Clinical characteristics of non‐infectious inflammatory myopathy in the boxer dog

Objectives To evaluate the clinical characteristics, treatment, outcome and potential association between non‐infectious inflammatory myopathy and malignancy in boxer dogs. Materials and Methods Boxer dogs histologically diagnosed with non‐infectious inflammatory myopathy at the Comparative Neuromuscular Laboratory, University of California San Diego from 2010 to 2018 and with complete medical records were included in this retrospective study. Signalment, history, clinical signs, clinicopathologic findings, treatment and outcome were documented. Results Twenty‐eight boxer dogs with non‐infectious inflammatory myopathy, aged 1 to 11 years, were included. Eighteen were male (16 neutered; two entire) and 10 were female (seven spayed; three entire). Clinical signs included generalised weakness (n=17), dysphagia (n=11) and weight loss (n=10). Serum creatine kinase activity was elevated in all 20 cases tested (range 908 to 138,000 IU/L). One dog had undifferentiated round cell neoplastic infiltration within the muscle at the time of inflammatory myopathy diagnosis. Five dogs historically had mast cell tumours and 21 dogs were not diagnosed with neoplasia prior, at the time of or after inflammatory myopathy diagnosis. Treatment included glucocorticoid monotherapy (n=12), cyclosporine monotherapy (n=1) or multiple immune‐suppressive medications (n=14). Six dogs neurologically improved, 11 improved but relapsed while on treatment, seven did not improve. Eight dogs were euthanased, one died, four were lost to follow‐up. Clinical Significance Boxer dogs with non‐infectious inflammatory myopathy can present for generalised weakness and dysphagia; long‐term successful outcome is uncommon. The relationship between neoplasia and non‐infectious inflammatory myopathy in boxer dogs remains unclear; future prospective studies evaluating a larger cohort are warranted.