SETFAST (AZOTURIA, TYING UP, EXERTIONAL MYOPATHY, RHABDOMYOLYSIS)
In its mild form, this is a relatively common condition which affects the muscles of the horse’s hindquarters and back. Inflammation of these muscles causes pain and stiffness. Setfast is usually seen in horses on high energy diets after a rest from their normal exercise routine. It used to be called ‘Monday Morning Disease’ because it commonly affected working horses which had a rest day on Sunday. In some individuals which appear to be particularly susceptible, notably fillies, even a reduction in exercise for one day can result in the horse becoming setfast on resumption of normal exercise.
What are the symptoms?
Mild cases of setfast just appear ‘stiff behind’ after exercise. Usually both sides of the body are affected equally so the horse does not actually appear lame. In more severe cases the horse may be reluctant to move and show signs of pain such as sweating and scraping the ground. Some may be thought to have colic. Very severe cases may be unable to move and may even collapse.
What should I do?
If you suspect that your horse is setfast, stop exercise immediately and dismount. Sometimes a rider can detect very early signs by a change in action and this should not be ignored. If the horse can walk, return him to his stable. Do not keep him walking as this may worsen the muscle damage. If he is very stiff or unable to walk, get a horsebox or trailer to transport him to a stable. Moderate to severe cases should be seen by a veterinary surgeon as soon as possible. Initial treatment consists of anti-inflammatory medication and rest. Very mild cases sometimes respond quickly to tranquillisation with acetylpromazine. Severe cases may require intravenous fluids and intensive supportive treatment. Your veterinary surgeon will collect a blood sample to measure muscle enzymes to confirm the diagnosis and to determine the severity of the muscle damage. Follow-up blood samples will monitor response to treatment and confirm when it is safe to resume exercise.
Why do horses get setfast?
This is a very complex condition and one which is not completely understood. In some cases it appears to be due to excess energy intake relative to the amount of exercise being received. In others, particularly affecting fillies and mares, it appears to be associated with changing levels of electrolytes (salts) in the blood. Imbalances in certain minerals and vitamins (especially calcium, phosphorus, vitamin E and selenium) are believed to be involved in some cases.
How can I prevent setfast?
Ensure that your horse’s diet is providing him with the right amounts of energy, protein, vitamins and minerals for the job you are asking him to do. In the first instance we recommend that he is fed a low starch high fat diet (ie. Cubes rather than mixes). A pony being hacked out at weekends should not be fed the same diet as a racehorse in full work. On ‘easy’ days, reduce the amount of feed you give your horse to reflect the reduction in exercise. If possible, ensure your horse gets some exercise every day, even if this is being turned out or hand walking. Some horses who are prone to setfast can never have a complete day of rest.
If your horse has been setfast it is important to give the muscles time to recover. This can be monitored using blood tests. A gradual return to exercise after recovery is recommended so that there is no sudden increase to trigger another episode of the condition. Some horses can safely resume exercise following treatment with dantrolene sodium, given by mouth. This is given approximately one hour before exercise and should be administered until the horse has returned to the level of exercise reached before becoming setfast. Dantrolene cannot be used while horses are racing or performing under Jockey Club or FEI rules.
If your horse has a recurrent problem with setfast, your veterinary surgeon can collect samples of blood, urine and feed stuffs, for laboratory testing to determine if there are electrolyte, mineral or dietary imbalances which may contribute to the condition and require correction.
Atypical Myoglobinuria
This is a similar condition which occurs sporadically (commonly in the autumn and often after a period of inclemental weather) and is not necessarily associated with exercise. Affected horses show sudden onset stiffness /weakness, which rapidly progresses to recumbency and often death. Single cases can occur, but it is more common for two or three cases to occur on the same premises over a period of several days. Recent evidence suggests an association between this condition and maple and sycamore trees, (possibly due to a mycotoxin found on these trees?). Treatment is often unrewarding and is symptomatic at present
The Acorns Equine Clinic, Pleshey, CHELMSFORD, Essex. CM3 1HU.
Telephone (01245) 231151, Fax. (01245) 231601.