Canine masticatory muscle disorders: a study of 29 cases.

The histopathologic features in temporalis muscle biopsies from 29 dogs with masticatory muscle disorders were characterized and used for their subgrouping: 2 without lesions, 3 with nonspecific changes, 7 with neurogenic atrophy, and 16 with myositis. The immunocytochemical and immunochemical features of the muscle biopsies and sera from those dogs were compared among the histopathologic subgroupings and compared with biopsies and sera from healthy dogs and dogs with polymyositis. Of the 14 biopsies from dogs with masticatory muscle myositis, 12 had immune complexes limited to type 2M fibers, whereas 13 of 16 sera samples had detectable antibodies against type 2M fibers. The immune complex deposition was found only in biopsies of dogs with masticatory muscle myositis, and the antibodies were detected in the sera of only one dog that did not have masticatory muscle myositis. Immunoblot assays revealed that the antibodies were most often directed against a 185 K protein, myosin heavy chain, and a band that appeared to be LC2-M (myosin light chain 2-masticatory).