Feeding Horses With Exertional Rhabdomyolysis (ER)


Previously known as Tying-Up, Azoturia and Monday Morning Disease, Equine Rhabdomyolysis Syndrome or ERS is the most common name currently used to describe muscle disorders in the horse. Studies and research have identified some distinct disease pathways for tying up in horsesand we will explain more about what those are. But first….

How big a problem is it?

Surveys in leisure horses suggest up to 3% of the population may be affected by tying up and other exercise-related muscle disorders,whilst in performance horses the incidence rises to between 5 and 7% of racehorses, 8% in polo ponies and up to 14% in eventers. The differences between studies of different disciplines probably reflect variability in study designs etc and it is probably fair to suggest that an incidence of around 10% in exercising horses is a reasonably accurate assessment based on research to date.

Why does it happen?

Horses affected have an underlying susceptibility that is triggered by other things. It is generally accepted that movement or exercise is the final trigger factor that results in the muscle seizing or cramping but there are other risk factors also involved including diet, sudden changes to work without adjusting the diet, electrolyte imbalances, infections and weather. Several of these may combine to create a “perfect storm”.

How are muscle problems defined?

Some horses have an inherent muscle defect which can either be a disorder relating to the contracting and relaxing of muscles which tends to be called RER, or a defect in how carbohydrates are stored and/or utilised in the muscles which is referred to as PSSM. The latter is further sub-divided into two groups – Type 1 and Type 2.

There appears to be a group of sufferers that don’t have an underlying muscle defect. Research suggests that inappropriate diets and management issues are most likely to be the cause of problems in this group.

More about the different disease types

Polysaccharide Storage Myopathy (PSSM)

There are two sub-groups of PSSM cases

PSSM1– caused by a genetic mutation found in more than 20 breeds of horses with the highest prevalence being in European draft horses. Quarter horses and Appaloosas are also affected by PSSM1. These horses don’t break down glycogen in their muscles as easily as normal horses due to a mutation of the enzyme Glycogen Synthase 1 (GYS1). This means they can have 1.5 – 5 times higher levels of muscle glycogen in their muscles compared to normal horses. This can make it very difficult and painful for them to move.

Whilst some PSSM1 horses are asymptomatic, diets high in non-structural carbohydrates (sugar and starch) can exacerbate clinical signs as these nutrients are stored and accumulate. Signs include apparent laziness, shifting lameness, tensed up abdomen, tremors in flank area and firm hard muscles over hindquarters. More chronic signs such as reluctance to move forward, muscle loss and lack of energy are seen in Draft Breeds and gait abnormalities can also be seen in Quarter Horses.

PSSM2 – refers to all PSSM cases that aren’t caused by genetic mutation GYS1 and so there may be more than one further subset identified in due course. PSSM2 is more prevalent in Warmbloods and is often detected via poor performance.

PSSM2 – The most common signs of PSSM2 are poor performance and a drop in energy levels after a short period of exercise. Unwillingness to perform can be highlighted by the horse’s reluctance to collect and engage hindquarters, as well as poor rounding over fences. Firm back and hindquarter muscles can also be seen in some cases.

Recurrent Exertional Rhabdomyolysis (RER)

RER is the term used to describe horses that have an abnormal process of muscle contraction. As the name suggests it commonly happens when horses are working at speed and so is most commonly seen in Thoroughbreds, Standardbreds and Arabs. It also tends to be associated with nervous, excitable or stressy individuals and in younger horses, fillies are more prone to the problem but this difference between the sexes seems to reduce with age. The trigger factors for RER include high cereal diets and being held back when training at speed.

If we use ERS to refer to those horses that don’t have a muscle abnormality,then the most commonly reported trigger factors include:

  • High starch diets and often too little fibre
  • Electrolyte imbalances or deficiencies
  • Low levels of selenium and/or vitamin E
  • Exercising after time off especially if starch intakes were high or not reduced

Feeding and Management Tips:

Much of the advice applies to any of the different disease types. The extent to which you may be willing or able to implement the advice is often determined by the type and level of work the horse is doing. It is important to note that research in Scandinavia has shown that Standardbreds could perform racing level exercise on a forage based ration without compromising performance.

  • Feed diets that are low in starch and high in fibre.
  • Use oil for additional energy or calories – Dengie Alfa A Oil, Performance Fibre, Alfa A Molasses Free and Healthy Tummy are all equivalent to conditioning/competition mixes in terms of their energy content but have much lower levels of starch
  • When concentrates are fed be sure to reduce the quantity fed when the horse is having time off, even if only for one day
  • Do not increase concentrates ahead of increased workload – wait until the work has been done
  • Ensure you are feeding a ‘balanced diet’. This can be provided in the form of a good quality vitamin and mineral supplement or balancer, such as Dengie Performance Vits & Mins or Dengie Performance Plus Balancer. If feeding less than the manufacturers recommended amount of mix or cubes, remember to ‘top up’ your vitamins and minerals with one of the above.
  • Feed salt daily and electrolytes when necessary
  • Ensure you warm up and cool down sufficiently
  • Try to keep work intensity and duration consistent when possible
  • Try to exercise daily and little and often if your time is limited – ie two or three sessions of 10 or 15 minutes.
  • Keep stress to a minimum
  • Allow regular turn out – this is especially important when the horse is not being exercised
  • If there is any sign of viral infection on the yard,then reduce workload immediately
  • Always seek veterinary advice if your horse has any signs of muscle problems