South Africa is renowned for breeding magnificent endurance and race horses, especially of the Arabian breed. Arabian horses are typically bred for their genetic ability to withstand fatigue and endurance capacity. There are a number of endurance events worldwide, each differing in distance and difficulty. South Africa, with its sub-saharan to desert-like climate, plays host to a number of these endurance events throughout the year.
Many of these horses suffer from muscle breakdown (rhabdomyolysis), even before the race has started. The rhabdomyolysis might be so severe, that some horses may require assistance from slings. Diagnosis of muscle rhabdomyolysis is primarily determined from visually inspecting the urine (a dark brown-like colour indicates severe rhabdomyolysis). Another method is to measure the level of blood creatine kinase, the latter being a protein enzyme that is abundant in skeletal muscle.
What causes the muscle to break down?
The term exertional rhabdomyolysis literally means muscle breakdown caused by exercise. During long duration and sometimes intense exercise bouts, the muscle fibres can tear and leach their content into the blood stream. In some cases, exercise may merely be the aggravating trigger for rhabdomyolysis, with other biological factors predisposing some horses (or even certain breeds) to develop rhabdomyolysis. Genetic mutations in metabolism are known to cause muscle rhabdomyolysis in many species, including humans. There are hundreds of these mutations already identified in the likes of the glycolytic pathway (e.g. McArdle’s disease, polysaccharide storage myopathy (PSSM), Tauri’s disease), certain mitochondrial defects (e.g. carnitine palmitoyltransferase II deficiency), and fatty acid oxidation defects (for an excellent review on all the myopathies, please refer to Van Adel and Tarnopolsky, 2009). A few of these have been identified in horses, and may only appear at a later stage in life. Viral infections may also trigger rhabdomyolysis.
Apart from diseases, other factors may increase the risk of rhabdomyolysis. Factors such as sleep deprivation, stress, drugs (e.g. non-steroidal anti-inflammatory drugs, amphetamines), and a compromised liver have been linked to causing rhabdomyolysis in both humans and animals.
What are we doing?
Our first aim is to monitor the incidence of rhabdomyolysis in endurance horses within the Southern African region, which includes South Africa, Namibia and Botswana. Bloods will be collected to search for known genetic mutations. With consent from the owner, a muscle biopsy will be obtained and screened for various muscle properties, including metabolic enzyme abnormalities, abnormal glycogen deposits and structural defects.
★ Michael Hewetson (University of Pretoria)