Complex polysaccharide inclusions in skeletal muscle of stranded cetaceans resembling equine polysaccharide storage myopathy: necropsy study of 148 specimens in the Canary islands

Abstract

Equine polysaccharide storage myopathy (EPSSM) is a metabolic myopathy characterized by abnormal accumulation of glycogen and glycogen-related polysaccharide in skeletal muscle fibers of affected animals. The presence of chronic myopathic changes (fiber size variation and internal nuclei) is also considered criteria diagnostic. Skeletal muscle of small and large odontocetes and misticetes (148) of 19 different species was examined. The animals were of both sexes and ranged from neonatal to old age stimated by length and histopathological findings. Samples were obtained during necropsy procedure, fixed in 4% neutral buffered formalin, routinely processed and stained with hematoxylin and eosin (HE) and periodic acid-Schiff (PAS) for glycogen. In addition, skeletal muscle samples were immunostained to detect ubiquitin and fast and slow myosin heavy chain isoforms, type II and type I, respectively, using the avidin-biotin-peroxidase method. Sections were examined for abnormal glycogen content, amylase resistant complex polysaccharide and chronic myopathic changes. A total of 26 cetaceans (14 females and 12 males) with lesions of polysaccharide storage myopathy were identified. Affected animals belonged to 11 different species with the highest incidence recorded in family Delphinidae. The histopathological diagnosis was made for samples containing few to multiple intramyofiber inclusions of blue (basophilic) material with HE stain. These inclusions were intensely PAS positive and amylase resistant, which replaced all or part of the fiber segment. The number of affected fibers ranged from few to many, which most often occurred singly or in small cluster at the periphery of fascicles. Even if type II fibers, immunostained against fast-myosin antibody, were preferentially involved, type I fibers were also affected in some cases. All amylase resistant inclusions contained ubiquitin, while this protein was not detected within normal myofibers, as has been previously reported in affected horses. Results of this study are the first description of skeletal muscle complex polysaccharide inclusions resembling EPSSM in cetaceans.