Prevalence of exertional rhabdomyolysis in endurance horses in the Pacific Northwestern United States.

This research aims to understand the prevalence and underlying causes of exertional rhabdomyolysis (ER), a muscle disorder, in endurance racing horses in the Pacific Northwestern United States. It was found that the condition occurred in 4.0% of the studied sample and the cause remains unknown.

Study Methodology

• The research employed a prospective clinical study to analyze data from 101 horses participating in one of four 50-mile distance races.
• The riders filled out detailed questionnaires about their horse’s medical history, management, and performance, providing a thorough background for each horse in the study.
• A key aspect of the research was the measurement of the horses’ serum creatine kinase activity (CK), a marker of muscle damage, both before and four hours after they had completed the exercise.
• Additionally, hair samples from the horses were analysed for two genetic defects associated with muscle disorders: the R309H mutation in the glycogen synthase gene (GYS1) which causes type 1 polysaccharide storage myopathy (PSSM), and the C7360G mutation in the ryanodine receptor 1 (RYR1) gene which causes malignant hyperthermia (MH).

Study Results

• The majority of the studied horses were Arabians (68), with 20 half-Arabians and 13 of other breeds.
• After racing, serum CK levels of 38 horses were found to be above the normal baseline, with a median value of 883 u/l, signaling possible muscle damage. However, these values were in line with previously reported values in apparently healthy endurance horses.
• Four horses (3 Arabians and one half-Arabian) were suspected to have pathological ER, a severe form of the syndrome, with serum CK activities exceeding 10,000 u/l 4 hours after the race.
• Interestingly, none of the horses presented the two genetic defects studied (R309H and C7360G mutations), indicating these mutations were unlikely to be responsible for the observed ER rates.

Study Conclusions

• The study concluded that exertional rhabdomyolysis occurred at a prevalence of 4.0% in the studied sample, with all affected horses being Arabian or half-Arabian.
• The underlying cause of ER in the endurance horse population remains unknown. However, the study ruled out type 1 PSSM and MH as potential causes of ER in competing Arabian endurance horses.