Exertional rhabdomyolsis (also known as tying up, azoturia, or Monday morning disease) is a condition that causes painful muscle cramping in horses. It can affect the muscles of apparently any age, breed or gender of horse and results in a partial or complete inability to move. In the severest of cases even death can occur.
These can be highly variable in severity. In mild cases there is a hind leg stiffness and a shuffling hind limb gait. However sometimes owners of competition horses may just report poor performance. There may be pain/firmness on palpation of the affected muscle groups, most commonly of the back and gluteal muscles (rump), however any muscle group can potentially be affected. Muscle trembling and fasciculations can be seen. This can progress to signs of severe pain, elevated heart rate, sweating and the passage of a dark brown coloured urine (myoglobinuria), which is formed from the breakdown of the damaged muscles . Eventually there can be a temporary paralysis of gut movement , colic, lying down and death. For this reason the condition can sometimes be mistakenly diagnosed as an episode of colic. Other possible differential diagnoses could be various causes of acute back pain, laminitis, tetanus and iliac thrombosis.
So what causes the condition and which horses are predisposed?
The exact mechanism of the condition is not fully understood and there are undoubtedly several different conditions involved. Tying up occurs due to two main reasons, either the horse has an underlying muscle disease (myopathy), thought to have a genetic component or the horse has been over exerted physically. Exercise is seen in every case, but it is always accompanied by another factor.
Possible factors include:
- The overfeeding of carbohydrates
- Poor fitness, followed by a sudden increase in work load
- A period of rest and the level of concentrates not reduced
- Electrolyte or mineral imbalances
- A deficiency in selenium or vitamin E
- Imbalance of hormones, often nervous fillies and mares are affected more.
- Wet/windy/cold weather or stress.
The most common predisposing factor is an imbalance between a horse’s diet and it’s work load.
There is a genetic/hereditary predisposition. Quarter horses, thoroughbreds and related breeds such as the Paint Horse or Appaloosa are more prone to polysaccharide storage myopathy, which is characterised by repeated episodes of rhabdomyolysis. A muscle biopsy is required to diagnose this group of diseases.
There is currently extensive research being carried out into the underlying disease process. The end point of rhabdomyolysis involves the break down of the muscle cells and release of muscle enzymes. It has shown in many cases that there are excessive amounts of glycogen stored in the muscle and it is suggested that when it undergoes rapid anaerobic metabolism brought on by exercise, there is a build up of lactic acid in the muscles, which might damage the muscle cells. However, this theory has been discounted as the levels of lactic acid in clinical cases has not been high enough to cause harm.
The condition can be diagnosed on the clinical signs alone. The horse will become very painful and unwilling to move. Symptoms are usually seen at the start of exercise, although for milder episodes, the symptoms may not be seen until after the horse has cooled down. If your horse displays any of the above clinical signs, then it is important to move the horse as little as possible. If you are out on a hack, telephone for help and get the horse transported back to the yard. This will help to limit further muscle damage. Once the horse is in a stable, keep it warm with sweat rugs and call your vet. Your vet will be able to carry out a full clinical examination and take a blood sample which will help to further confirm the diagnosis. The blood sample looks at the levels of different enzymes, which are released from the inside of the muscle cells when they are damaged due to azoturia. Creatinine kinase (CK) is a sensitive indicator of muscle damage and it’s levels peak over 4-6 hours and decrease over 24-48 hours if there is no further muscle damage. Aspartate aminotransferase (AST) peaks at 12-24 hours and declines over the next 21 days. When investigating horses suspected of being prone to the condition, it may be helpful to measure the enzyme levels, before, immediately after and 24 hours after a known amount of exercise to confirm the diagnosis. A urine sample may also be analysed to look for the myoglobin which is excreted by the kidneys and subsequently cause damage.
Treatment and Prevention
Treatment will depend on the severity of the attack. It is important to keep the horse box rested and your vet will administer pain relief in the form of anti inflammatory drugs. Feed should be reduced to good hay and little or no carbohydrates in the initial period. Your vet will then check the enzyme levels in the blood about 14 days after and during this period whilst the enzyme levels are raised, no exercise should be attempted.
In severe cases, intravenous fluid therapy may be necessary. This is to counteract the dehydration and help flush the excess and potentially damaging myoglobin from the kidneys. Other drugs which help increase the blood flow to the muscles may also be administered, along with electrolytes and other supplements including antioxidants such as vitamin E.
Blood tests should be normal before a horse gradually returns to work. Feed levels are gradually reintroduced as exercise resumes.
Prevention of recurrence is essential. This will rely on a careful balance between feeding and exercise. The major part of the diet of a horse suffering from recurrent episodes of rhabdomyolysis should be made up of forage, either grass or hay. If they require further energy, then a high fat:low carbohydrate diet should be fed. Adding in corn oil to the diet can help meet these requirements. Appropriate electrolyte supplementation may be necessary depending on the results of a urine test.
It is important to make sure that the horse is warmed up properly before any hard work and then is always given a period of cool down after the hard work. Making sure the horse is worked on a daily basis helps to prevent episodes of tying up. If the horse is off work for any period of time, ensure that the return to work is gradual.
If you have have concerns about your horse suffering from this syndrome, please contact your vet for further advice.
If you have any questions about any aspects of your horse’s health, or indeed your pet, farm or smallholding animals, please do not hesitate to contact us on 01892 835456. Putlands Veterinary Surgery is based in Paddock Wood in Kent, and offers a friendly, professional and personal service for all species of animal. We have four dedicated large animal vets with a wealth of equine experience between us, and are always happy to help.