatory myositis treated with oclacitinib as an alternative to corticosteroids (4). Although the mechanism of action remains unclear, the off-label dose of oclacitinib reportedly inhibits T lymphocytes and cytokines, which are elevated in dogs with inflammation of the masticatory muscles (3,4,6). In this case, oclacitinib was used for idiopathic polymyositis for the first time, to our knowledge, and it increased the dog’s mouth opening and appetite before the results of serology and histopathology were available. This suggests that oclacitinib may serve as not only an alternative when traditional immunosuppressants are ineffective or contraindicated but also a potential therapeutic option for immune-mediated diseases with complex diagnosis. Further research and clinical trials are needed to evaluate its efficacy and safety in a larger population of dogs with inflammatory myopathies.

Myopathy including polymyositis is relatively uncommon compared to orthopedic or neurological diseases in dogs. Clinical signs, including generalized weakness that worsens with exercise, a stiff gait, pain upon palpation of affected muscles, dysphagia, and generalized muscle atrophy, may overlap with signs of other inflammatory myopathies and initially manifest as nonspecific symptoms such as depression and fever (5,11,13). In addition, certain dogs with polymyositis also show signs of masticatory myositis. The goal of treatment is return to normal muscle function and a normal quality of life. In the presence of severe fibrotic changes in affected muscles, atrophy persists despite immunosuppressive treatment and is exacerbated by glucocorticoid therapy (5,9). Therefore, early recognition and diagnosis are crucial and directly impact the prognosis (1,13). Despite the presence of muscle fibroplasia, the dog in this case responded well to immunosuppressive treatment; this emphasizes the importance of accurate diagnosis and prompt intervention. Comprehensive diagnostic evaluations, including electromyography and multiple muscle biopsies, should be performed to improve diagnostic accuracy and treatment outcomes (10,13,16). Furthermore, ongoing education and awareness regarding the clinical presentation and management of idiopathic polymyositis are needed to ensure that these conditions are not overlooked as differential diagnoses for dogs exhibiting atypical muscular signs (5).

We described a case involving a dog with idiopathic polymyositis that initially presented with nonspecific clinical signs and subsequently developed trismus. Oclacitinib, prescribed on suspicion of masticatory myositis, improved clinical symptoms until a diagnosis of idiopathic polymyositis was confirmed. The dog recovered well following the administration of immunosuppressive drugs, despite muscle fibroplasia. This report highlights the importance of accurate diagnosis and timely treatment as well as the therapeutic potential of oclacitinib for dogs with idiopathic polymyositis.

The authors have no conflicting interests.