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One of the most painful and distressing conditions that can befall a horse during exercise is equine rhabdomyolysis syndrome (ERS) – also known as tying up, azoturia, setfast and Monday morning disease.

It describes a tissue-level breakdown of muscle fibres, usually in the large muscle masses of the horse, around the croup, loins and thigh area. ERS often appears within half an hour of the beginning of exercise, particularly after an enforced period of rest.

Typically, a horse will become anxious and sweat profusely, its gait will stiffen and it may be reluctant to move. The affected muscles become stiff, swollen and painful to the touch, and the pulse and respiration rates rise.

The severity of the attack ranges from mild stiffness and a shortened stride to total muscle seizure and even death. It occurs in horses and ponies of all types and ages and can appear at any stage in their life. Fillies and mares seem to be more prone. This may to relate hormones, as progesterone is an “electrolyte-leeching” body chemical.

The trigger for an attack varies between horses. It seems to be either increased muscle energy (glycogen) stores, caused by supplying more energy through the feed than the muscles need, such as in a period of rest, or poor or decreased blood circulation in the muscles.

This is important, because a muscle cell is 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.

These can be influenced by genetics, but management plays the largest part and is mainly linked to feeding practices.

Historically, equine rhabdomyolysis syndrome occurred in working horses rested on a Sunday on full rations andthen brought out on a Monday. The equivalent of this still happens today. It is a classic case of supplying more energy than the muscles need, often of a high starch nature. ERS is frequently seen in horses whose diet teeters on a knife edge of minimal fibre, high starch and resulting low electrolyte status.

Moving away from fibre in a performance horse diet has its dangers. First, fibre binds water as it passes through the gut, acting as a fluid and electrolyte reservoir. Reducing this can have a knock-on effect on circulating electrolyte levels. Second, low fibre usually means high starch and these diets 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 is not good

What you can do to help avoid ERS

  • Cut the feed, or move to a low energy product on days off or during rest periods.
  • Feed a high fibre, low starch diet. Feed plenty of hay or turn out on decent grass. Many racehorses which tie up on low fibre, high starch diets come right when turned out – the secret is an increased fibre supply. Cut down on oats as the main energy source. They are high starch, low fibre and have a low mineral and vitamin content.
  • Increase workload before increasing feed levels. Feed according to work done, not according to work about to be done.
  • Lengthen the period you spending warming the horse up before starting harder work, particularly after a day off.