Riders of 101 horses participating in one of four 50-mile (80.5 km) distance races completed a comprehensive questionnaire regarding the medical history, management and performance of their horse.
Serum creatine kinase activity (CK) was measured before and 4 h after completion of exercise.
Hair samples were analysed by PCR for the R309H mutation in the glycogen synthase gene (GYS1) responsible for type 1 polysaccharide storage myopathy (PSSM) and the C7360G mutation in the ryanodine receptor 1 (RYR1) gene causing malignant hyperthermia (MH).
Samples were obtained from 68 Arabians, 20 half-Arabians and 13 horses of other breeds.
Serum CK was above the resting reference interval (145–633 u/l) in 38 horses after racing (median 883 u/l, range 658–3739) but was compatible with values previously reported in apparently healthy endurance horses.
Pathological ER was suspected to occur in 4 horses with serum CK activities exceeding 10,000 u/l 4 h after racing (median 84,825 u/l; range 10,846–381,790) including 3 Arabians and one half-Arabian horse.
GYS1 and RYR1 mutations were not present in hair samples from any horses.
Exertional rhabdomyolysis occurred at a prevalence of 4.0% in a sample of horses participating in 50 mile distance events and all affected horses were Arabian or half-Arabian.
The cause of ER in the endurance horse population remains unknown; however, ER in competing Arabian endurance horses is unlikely to be due to type 1 PSSM or MH.
Source: Wilberger, M. S., McKenzie, E. C., Payton, M. E., Rigas, J. D. and Valberg, S. J. (2015), Prevalence of exertional rhabdomyolysis in endurance horses in the Pacific Northwestern United States. Equine Veterinary Journal, 47: 165–170. doi: 10.1111/evj.12255
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