Pathology of muscular steatosis in the bovine species: report of two spontaneously arising cases and comparative overview of the condition

Muscular steatosis, a condition characterized by prominent fatty replacement of skeletal muscles, has been known from a long time ago to mainly involve meat animals, such as pig, sheep, and cattle. Although a variety of hypotheses are proposed for explaining the condition, its etiology and pathogenesis are still poorly understood. Spontaneously occurring lesions of muscular steatosis were recognized in two 2-year-old steers that were slaughtered for food. Significant clinical abnormalities indicative of motor dysfunction were not observed in these animals. Postmortem examination revealed a remarkable and bilaterally symmetrical deposition of adipose tissue in the semitendinosus muscle of one steer and the gluteus muscle of the other steer. Affected muscles were of normal volume but exhibited an irregularly streaked appearance due to ill-proportioned fat infiltration. On histopathology, the lesioned muscles were extensively replaced by much adipose tissue, showing complete loss of muscle fibers in some fascicles. Structure of the perimysial connective tissue network was well preserved. All fat cells did not exhibit immature morphology but were of a well-differentiated mature appearance. Although a small number of muscle fibers represented atrophic or degenerative changes, regenerative muscle fibers and proliferation of the perimysial connective tissue were not demonstrated elsewhere in the lesions. Intramuscular blood vessels and nerves showed no significant changes. Precise pathogenesis of muscular steatosis, such as that found in these steers, remained uncertain. These fatty muscular lesions were distinguished from those of advanced or end stages in previously described muscle disorders in cattle, including neurogenic muscular atrophy, nutritional myopathy, or muscular dystrophy. The present article makes a comment on the pathology of muscular steatosis involving the bovine species from the comparative viewpoint.