Pathologic findings in cetacean skeletal muscle.

Abstract

Incidence and characterization of skeletal muscle lesions in stranded cetaceans is unknown. The purpose of the current study was to determine the prevalence and nature of skeletal muscle lesions in cetaceans. Histopathologic evaluation of skeletal muscle was performed in 162 stranded cetaceans belong to 21 different species, undergoing post-mortem examination in Canary Islands between 1996 and 2008. Skeletal muscle samples were taken from the middle portion of Longissimus dorsi (near the last thoracic vertebrae). Muscle samples were fixed in formalin, processed routinely, and stained with haematoxylin and eosin, Phosphotungstic Acid Haematoxylin, periodic acid-Schiff for glucogen, and Von Kossa for calcium. Samples were also postfixed in Osmium tetroxide for lipids. In addition, immunohistochemical examination was performed using primary monoclonal (fast and anti-slow myosin heavy chain) and polyclonal antibodies (myoglobin and fibrinogen). Skeletal muscle lesions were common in stranded cetaceans examined at post-mortem (88.8%). In our opinion, consecutive longitudinal and transverse sections stained with a combination of different histological, histochemical and immunohistochemical techniques were necessary to establish a definitive morphologic diagnosis in muscular pathology of stranded cetaceans. Acute to subacute segmentary myonecrosis represented the main finding (51.8%) and it was related to the stress of active stranding. Other common causes included septicaemia, fishery interactions and ship collision. Chronic myopathic changes including excessive fibre size variation, internal nuclei, yuxtanuclear lipofuscin-like pigment, and different stages of myofibre regeneration were also a frequent group of muscular changes (13.5%). These are non-specific findings and can be associated with a variety of neuromuscular disorders, and also with increasing age in cetaceans. Generalized or focal atrophy represented the 11% and was related to a poor body condition or senility. Other findings were myositis (1.8%), ring fibres (1.23%), fibre splitting (2.46%) and presence of intramyofibre sarcocystis sp. (8%). Finally, intramyofibre protozoa (Toxoplasma sp.) were only detected in one animal.