Clinical, histopathological and metabolic responses following exercise in Arabian horses with a history of exertional rhabdomyolysis

• Exertional rhabdomyolysis (ER) is a common problem in endurance racing Arabian horses.
• Arabian horses with a distant history of ER had chronic myopathic changes in muscle biopsies.
• Metabolic responses to exercise in ER-susceptible and matched control Arabians were very similar.
• Findings suggest a lingering underlying myopathy, not a consistent metabolic disorder.

A previous report suggests a substantial incidence of exertional rhabdomyolysis (ER) in Arabian horses performing endurance racing. This study compared formalin histopathology and clinical and metabolic responses to a standardised field exercise test (SET) between Arabians with and without ER. Arabian horses with (n = 10; age 15.4 ± 5.6 years) and without (n = 9; 12.9 ± 6.1 years) prior ER were stall-rested for 24–48 h, after which paired ER and control horses were fitted with a telemetric ECG and performed a 47 min submaximal SET. Plasma glucose, lactate, electrolyte and total protein concentrations and packed cell volume were measured before and immediately after exercise. Blood and percutaneous gluteal muscle samples were also obtained before and 3 h after exercise for measurement of plasma creatine kinase (CK) activity and muscle glycogen concentration, respectively. Histopathologic analysis of formalin-fixed pre-exercise muscle sections was performed. Data were analyzed by ANOVA and non-parametric tests (P < 0.05).No horses displayed clinical signs of ER during exercise, and plasma CK increased similarly in ER and control Arabians. Muscle glycogen, heart rate, and remaining plasma variables did not differ between horses with ER and control horses. Horses with ER had more internalised nuclei in mature myofibers, more aggregates of cytoplasmic glycogen and desmin, and higher myopathic scores than control horses. Although many horses with ER had histopathologic evidence of chronic myopathy, muscle glycogen concentrations and metabolic exercise responses were normal. Results did not support a consistent metabolic myopathy or a glycogen storage disorder in Arabians with ER.