Spontaneous Complex Polysaccharide Inclusions in the Skeletal Muscle of Purpose-bred Beagle Dogs

Amylase-resistant, periodic acid-Schiff (PAS)–positive inclusions were identified in the skeletal muscle of four of twenty-four purpose-bred beagle dogs from a routine toxicology study. Affected myofibers contained amorphous material filling up to 20% of the sarcoplasm that stained lightly basophilic with hematoxylin and eosin and was strongly PAS–positive with amylase resistance. Transmission electron micrographic examination of the inclusions revealed granular, non-membrane–bound, electron-dense material, consistent with polysaccharide. Although skeletal muscle inclusions with similar features have been reported in dogs in conjunction with systemic metabolic disorders and less often in muscle adjacent to nonmyogenic sarcomas, all four of these dogs lacked clinical or pathological findings diagnostic of a concurrent systemic metabolic or localized skeletal muscle disorder. Furthermore, these skeletal muscle inclusions were present in both vehicle- and test article–treated dogs and were considered an incidental finding that may occur spontaneously in clinically normal beagle dogs; as such, their presence in drug-treated animals should be interpreted with caution.