A racehorse exhibiting colic-like signs after a workout or an Arab endurance horse quitting in the final mile could both have something in common: exertional rhabdomyolysis (ER) or as it is more commonly known tying up. This is an equine “myopathy”, a disorder of the muscle tissues that is also known as azoturia or set fast.
The condition was historically know as Monday-morning disease because it commonly used to occur in working horses on Monday morning after they had been rested on Sunday on full rations.
Although the reasons for the condition are still not fully understood, symptoms of tying up include:
- Short strides and general discomfort
- Reluctance or refusal to move
- Severe contraction or spasm of the large muscles in the horse’s hindquarters or back
- Sweating and anxiety
- Increased respiration, pulse rate and anxiety
- In severe cases, the muscle protein myoglobin leaks into the bloodstream, staining the urine brown or red
Diagnosis is made after the observation of clinical signs and lab testing of the blood, urine and muscle.
What causes tying up in horses?
It was once thought that returning to work after a day of rest on full rations caused excess deposits of glycogen in the muscles, increasing lactic acid levels, which led to muscle fatigue. More recently, experts concluded that ER is a result of metabolic deformities in the muscle cells, but this broad diagnosis involves many trigger factors. It is important to work with your vet to identify why your horse has ER and make the right management changes.
“The general school of thought is that the condition is associated with exercise,” explains equine nutritionist Michaela Bowles. “A controlled exercise programme is vital in managing it.” This includes warming-up and cooling-down periods of at least 20 minutes, as well as restoring important electrolytes after hard work.
- Polysaccharide Storage Myopathy Syndrome
“Research has also suggested that an abnormal polysaccharide [a complex carbohydrate composed of sugar compounds] is present in some animals and cannot be metabolised by the organs,” explains Michaela. “This results in Polysaccharide Storage Myopathy Syndrome (PSMS), a form of ER causing abnormal build-up of polysaccharide in the skeletal muscles and problems delivering energy to them.”Owners of diagnosed horses should increase work gradually and feed a high-fat, low-starch diet, replacing part of it with oil, an efficient energy source.
- Carbohydrate overload
If excess soluble carbohydrate leaks into the horse’s hindgut – for example, due to too much spring grass or a high-starch cereal diet – rapid fermentation can occur and boost lactic acid production. The acidity can upset the gut’s bacterial balance and cause the release of endotoxins – this can affect bloodflow to the feet, resulting in laminitis; it can also be conducive to ER.
- Free radical damage
Free radicals, reactive forms of oxygen, damage structural cellular components in the horse’s muscles, which can lead to ER. They occur during exercise as the body breaks down dietary compounds for energy, utilises oxygen and produces carbon dioxide.”Antioxidants protect tissues, cells and organs and boost the immune system,” says Michaela. “Key antioxidants – vitamins E, A, C and trace elements copper and zinc, all found in small amounts in a good maintenance diet – and selenium, which is often given to horses on a supplementary basis, fight free radicals and help protect cells.”
- Calcium dysfunction
ER can be caused by dysfunction in the channels carrying calcium to the muscles. “In some horses, the muscle loses its ability to maintain concentrations of calcium during exercise,” Michaela says. “In these cases, calcium levels in the part of the cell that converts nutrients into energy increase to the extent that muscle fibres can be damaged.”
- Inadequate blood supply to the muscles
Changes to a horse’s exercise regime may decrease blood supply to the muscles and cause toxin build-up – according to Michaela, horses suffering from an inactive thyroid (hypothyroidism), have been known to suffer from recurrent attacks of ER.
- Breed susceptibility
ER can occur in all horses, although there is a higher incidence among Thoroughbreds, which may reflect the number used for competition. The Arab can suffer similar symptoms, as it has a metabolic defect caused by a deficiency of an enzyme in the oxygen-utilising pathways of its skeletal muscles.
- Loss of electrolytes
Electrolytes are essential body salts. Research has identified electrolyte loss as a major trigger of ER, as they are critical in maintaining body temperature. The salts are lost via sweat and urine, potentially causing muscle damage – the Animal Health Trust says horses can lose 10-15 litres of sweat per hour in hot weather.
- Other causes
There is often a deficiency of the amino acids leucine, isoleucine and valine at the sites of muscle oxidisation, suggesting that supplementation of these could postpone the onset of ER – consult your vet for advice. Other potential triggers include a combination of raised body temperature, dehydration, stress and anxiety, physical trauma such as an accident involving muscle damage and consuming toxic plant agents.
Preventing an attack of tying up
Michaela states that the best way to prevent ER is through consistent, controlled exercise and only the necessary amount of feed rations. However, MSM (a sulphur donor that can aid tissue repair) is given to racehorses to reduce stress and has been found to reduce inflammation, boost circulation and help ease pain. It could be a useful preventative for horses prone to ER. Again, ask your vet for advice.
Horses with continual bouts of ER that display recurrent symptoms from an early age or after minimal exercise will need veterinary investigation.
Top tips for horse with a history of tying up
- Walk for 20 minutes on commencing exercise, in order to warm up the horse’s muscles
- Allow for a cooling-down period after work – in other words, walk the horse for at least 20 minutes to ensure the removal of lactic acid and allow respiration to normalise
- Horses should never be left in their stables all day on rest days – susceptible horses should have field rest or exercise in-hand
- Try to provide a stress-free environment in order to reduce anxiety, as this could provoke metabolic disturbances, resulting in an attack
- If considering antioxidant supplementation, make sure you are not exceeding daily recommended vitamin and mineral amounts – if you are in any doubt, always check with a nutritionist. Consider using a feed supplement formulated to match your individual horse’s situation
- Seek advice from a nutritionist about feeds designed for those horses that are prone o ER, with extra antioxidants and pre-determined levels of vitamins and minerals
This article was first published in HORSE magazine