VET CLINIC
In association with
Mark Andrews, BVM&S CertEP MRCVS, of
Equine Science Update
we are pleased to provide the latest Equine Veterinary Information
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EQUINE RHABDOMYOLYSIS SYNDROME
Equine Rhabdomyolysis Syndrome (ERS) is also known as Monday morning disease, setfast, azoturia, tying-up and exertional rhabdomyolysis. The condition is characterised by damage ("lysis") of striated muscle fibres (especially type II muscle fibres). Recent surveys have shown that, in the UK, about 7% of flat racing Thoroughbred horses, polo horses and competition horses are affected.
The signs vary in degree from a mild abnormality of gait to recumbency. ERS may cause behavioural problems during backing.
Measurement of “muscle enzymes” in the blood can give a guide to the severity of the muscle damage. The most specific indicator of muscle damage is creatine kinase (CK) . Levels in the blood increase rapidly after the onset of ERS. Values above 20,000iu/l are not uncommon in severe cases. Aspartate aminotransferase (AST) is less specific for muscle damage, and can be increased as a result of other conditions such as liver problems. It reaches peak levels more slowly than CK and persists longer.
Mild cases of ERS can be detected by using an exercise test in which blood levels of the muscle enzyme CK are checked before, and four hours after, 20 minutes of trotting exercise. An increase of more than twice pre-exercise values, confirms that the horse has ERS.
Our understanding of ERS has grown in the last few years. Specific causes of the syndrome have been identified, and the factors influencing the occurrence of the disease have been further investigated.
Equine rhabdomyolysis syndrome appears in two forms: sporadic and recurrent. It is important to differentiate the sporadic cases from the acute recurrent episodes because the management of each differs. Various causes of the sporadic form have been described - such as carbohydrate overload, viral myositis, vitamin E / Selenium deficiency and exertional myopathy. These factors may also act as triggers for the recurrent form.
Two specific causes of recurrent ERS have been identified so far. Equine polysaccharide storage myopathy, and a calcium channel disorder.
Equine polysaccharide storage myopathy (EPSM) was first seen in Quarter horses in the USA, but has since been recorded in other breeds - including draft horses and draft horse crosses, and warmbloods. The diagnosis is confirmed using a muscle biopsy, which shows characteristic accumulation of abnormal levels of polysaccharides (glycogen) in the muscle. Researchers at Cornell University in the USA suggest that 50% of draft horses and draft cross horses are affected to some extent.
Recent work shows that EPSM definitely occurs in the UK. (1) It has been found in samples of muscle taken from clinical cases and accounts for about 25% of cases of ERS. A random sample of horses at abattoirs showed that 5 -10% of horses were affected. The cause of EPSM is unknown. Affected horses have high resting glycogen levels in the muscle. But this is not because they are unable to break the glycogen down - rather it appears that they manufacture more of it.
The calcium channel disorder is a defect in calcium metabolism within the muscle cell that interferes with normal control of muscle contractions. This form is found in Thoroughbreds and appears to be inherited.
Trigger factors depend on the horse and the type of work it does. For example, in Thoroughbred horses in the UK, nervous two-year old fillies are most likely to be affected. In one study, ERS recurred within a season in most of the affected horses, and 70% of affected horses were unable to race that season. So it is debateable whether it is actually worth trying to train these horses. Interestingly, diet and previous lameness have only been associated with an increased risk of ERS in the USA, but not in the UK.
In contrast to the situation in Thoroughbreds, in eventing horses the only factor that has been shown to affect the risk of the disease is turnout time.
The major trigger factor in polo horses is lack of fitness. ERS tends to be seen early in the season; after a rest period prior to exercise and if the horse has had more strenuous exercise than normal. Horses with an excitable temperament are more likely to be affected. Typically the signs of ERS in polo horses are severe. (2)
Management of horses affected with ERS plays an important role in reducing the risk of recurrence. The carbohydrate content of the diet should be reduced as much as possible and replaced by oil. To replace 20% of the caloric intake with oil would require about 500ml for a 500kg horse. Oil should be added to the diet even if the horse is turned out and only eating roughage. This lowers the muscle glycogen and is unlikely to adversely affect performance. Horses should receive a daily minimum roughage intake of 1% of body weight .
Attention should be given to vitamin and mineral levels. This is especially so if large amounts of oil are added to the diet( in which case the requirements for vitamin E and selenium may be increased.)
It is important to resume exercise early. According to Dr Cathy McGowan, formerly of the Royal Veterinary College, it is not necessary to wait until the muscle enzymes return to normal before starting work again."If your horse is walking freely I`m happy for you to exercise it." She advises that warm-up work should be relaxed rather than too collected. It is also best to have regular exercise with no days off. Horses should be turned out as much as possible without exposing them to too much rich pasture.
Dantrolene, a drug that decreases the release of calcium within the muscle, is sometimes helpful, especially in Thoroughbreds, to allow the horse to get back to regular work.
References.
1. Four cases of equine polysaccharide storage myopathy in the United Kingdom.McGowan CM, Menzies-Gow NJ, McDiarmid AM, Patterson-Kane.JC. Veterinary Record (2003) 152, 109 - 112
2. Incidence of exertional rhabdomyolysis in polo horses in the USA and the United Kingdom in the 1999/2000 season. McGowan CM, Posner RE, Christley RM. Veterinary Record (2002) 150, 535 - 537.
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