Recurrent exertional rhabdomyolysis

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Recurrent Exertional Rhabdomyolysis

Chronic Exertional Rhabdomyolysis

Recurrent Exertional Rhabdomyolysis (RER) refers to a type of muscle disease that occurs in sport horses, that is actually a subset of recurring "tying-up" episodes. It is a clinical syndrome which occurs when horses exercise, resulting in painful muscle contractions and necrosis of the skeletal muscle fibers. RER occurs most commonly in young, skittish female Thoroughbred horses, especially racehorses. It is also seen in Arabians, Warmbloods, and Standardbreds. According to the University of Minnesota's Equine Genetics and Genomics Laboratory, 5-10% of all Thoroughbred racehorses suffer from RER during the racing season.

Horses with RER have a history of repeated signs of muscle stiffness during exercise. Episodes most frequently manifest once horses have achieved a certain level of fitness, when asked to hold back during a race, following a steeplechase race, or at the beginning of the last portion of the three-day-event, which is the cross-country phase of eventing.

RER is believed to be caused by an abnormality in intracellular calcium regulation that occasionally acts up while the horse is exercising. A genetic susceptibility to RER has been shown to exist as well, where affected horses pass the trait to 50% or more of their offspring.


Muscle stiffness
Rapid breathing
Reluctance to move for several hours
Short strides
Shifting hind limb lameness
Firm, painful hindquarter muscles
Elevated heart rate


  • History
  • Clinical signs
  • Physical exam
  • High serum levels of creatine kinase (CK) and aspartate aminotransferase (AST)
  • Urinalysis
  • Muscle biopsy

While waiting for your veterinarian

  • Do not move the horse until your vet arrives.



Diet managementModify diet to include less concentrated feed and more quality hay. Switch to a low sugar/ low starch feed. If needed, provide supplmental fat for increase of carbohydrate intake.
Exercise managementEnsure regular, consistent exercise without any days off and daily turnout.
Muscle relaxantsDantrolene and phenytoin
Pain reliving medicationBanamine or Bute
Tranquilizersto help control anxiety
FluidsProvided orally or intravenously


  • Diet - Feed horses a low sugar, low starch (NSC) grain with plenty of high quality hay. If needed, such as for performance horses, extra energy can be obtained by fat supplementation, in order to increase their carbohydrate intake.
  • Activity level - Ensure horses have daily turnout and consistent exercise.
  • Minimize stress and excitability - Turn out, feed, or exercising RER horses before other horses and providing pasture mates for comfort
  • Do not over supplement horses with more than the recommended amount of selenium in their diet

Scientific Research

General Overviews

Risk Factors

  • Genetics: Thoroughbred, Standardbred and Arabian horse breeds.
  • High-strung, female horses
  • Diet: Horses fed large amounts of sweet feed
  • Chronic lameness

Commonly Affected Breeds

Thoroughbred icon