ERS, also known as Azoturia, tying-up, set-fast and Monday morning disease

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    This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.
  • Azoturia, which is more correctly described as equine or exertional rhabdomyolysis syndrome (ERS) and is also referred to as set-fast, tying-up and Monday morning disease, is a disturbance of the horse’s normal muscle function. It can be compared to severe muscle cramp and it tends to happen during the early part of a horse’s exercise session, typically in the first 10-30 minutes, and often – but not always – following a period of rest.

    Azoturia/ERS: Signs | Causes | Immediate management | Diagnosis | Treatment | Recovery management | Avoidance | Prognosis

    When a horse has a severe azoturia attack, the muscle fibres are damaged and release muscle breakdown products into the bloodstream. Usually it is the large muscle masses of the horse, around the croup, loins and thigh area that are affected. Depending on how many muscle fibres are affected, this can be extremely painful so unsurprisingly the horse will be unwilling to move, may appear distressed and often sweats profusely, hence the name tying-up, which describes the condition clearly.

    Typical signs of azoturia/ERS in horses

    • The horse is unwilling to move, may take short steps and feel unsteady or stiff on his back legs
    • The muscles of the hindquarters feel hot and hard
    • The horse is likely to appear distressed and may sweat heavily
    • There may be frequent attempts to urinate, although the horse may struggle to take the normal stance to stale
    • Raised pulse and respiration rate
    • A slight increase in temperature
    • Urine may be a red-brown to dark chocolate in colour due to the release of myoglobin, a muscle breakdown product.
    • In extreme cases the horse will be unable to move and may collapse

    The severity of an attack can range from mild stiffness and a shortened stride to total muscle seizure and even death, as a result of failure of the kidney and circulatory systems.

    It can occur in horses and ponies of all types and ages and can appear at any stage in their life, while fillies and mares seem to be more prone than geldings.

    Causes of azoturia

    The trigger for an attack of azoturia/ERS varies between horses. Research has shown that there are multiple causes with different levels of complexity.The simplest cause is a dietary imbalance where a horse is fed more than is appropriate for their level of fitness.

    Some cases are associated with either increased muscle energy (glycogen) stores, caused by supplying more energy through the feed than the muscles need, such as during a period of rest, or poor or decreased blood circulation in the muscles.

    There are suggestions that individual muscle cells are like a mini reactor, and a good blood supply is vital to supply essential nutrients, including water and electrolytes, to the tissues and also to cart away toxic waste. When there is a mismatch between supply and demand, muscle problems develop.

    Genetics also play a role, with some horses being more prone than others, particularly those that suffer from polysaccharide storage myopathy (PSSM).

    Management is an important factor and the condition is mainly linked to feeding practices involving minimal fibre and high levels of starch associated with low electrolyte status.

    Fibre is key to maintaining the correct levels of electrolytes in the horse’s body as it binds water as it passes through the gut, acting as a fluid and electrolyte reservoir. A reduction in fibre can have a knock-on negative effect on circulating electrolyte levels.

    A low-fibre diet usually means high levels of starch that can cause an increase in muscle fuel stores, which, if rapidly used, can cause increased acidity in the blood. This can lead to altered muscle metabolism and increased excretion of calcium and phosphorus. Electrolytes are essential in optimum muscle function, so any reduction in them is a potential cause for concern.

    Scientific understanding of ERS has disproved the popular belief that lactic acid build-up in the muscles is the cause of the condition. Susceptible horses have an underlying, most likely genetic, predisposition to ERS, which leads to an attack when some management factor tips them over the edge.

    Stress is one of the triggers: it can arise from travelling long distances to a competition, not only from the journey itself, but also due to excitement on arrival.

    Dietary stress is another factor. Many incidences are caused by a change in routine that (sometimes without your knowledge) means that the amount of fibre in the diet is reduced. This can happen with an increase in workload, a change in forage or by a horse being too excited to eat before, during or after a competition.

    Classically it was called Monday morning disease as working horses developed the condition at the beginning of the week, after having the Sunday off with no reduction in rations.

    If an attack occurs…

    If you suspect your horse may be suffering from azoturia you should keep him still, cover his loins and quarters with a rug or blanket and seek urgent veterinary advice.

    If the horse is at home move him quietly to his stable, ensure there is plenty of bedding and allow him to lie down if he wishes. If out riding, the horse should be transported home. A trailer or a horsebox with a lower ramp is preferable.

    Try to encourage the horse to drink, if possible. Fluids will help flush out the kidneys and reduce the problems associated with muscle breakdown. Also monitor if the horse is able to urinate normally and catch a sample of any urine passed share for your vet to check.

    Offer the horse hay to eat to help reduce its stress levels, but do not feed any concentrates until you have spoken to your vet.


    The condition is diagnosed based on the clinical signs, the horse’s history and the results of blood tests. The timing of the blood tests after the initial signs are seen is important.

    Muscle biopsies and sometimes exercise tests, can be used to help diagnosis the condition in chronic cases that suffer repeat mild attacks.

    There are also genetic tests available using blood samples or sometimes hair plucks , which can be use to show up some forms of PSSM.


    Initial treatment is given to reduce the horse’s pain and anxiety, prevent further muscle damage and restore the fluid and electrolytes balance to normal levels. It is important that adequate blood flow is maintained to the horse’s kidneys to prevent permanent damage from occurring.

    Treatment will vary depend on the cause , sometimes the vet may use medication such as acepromazine (ACP) to help the horse to relax and reduce its anxiety, combined with analgesic and anti-inflammatories to relieve pain and inflammation. Sometimes specific muscle relaxants are prescribed.

    Intravenous fluids may be given, or sometimes fluids and electrolytes via a stomach tube. If electrolytes are offered in drinking water, then plain water should also be made available.

    Treatment should only be given under veterinary supervision as the use of analgesic and sedation in a dehydrated horse can result in kidney damage.

    Management during recovery

    A horse that has experienced a bout of ERS should be kept stabled with a thick dry bed until signs of stiffness have passed. Feed should be restricted to hay combined with a broad-spectrum vitamin and mineral supplement.

    As the stiffness and discomfort passes the horse can be led out in hand to graze before being turned out for short periods in a small paddock as gentle motion will encourage blood flow to the muscles and therefore aid repair. If the horse is likely to gallop around when loose, suitable steps should be taken to avoid this as extreme exercise may slow recovery of the damaged areas.

    Return to work should not take place until the muscle enzymes have returned to normal, which can take a surprisingly long period of time, often up to two weeks or more. The reintroduction of work should be undertaken gradually and regularly.

    How to avoid an azoturia attack

    • Reduce the horse’s feed on days off or during rest periods
    • Feed a high fibre, low starch diet with plenty of hay or turn out on decent grass, with a broad spectrum vitamin and mineral supplement
    • Increase your horse’s workload before increasing feed levels, do not increase feed in advance of work
    • Several small feeds per day are better than fewer larger ones
    • Avoid making sudden changes to the horse’s diet
    • A bio-available source of calcium may be beneficial
    • Use electrolytes, especially when travel and workload are high
    • Lengthen the period you spending warming the horse up before starting harder work, particularly after a day off
    • Use an exercise sheet while warming up in cold or wet weather
    • Training programs should be planned to ensure increase in workload and associated fitness is achieved gradually
    • Ensure regular daily exercise – days off should be avoided
    • Turn the horse out as much as possible
    • Stick to a routine and minimise stress for the horse as far as possible


    Some horses have a single episode of ERS and go to make a full recovery, never suffering the condition again. Others suffer repeated bouts despite careful management. In chronic cases where the muscles are repeatedly stressed, muscle damage can occur which limits the horse’s ability to work.


    New RVC study on tying up in horses and ponies Published: 19 Feb 2019