Last Updated on April 15, 2022 by Allison Price

Originally known as Monday Morning Disease, ER is more common in horses after exertion such as hunters, racehorses and endurance horses. However, it can happen in any horse.

We’ll discuss the symptoms, causes, how to diagnose and treat it, and what we can do prevent future episodes. We will also examine other less common muscle disorders.

What is Exertional Rhabdomyolysis (ER)?

ER is the process of muscle breaking down after exercise. It was once believed that lactic acid buildup in muscles caused the problem. However, this is no longer the case. It is now believed that the problem is in the function of muscle cells, which causes damage. This causes muscles to become rigid and then to spasm, and cells to begin to fall apart, releasing myoglobin to the bloodstream. Myoglobin, which is toxic, is not only painful for horses.

It is important that you understand that ER is a sign of underlying disease and not a disease.

Horses are more susceptible to ER than others. Thoroughbred mares are especially at risk. However, there are also external factors that can lead to ER. These causes will be discussed later.

Signs that a horse is tied up

Tight muscles (R), and tying up (L).

A short stride and discomfort when moving- often an awkward gait.

Reluctance to or refusal to move

The horse’s large muscles, which are located in his hindquarters and back, will contract or spasm severely. These muscles can feel painful to touch and often feel stiff.


Anxiety, panic, and trembling.

Temperature, heart rate, and respiration increase

The production of urine will decrease. This could cause urine to turn reddish or brown. This is a sign of something wrong

These signs may not be specific and could be mistakenly attributed to other diseases such as laminitis or colic. If your horse shows any of these symptoms, it is important to get a prompt diagnosis.


The vet will suspect the horse has ER based on clinical signs and past history. However, blood testing is required to confirm this diagnosis. The enzymes Creatinine Kinase and Aspartate aminotransferase in blood will be checked. These are markers for muscle damage. You can also test for genetics and urine samples. These additional information can help determine the cause of a horse’s tie-up and are especially useful for horses who have recurrent episodes.


Muscle biopsy

The treatment aims to prevent further damage, control pain and possibly address the underlying causes.

  • Horses need to be rest. Movement can cause more muscle damage. Therefore, horses should be initially kept in a stable.
  • Horses may need to be treated for dehydration. They can be stomach tubed to provide a bolus water and electrolytes. To minimize damage, horses may require IV fluids (drip) in severe cases.
  • The administration of pain relief will be done.
  • You may also be offered a tranquilliser to help with muscle relaxation.
  • Keep the horse warm to prevent further injury to its muscles.
  • Horses should be encouraged and allowed to drink.
  • Horses should be fed low-sugar and starch diets.
  • Massage can be used to relax and relieve pain in the large muscles, especially around the rump.

Your horse’s response will guide your treatment. This can be best monitored through clinical examination and sequential blood tests.

Your vet will design a recovery and rehabilitation program. It will likely include a gradual return to work and preventative measures (see below), to avoid further tie-ups.

Muscles typically recover in 3-4 weeks. However, after a severe episode, it is possible for the muscle to appear shrunken.


A careful and responsible management strategy can significantly reduce the chance of an episode in the ER recurring.

  • Horses suffering from ER should be kept in the stable for longer periods. A warm-up of 20 minutes of gentle walking is a good idea. It can reduce the likelihood of an ER episode and prevent other injuries.
  • It is a good idea to use an exercise sheet to cover the quarters of your horse while you warm up.
  • It is important to make sure that your horse is properly warmed up after work. Gentle walking and stretching are two of the best ways to do this. The period should last approximately 20 minutes or more if the horse’s respiratory rate remains high.
  • It is important to walk your horse gently the next day if you are certain you have worked your horse hard, such as for competitions or day’s hunting. You should allow your horse to relax and stretch.
  • Horses that have been working hard should not be left unattended in the stable. It is important that horses are taken out of the stable if they must stay.

Stable Management

  • Feed only the required amount. Horses that are fed too much can be at significantly higher risk for ER.
  • Horses with ER should be fed a low-starch diet and avoid cereals’ quick release carbohydrates. There are many commercially available diets, but you should look out for low sugar, high fibre and cereal-free. Many feed companies have nutritionists who can help you choose the right diet for your horse. (Re-Leve By Saracen, Dodson or Horrell ERS Pellets).
  • Make sure your horse has a healthy, balanced diet. ER can be caused by deficiencies in vitamins and mineral. A balancer, at the right rate, will prevent deficiencies.
  • Horses who do heavy work or moderately will need additional electrolyte supplements. It is not advisable to feed electrolytes one-time. Instead, give a maintenance dose and continue feeding after the horse has swelled. You can add electrolytes to water or feed as a powder, or as a paste.
  • Make sure your horse is always hydrated. Make sure to give your horse regular water if you are giving electrolytes in water. You can encourage your horse to drink even when they’re away from home by adding peppermint or apple chunks to the water. To increase your horse’s water intake, you can add water to their feeds by making a mash.
  • Avoid chilling! Avoid chilling!

There are many causes of ER, we will explore the most common. If you suspect your horse is showing signs of ER, prompt examination and treatment are vital.

Carbohydrate overload

ER can be caused by overfeeding on high-carbohydrate foods that have high sugar content (e.g. cereal mixes) and insufficient exercise. Accident overload, such as a break-in at a food store, can also lead to ER.

Vitamin Deficiency

ER can be caused by a deficiency in certain vitamins that protect muscles from oxidative damage. Exposed to free-radicals can cause damage to the muscles by causing the breakdown of normal diet components. This damage can be prevented by antioxidants, such as Vitamin C, Vitamin E and copper. If this is suspected, additional antioxidants such as selenium may be added to your diet, but only after veterinary advice.

Electrolyte deficiency

ER can be caused by fluid and electrolyte deficiency. Horses that sweat a lot, especially if they are working hard, can lose large amounts of salts. These salts must be replaced. ER can be caused by electrolyte imbalances.

Recurrent External Rhabdomyolysis

Horses suffering from recurrent exertional rhabdomyolysis (RER), an irregular form of tying, have been shown to be caused by an abnormality in intracellular calcium regulation. Past research has supported this possibility. There is currently no diagnostic test for RER. A history of tying up in horses who are fit and on a balanced diet with reasonable training is a good indicator of RER. Muscle biopsy abnormalities should not be considered. Horses suffering from RER are more likely to tie up when they get too excited.

Although it is believed that RER may be an inherited trait, horses might not develop clinical signs if they are exposed to risk factors.

RER is most common in thoroughbreds. However, it can also affect other breeds.

The management of RER should be focused on controlling the stress triggers. These can include:

  • Place the horse in a quiet part of the yard
  • Training time (first, not last)
  • If you have the opportunity to turn-out, it is a huge advantage. The more time you can move around and let off steam, the better.
  • Avoid training regimens such as retraining at a gallop, or doing intervals that excite horses.
  • Pay attention to lameness and how it can be treated
  • If possible, avoid box rest and lay-up. Provide calm exercise for horses who have been rested the previous day.

Polysaccharide Storage Myopathy

In 1992, quarter horses and appaloosas were first able to identify PSSM. This is characterized by the accumulation of glycogen (and related monosaccharides), within muscle fibers that are resistant to the enzyme diastase.

The disease was first discovered in horses of all breeds, including European ones. There are two types of PSSM. Type 1 is caused by a genetic mutation in the glycogen gene. Type 2 is not affected by this mutation. You can test for Type 1.

Signs for PSSM:

The most common and easily recognized clinical syndrome is exertional rhabdomyolysis. In one study, more than 90% of cases with exertional rhabdomyolysis were reported.

You may also see the following sub-clinical signs:

  • Poor Performance
  • Muscle fasciculations (shivering).
  • Muscle atrophy (wastage).
  • Gait abnormalities
  • Stiffness in the pelvis or generalized limbs
  • Undiagnosed back or lameness
  • PSSM-affected horses often have calm demeanours.
Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!