Tying up was once thought of as a racehorse-only muscle disease, but we now know that more equines are at risk of tying up than we originally thought, including those in low levels of work.
What is tying up?
Tying up is a muscle disease of horses, known in technical terminology as exertional rhabdomyolysis. More colloquially, it is referred to as setfast or Monday-morning disease.
Tying up is usually characterised by repeated ‘episodes’ of muscle damage and usually happens during or shortly after exercise. Affected horses become stiff and painful, often sweat profusely and might be totally unwilling to move. In the worst cases, affected horses will lie down and may be unable to get up. Some horses do die from the condition.
Tying up has classically been described in racing Thoroughbreds and has been best studied in this breed. However, a variety of other breeds are also diagnosed with similar if not identical disorders that might or might not have a common cause.
Tying-up is seen in all athletic equestrian disciplines such as showjumping, dressage, eventing and endurance riding, and can even occur while out for a hack.
A horse may tie up just once in their life, but some horses are prone to repeated episodes. This is a welfare issue and can become extremely challenging to manage.
Signs
Tying-up is often associated with exercise and tends to occur during or immediately after a horse is worked. The signs of tying-up vary in severity but can include:
- Muscle stiffness or spasms
- Muscles that suddenly appear hard or stiff and can also start trembling and spasming.
- High heart rate or breathing rate.
- Horses that are tying up can suddenly sweat far more than the level of exercise would normally cause them to, and generally, the sweat will be found all over the body.
- Hesitance to move.
- A horse may become reluctant to walk, becoming rooted to the spot or stiff during an episode. They may also lie down and may not be able to get up.
In some horses that tie up, the episode is severe enough to result in urine that appears very dark brown or red. This is not caused by blood in the urine but rather by myoglobin, a substance found inside the horse’s muscles. When a horse ties up, and the muscle is damaged, the myoglobin is released into the circulation and removed from the body in the urine. Myoglobin at high concentrations in urine can damage the kidneys, and on rare occasions, can cause kidney failure.
Causes
The causes of tying-up are varied and not completely understood. A one-off episode can probably be caused by triggers such as over-exertion or eating something toxic, but repeated episodes of tying-up after exercise seem to be a syndrome with both genetic and environmental elements. These animals are said to have recurrent exertional rhabdomyolysis syndrome (RER).
Despite this apparent genetic factor to tying-up, the only currently identified genetic disorder commonly identified to date is type one polysaccharide storage myopathy 1 (PSSM1). This condition is caused by a mutation in an enzyme that is involved in muscle energy metabolism. However, most horses that tie up, particularly of certain pleasure horse breeds, do not have this mutation. The gene or genes causing RER have therefore not yet been identified but studies are ongoing to identify them.
Treatment of an episode
If you suspect your horse is having an episode, you must contact your vet immediately. While waiting, try to keep your horse calm.
Often, a vet will confirm the diagnosis with a blood sample and might administer fluids via a stomach tube or directly into the vein. A vet can also give painkillers, and in severe cases may hospitalise your horse for close observation.
In horses that seem susceptible to multiple episodes, your vet might recommend a muscle biopsy or other tests to try and find the underlying cause.
Management
There is currently no cure for this disorder, although horses can continue to perform well with a good quality of life if managed appropriately in the long term. Aspects of long-term management include diet, exercise, administration of certain preventative drugs and reducing anxiety. Anxiety is a key factor as research has shown that horses with a more excitable temperament are more likely to have an episode.
Unfortunately, despite the best management some horses will struggle to maintain any kind of athleticism.
Ways to prevent tying up
If your horse is at risk of tying up the following preventative measures are a good idea:
- A regular exercise programme needs putting in place, with plenty of time to properly warm up and cool down.
- Avoid box rest if you can, and ensure a careful return to work after a break.
- Provide plenty of turn-out (but avoid lush grazing) to help keep your horse active.
- Reduce your horse’s stress levels during exercise, stabling, turn-out and during transport.
- Be vigilant when any of the above preventative measures can suddenly not be implemented, e.g. turn-out is reduced, stress levels increase, or box rest is necessary.
Correct nutrition
It is advised to maintain horses that tie up on diets that are low in soluble carbohydrate, replacing any required additional calories with fat. The mechanism by which a higher fat diet benefits horses that tie up remains unclear, but it has been suggested that this could be linked to a calming effect.
This kind of diet would involve replacing calories from high sugar/starch feeds like oats with high fat feeds like rice bran or their commercial feed equivalents.
It is also important that caloric intake is reduced during periods when the horse is doing less work, for whatever reason.
Medication
Studies have shown that dantrolene, a muscle relaxant medication given before exercise, can help prevent episodes in some horses that are prone to them. Depending on the circumstances, your vet might recommend this medication.
Take home message
Tying-up is a serious and debilitating condition, but with the correct measures can be management to give your horse a good quality of life, and a chance at performing at the top levels.
