Musculoskeletal System
March 2019

Signalment (JPC #1535964):  Five-year-old quarter horse mare

HISTORY:  This mare showed signs of "tying up" after being exercised. She became recumbent and began to pass coffee-colored urine.  The mare was euthanatized 8 days after onset of signs.  Clinical pathological findings include: BUN-25 (11-27); AST 1250 (160-412); ALT-1750 (no reference interval given); LDH-7200 (112-456).

HISTOPATHOLOGIC DESCRIPTION:  Skeletal muscle:  Multifocally and randomly affecting up to 75% of muscle fibers, individual myocytes are markedly swollen with pale vacuolated sarcoplasm (degeneration); hypereosinophilic, and hyalinzed with fragmented myofibrils, disruption of the endomysium, loss of cross-striations, occasional contraction bands, and nuclear pyknosis or karyolysis (necrosis); or lost with replacement by karyorrhectic debris, moderate numbers of macrophages and few fibroblasts.  Multifocally few myocytes have nuclear internalization and increased basophilia with rowing of nuclei that are large with prominent nucleoli (regeneration), and there is moderate proliferation of satellite cells.  Fibrin, edema, moderate numbers of histiocytes, lymphocytes and plasma cells multifocally infiltrate and expand the perimysium and endomysium.

MORPHOLOGIC DIAGNOSIS: Skeletal muscle:  Degeneration and necrosis, monophasic, multifocal, random, moderate, with myofiber regeneration, and moderate histiocytic and lymphoplasmacytic myositis, quarter horse, equine.

CONDITION:  Equine exertional rhabdomyolysis

SYNONYMS:  Exertional rhabdomyolysis syndrome (ERS), exertional myopathy, ‘tying-up’, ‘Monday-morning disease’, ‘set fast’, azoturia, black water, paralytic myoglobinuria






Clinicopathological findings:

UltrastructurAL FINDINGS:




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