Bit & Bridle

Equine Azoturia

Other Names

  • Tying-Up
  • Monday Morning Disease
  • Set-Fast
  • Exertional Rhabdomyolysis Syndrome (ERS)
  • Sporadic Exertional Rhabdomyolysis
  • Recurrent Exertional Rhabdomyolysis (RER)

What Is Azoturia in Horses?

Azoturia — most commonly known today as exertional rhabdomyolysis syndrome (ERS), or simply “tying-up” — is a condition involving acute muscle damage and breakdown triggered by exercise. During an episode, muscle fibres in the hindquarters, back, and sometimes other large muscle groups undergo injury or necrosis (cell death), releasing their contents — including the protein myoglobin — into the bloodstream. Myoglobin is subsequently filtered by the kidneys and excreted in the urine, which turns coffee-coloured or dark brown (a process called myoglobinuria, which gives the condition its historical name: azoturia, from the presence of nitrogen-containing compounds in the urine).

The historical name “Monday Morning Disease” originates from the era of heavy working horses, when the condition was observed to occur at the start of the working week in horses that had been rested on Sunday but fed their full working ration. This pattern — high energy intake during rest followed by strenuous exercise — remains one of the classic triggers for sporadic tying-up today.

Tying-up is one of the most common muscular disorders in horses and affects animals across disciplines and breeds. Its causes are heterogeneous: some horses experience isolated episodes associated with identifiable triggering factors, while others suffer from chronic, recurrent disease rooted in heritable muscle biochemistry disorders.

Causes

ERS encompasses a spectrum of disorders with different underlying mechanisms:

Sporadic Exertional Rhabdomyolysis

  • Work-diet mismatch: Exercising a horse at intensity beyond its current conditioning level, or working a horse that has received high-starch/energy feed during a period of rest.
  • Dietary deficiencies: Selenium and vitamin E deficiency impairs antioxidant protection of muscle cell membranes, increasing susceptibility to exercise-induced damage. See our article on Equine Vitamin E Deficiency for more detail.
  • Electrolyte imbalances: Potassium, sodium, and chloride are lost through sweat during exercise. Inadequate electrolyte replacement — particularly in horses working in hot weather — can impair normal muscle function.
  • Dehydration: Impaired circulation to muscles during exercise reduces oxygen delivery and waste removal, predisposing to muscle damage.
  • Concurrent illness: Respiratory infections (particularly influenza and herpes virus) or other systemic illness can predispose horses to muscle problems during recovery and return to exercise.

Recurrent Exertional Rhabdomyolysis (RER)

  • Seen most commonly in Thoroughbreds and Standardbreds, RER is associated with abnormal intracellular calcium regulation in muscle fibres. Affected horses have episodes triggered by exercise, stress, or dietary factors regardless of conditioning level.
  • A nervous temperament and excitable disposition are strong risk factors for RER in these breeds.

Polysaccharide Storage Myopathy (PSSM)

  • A distinct inherited muscle disorder seen predominantly in Quarter Horses, draught breeds, warmbloods, and their crosses, characterised by abnormal accumulation of glycogen and abnormal polysaccharide in muscle fibres.
  • PSSM horses often have episodes precipitated by even minimal exercise, gait abnormalities, and muscle firmness.

Symptoms and Signs

Clinical signs typically appear during or shortly after exercise:

  • Stiffness and reluctance to move: The horse becomes unwilling to move forward, particularly in the hindquarters. This can range from subtle stiffness to complete inability to walk.
  • Muscle hardness and pain: The large muscle groups of the hindquarters — particularly the gluteal and longissimus muscles — become hard, firm, and painful to touch. In severe cases, the muscles may feel rock-hard.
  • Sweating: Profuse sweating disproportionate to the level of work performed is common.
  • Anxiety and distress: Affected horses often appear anxious, uncomfortable, or show signs consistent with pain — pawing, looking at their flanks, shifting weight.
  • Dark-coloured urine: Myoglobinuria produces characteristic coffee-brown or port-wine-coloured urine. This is a serious sign indicating significant muscle breakdown and a high risk of kidney damage.
  • Elevated heart and respiratory rate: Consistent with pain and systemic disturbance.
  • Reluctance or inability to stand: Severely affected horses may be unable to stand or may tremble continuously.

Diagnosis

  • Clinical examination: History of exercise followed by sudden stiffness, palpation of affected muscle groups, observation of gait and demeanour.
  • Serum muscle enzymes: Elevated creatine kinase (CK) and aspartate aminotransferase (AST) confirm muscle damage. CK peaks within 4–6 hours of muscle injury; AST peaks later and persists for longer. In moderate to severe cases, levels may be thousands to tens of thousands of units above normal.
  • Urinalysis: Dark urine should be tested immediately for myoglobin. Significant myoglobinuria indicates severe muscle damage and urgent risk to renal function.
  • Electrolyte panel: Identifies any concurrent electrolyte imbalances contributing to or resulting from the episode.
  • Muscle biopsy: In horses with recurrent episodes, biopsy of the semimembranosus or semitendinosus muscles provides histopathological diagnosis of PSSM, RER, or other specific myopathies.
  • Genetic testing: PSSM type 1 (caused by a GYS1 mutation) can be confirmed by a DNA test from a blood or hair sample.

Treatment

Acute management:

  • Stop exercise immediately: The horse must not be pushed to continue moving. If the horse is manageable, lead it quietly to shelter — if it cannot move, keep it still and wait for the vet.
  • Keep warm: Apply rugs or blankets to maintain core temperature and reduce muscle spasm.
  • Intravenous fluids: Essential in moderate to severe cases — supports kidney function and helps flush myoglobin from the renal tubules. Administered by the vet.
  • NSAIDs: Phenylbutazone or flunixin meglumine for pain relief and anti-inflammatory effect.
  • Muscle relaxants: Methocarbamol may be used in severe cases to reduce muscle spasm.
  • Acepromazine: Used judiciously for its vasodilatory effects, improving muscle perfusion — beneficial in horses with RER where stress and tension are contributing factors.

Long-term management for recurrent disease:

  • Dietary modification: For PSSM horses, a low-starch, high-fat diet is highly effective in reducing episode frequency. For RER horses, reducing simple starch and sugar in the diet whilst maintaining overall energy intake through fat and digestible fibre helps stabilise muscle cell function.
  • Exercise management: Consistent daily exercise rather than erratic work patterns; gradual conditioning programmes; turnout in preference to stabling.
  • Selenium and vitamin E supplementation: Address any documented deficiencies.
  • Stress reduction: RER Thoroughbreds benefit significantly from environmental enrichment, turnout, and routines that minimise anxiety.

Prevention

  • Match feed to workload — reduce hard feed on rest days and increase again when returning to full work.
  • Ensure adequate selenium and vitamin E in the diet year-round.
  • Provide electrolyte supplementation for horses in regular or heavy work, especially in warm weather.
  • Maintain consistent exercise schedules — avoid long periods of rest without corresponding reduction in energy intake.
  • Ensure horses are properly conditioned before high-intensity work.
  • Investigate and treat horses with recurrent episodes — a specific underlying diagnosis opens the door to targeted management that dramatically reduces episode frequency.

Prognosis

For sporadic tying-up with identifiable and correctable triggers, prognosis is excellent. The majority of horses recover fully from individual episodes with no lasting consequences, provided kidney function is protected through adequate fluid therapy when myoglobinuria is present. For horses with recurrent disease — particularly PSSM or RER — prognosis for return to full work is good to excellent with appropriate dietary and exercise management, but the underlying condition is not curable. Horses must be managed for life. Without specific management, recurrent cases will continue to have episodes that affect welfare, performance, and longevity.

When to Call the Vet

Call your vet immediately if:

  • Your horse becomes stiff, reluctant to move, or shows signs of muscle pain during or after exercise
  • The horse’s urine appears dark, brown, or discoloured — this indicates myoglobinuria and a risk to kidney function
  • The horse is unable to stand or is sweating profusely without proportionate exercise
  • The horse has had previous episodes and current signs appear more severe than usual
  • You are uncertain whether you are dealing with colic or tying-up — both can cause hindlimb discomfort and sweating, and veterinary assessment is needed to distinguish them

Frequently Asked Questions

Can I walk my horse out of an episode of tying-up?

No. The old practice of “walking a horse through” a tying-up episode is now known to be harmful. Forcing a horse with active rhabdomyolysis to continue moving increases muscle damage and worsens myoglobinuria. The correct response is to stop exercise immediately, keep the horse warm and calm, and call the vet. Light walking may be appropriate later in recovery under veterinary guidance, but not during an acute episode.

What does dark urine mean in a horse with tying-up?

Dark, coffee-coloured or red-brown urine in a tying-up horse indicates myoglobinuria — muscle breakdown protein being excreted by the kidneys. This is a serious sign because myoglobin in high concentrations damages the renal tubules and can cause acute kidney failure. Prompt intravenous fluid therapy is critical to dilute the myoglobin and protect kidney function. Any horse passing dark urine after an episode of muscle stiffness requires immediate veterinary attention.

Is tying-up the same as colic in horses?

They can be confused at first glance — both involve a horse showing signs of distress, sweating, and abdominal discomfort. However, tying-up is characterised by hindquarter muscle stiffness, reluctance to move the hindlimbs, and firm painful gluteal muscles. Colic involves primarily abdominal pain and gastrointestinal signs. Veterinary assessment distinguishes the two, which is important because the initial management differs.

Are some breeds more prone to tying-up?

Yes. Thoroughbreds and Standardbreds are predisposed to RER, particularly mares and horses with a nervous temperament. Quarter Horses, draught breeds, warmbloods, and draught crosses are predisposed to PSSM. Sporadic tying-up can occur in any breed in response to the right combination of dietary and exercise triggers.