Tying Up in Horses (Azoturia): Causes, Treatment and Prevention

Reviewed by Dr. Khurrum Shahzad Khosa, DVM

Tying up is a distressing and potentially dangerous condition that every horse owner, trainer, and groom needs to recognise and respond to correctly. A horse that ties up during or after exercise is in genuine pain and is at risk of serious muscle damage — and in severe cases, kidney injury. It requires immediate veterinary attention, not well-intentioned walking to "loosen up." Understanding what tying up is, why it happens in different types of horses, and how to prevent it is essential knowledge for anyone working with horses in regular training or competition.

What Is Tying Up? Understanding Azoturia

Tying up is the common name for exertional rhabdomyolysis — the breakdown (lysis) of skeletal muscle tissue (rhabdo = striated muscle) during or shortly after exercise. The condition has accumulated several historic names: azoturia (from the excess nitrogen found in the dark urine of affected horses), Monday morning disease (from the classic presentation in working horses returned to heavy work after a rest day), and setfast (from the horse that becomes unable to move).

The central event is the destruction of muscle cells — myocytes — releasing their contents into the bloodstream. Myoglobin — the red, oxygen-carrying protein in muscle cells — is released in quantity and filtered by the kidneys, causing the characteristic dark brown, coffee-coloured, or port wine-coloured urine that is one of the most recognisable signs of a severe episode. This myoglobinuria is not just a diagnostic clue — it is evidence of significant renal load. In serious cases, the volume of myoglobin released can overwhelm kidney filtration capacity and cause acute kidney injury — myoglobinuric renal failure — making severe rhabdomyolysis a potential systemic emergency beyond the immediate muscle problem. The muscle enzymes creatine kinase (CK) and aspartate aminotransferase (AST) are released into the bloodstream in proportion to the degree of muscle damage, and their measurement in blood is central to diagnosis and monitoring.

Clinical Signs

The onset is typically during exercise or within 30 to 60 minutes of stopping work. Signs range from subtle to severe. In mild cases, the horse may show a slightly shortened stride, reluctance to move forward freely, mild stiffness particularly in the hindquarters, and a subtly uncomfortable expression. These mild episodes can easily be dismissed as "a bit stiff" if the owner is not alert to them.

In moderate to severe cases, signs are much more obvious: the horse slows dramatically or stops during work; the muscles over the croup and hindquarters feel hard, firm, or rock-like to palpation due to sustained muscular contraction and oedema; the horse sweats profusely; trembling occurs; there is a pained, anxious expression; and the horse may refuse to move or be physically unable to continue walking. In severe cases the horse may go down and be unable to rise. Urination during or after exercise may produce dark-coloured urine — a key sign that myoglobin is reaching the kidneys.

Immediate First Aid When a Horse Ties Up

Stop all exercise immediately. The single most important action is to stop exercise the moment tying up is suspected. Do not continue riding. Do not walk the horse back to the stable — forcing continued muscle contraction in already damaged tissue worsens the injury significantly. This is the most critical and most commonly misapplied first aid principle in the field. Stay where you are.

Keep the horse warm and calm. Apply a rug or cooler to prevent chilling, particularly in cold or windy weather. Keep the horse calm — stress increases muscle tension and worsens the episode, particularly in horses with RER. Avoid creating additional excitement around the horse.

Call your veterinarian immediately. Describe the signs clearly: the horse's location, level of distress, whether it is standing or down, whether you can see dark urine, and how long the episode has been going on. Your vet will advise whether to move the horse to the stable or wait where you are.

Do not give medication without veterinary guidance. Phenylbutazone (bute) is the most commonly used equine anti-inflammatory, but in cases of severe rhabdomyolysis with potential kidney involvement, NSAIDs can exacerbate renal injury. Your vet will make decisions about which drugs to give and when based on clinical assessment.

Veterinary Treatment

Veterinary treatment of tying up typically includes muscle relaxants (methocarbamol is commonly used where available); anti-inflammatory and analgesic medication chosen with attention to kidney status; intravenous fluids in moderate to severe cases to support kidney function and flush myoglobin from the renal tubules; and monitoring of urine colour and kidney function through serial blood tests measuring serum creatinine and urea. Serial CK measurements over 24 to 72 hours track the severity and trajectory of muscle damage — CK peaks at 24 hours after the insult and the rate of subsequent decline indicates the prognosis. Horses with very high CK values (hundreds of thousands of IU/L) or rising CK on serial testing have a more guarded prognosis for rapid return to work.

Types of Tying Up: Sporadic vs. Recurrent

Sporadic Exertional Rhabdomyolysis

Sporadic tying up refers to a single or occasional acute episode in an otherwise healthy horse without an underlying muscle disorder. Any horse can have a sporadic episode under the right circumstances. Common triggers include: working the horse significantly beyond its current fitness level; the Monday morning disease pattern of maintaining a high-grain ration through a rest day and then returning to full work; significant electrolyte imbalance, particularly sodium, chloride, and potassium after heavy sweating without adequate replacement; concurrent viral or bacterial illness at the time of exercise; or exercising in extreme heat and humidity without adequate hydration.

A horse that has a single well-explained sporadic episode, recovers fully, and has no further episodes when management is adjusted does not necessarily have a chronic underlying condition. However, a second episode warrants proper veterinary investigation.

Recurrent Exertional Rhabdomyolysis (RER)

RER is a genetic muscle condition predominantly affecting Thoroughbreds and Thoroughbred-related horses — Standardbreds, and Warmbloods with significant Thoroughbred breeding. Tense, anxious mares are classically overrepresented. The underlying mechanism involves an abnormality in intracellular calcium regulation within skeletal muscle cells: calcium handling during muscle contraction is disordered, leading to excessive or poorly controlled muscle contractions during exercise.

RER horses tie up repeatedly — often at similar points in their work, typically early in exercise during the warm-up or when the horse is stressed or excited rather than at peak exertion. The condition is stress-sensitive: horses that are nervous, excitable, or in a stressful environment tie up more frequently. Management involves both dietary changes and stress reduction — regular turnout, consistent routines, a calm stable environment, and minimising known stress triggers.

Polysaccharide Storage Myopathy (PSSM)

PSSM is a separate genetic muscle condition predominantly found in Quarter Horses, certain Warmblood lines, draft breeds, and their crosses. PSSM Type 1 is caused by a specific mutation in the GYS1 gene encoding glycogen synthase, leading to abnormally high accumulation of glycogen and an abnormal polysaccharide in muscle fibres. A DNA test for PSSM Type 1 is widely available through the APHA and independent genetics laboratories.

PSSM horses may tie up with even relatively modest levels of exercise, and some present with muscle atrophy, poor topline, and poor performance rather than acute tying-up episodes. The cornerstone of PSSM management is a low-starch, high-fat diet combined with a structured, consistent exercise programme. PSSM horses kept in stalls with minimal exercise and high-starch feed are at highest risk of severe episodes.

PSSM Dietary Management

For horses with confirmed PSSM Type 1, dietary modification is the most important management intervention. The fundamental principle is reducing non-structural carbohydrate (NSC) intake to the lowest practical level while maintaining sufficient caloric intake for the horse's work demands. This means eliminating cereal-based hard feeds — oats, barley, coarse mixes — from the diet entirely and replacing their caloric contribution with fat and fibre-based alternatives.

Added dietary fat — soya oil, rice bran oil, stabilised rice bran, or dedicated high-fat supplement products — provides a metabolically safer energy source for PSSM horses than starch. The horse's muscle cells handle fat as fuel more safely than the abnormal glycogen metabolism triggered by high-starch feeding. Good quality hay or haylage forms the dietary foundation, and a vitamin and mineral balancer ensures complete nutrition without the starch load of a concentrate feed. Consistent exercise is as important as diet — horses with PSSM maintained in regular low-level work have significantly fewer episodes than those in intermittent or variable exercise programmes.

Prevention Strategies

For all horses prone to tying up, consistent and progressive training is essential. Sudden large increases in workload, or full work sessions immediately after significant time off, are among the most common triggers. Build fitness gradually — even if the horse feels ready for more, the muscle physiology needs time to adapt. If a horse has had time off through illness, injury, or management changes, return to work progressively over a minimum of two to four weeks.

Reduce grain rations on rest days — a horse in no work does not require the same energy input as one in full training, and maintaining peak ration through rest periods is a well-established cause of Monday morning tying up. Electrolyte supplementation for horses in regular work, particularly in hot or humid conditions, supports normal muscle contraction and relaxation. For broader guidance on equine nutrition and fitness management, see our horse management guides.

Frequently Asked Questions

What causes tying up in horses?

Sporadic tying up is caused by overexertion, high-starch feeding before rest days, or electrolyte imbalance. Recurrent tying up is caused by specific genetic muscle disorders — RER predominantly in Thoroughbreds, or PSSM predominantly in Quarter Horses and draft breeds.

What should I do immediately if my horse ties up?

Stop all exercise immediately — do not walk the horse on. Keep them warm and calm. Call your vet. Do not administer medication without veterinary guidance as some drugs can worsen kidney injury in severe cases.

Is tying up the same as PSSM?

No — tying up is a clinical sign with multiple possible causes. PSSM is one specific cause of recurrent tying up, predominantly in Quarter Horses and draft breeds. RER is another cause, predominantly in Thoroughbreds. Veterinary investigation including blood tests, muscle biopsy, and genetic testing identifies which applies.

Can diet prevent tying up in horses?

Yes — reducing starch and sugar and increasing dietary fat is the most effective dietary approach for horses with RER and PSSM. Reducing grain on rest days prevents the classic Monday morning pattern. Low-starch, high-fat diet combined with consistent exercise is the cornerstone of PSSM management.

Why do some horses tie up more than others?

Genetic predisposition, individual stress sensitivity, diet composition, and training management all contribute. Thoroughbred mares and horses in tense, unpredictable environments are classic RER profiles. Draft and Quarter Horse breeds are classic PSSM profiles. Repeated episodes deserve thorough investigation to identify the specific underlying cause.

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Disclaimer: This article is for educational purposes only and does not constitute veterinary advice. Always consult a licensed equine veterinarian for diagnosis and treatment.