Tying up, or Exertional Rhabdomyolysis, in horses is characterized by muscle pain, stiffness, excessive sweating, and a reluctance to move associated with exercise. Once known as “Monday Morning Sickness”, it is not the simple muscle soreness a person might experience the day after running or lifting weights, especially if they hadn’t done so in a while.

Tying up in horses can be a serious, even life-threatening condition with multiple causes. This article will describe the two main types of tying up (sporadic and chronic), the underlying causes, the breeds of horse affected, and the signs and symptoms. The diagnosis and treatment, as well as specific diet and management practices in the hopes of preventing tying up, will also be covered.

Tying up has been recognized as a serious condition in horses for over a century. The name “Monday Morning Sickness” comes from the time when draft horses were worked six days a week and given Sunday off to rest in their stalls while fed their normal ration of grain. These horses often displayed signs of tying up when asked to work again on Monday morning. The trigger for an episode seemed to be the combination of returning to work or exercise after a period of enforced idleness while on a high-grain diet.

For many years, tying up was thought to be the result of the build-up of lactic acid in muscles. However, due to advances in exercise physiology and muscle diagnostic testing, it has now been shown that lactic acid build-up in horses does not cause tying up. In fact, veterinarians and scientists now know that tying up is a syndrome with two main types – sporadic and chronic -- and multiple causes within each type.

The classic signs of a horse experiencing an episode of tying up usually occur shortly after the beginning of exercise and include:

A horse exhibiting signs of tying up should be handled as an emergency. If the horse is currently under saddle or being exercised, any activity must immediately be stopped. Next, call the horse’s veterinarian and describe the situation. While waiting for the vet to arrive, make the horse more comfortable by removing tack, blanketing if it’s cold or providing shade if it’s hot, and holding up water for the horse to drink (he may not be able to lower his head to the ground). If possible, try not to move the horse. Instead, make his surroundings safe and quiet by removing other horses and activities. Follow any specific instructions from the vet such as administering medications, taking vital signs, etc. Do not offer any hay or grain at this time.

When the veterinarian arrives, they will assess the horse, take steps to relieve pain and anxiety, and possibly administer fluids if dehydration is an issue. With vet guidance, the horse may be put into a stall or other confined area since the horse should move as little as possible for the next 24 - 48 hours. Blood samples may be drawn immediately as well as later in the episode to confirm the diagnosis. Looking at levels of two indicators of muscle damage, creatine kinase (CK), which peaks 6-12 hours after an episode, and aspartate transaminase (AST), which peaks 24 - 48 hours after an episode may give a better understanding of the episode.

If this is the first time this horse has experienced a bout of tying up, the veterinarian may ask questions to try and figure out what triggered the episode. The diagnosis may be confirmed as “sporadic tying up” and therefore recommendations for the horse may include:

If the horse has had several episodes of tying up in the past, then the veterinarian may lean toward “chronic tying up” as a diagnosis. In this case, they may recommend additional diagnostic testing based on the horse’s history, age, breed, gender and other factors. Some of these extra tests include genetic assays (test) of blood or hair, muscle biopsy, exercise challenge, and more. It is important to get to the root cause of chronic tying up as the treatment and management for one type -- for example, RER -- is different from the treatment and management for other types, such as PSSM1 or PSSM2.

Horses prone to chronic tying up due to RER seem to do better when the sugars and starches in the diet are limited. This may be done by making high-quality forage the foundation of the diet; rounding out the protein, vitamins, and minerals with a low-NSC grain or ration balancer (not sweet feed) and supplementing with fat if the horse needs more calories. Sources of fat include stabilized rice bran, commercial feeds specifically made for these horses that are low in sugar and high in fat, and powdered fat or oil supplements. When adding fat to the diet of a horse with RER, here are three things to consider:

Start gradually, giving the horse’s digestive system time to adjust (helping to minimize the risk of loose stool)

Avoid fats and oils high in omega 6 fatty acids such as corn and sunflower oil

Provide additional vitamin E to offset that which is needed to help metabolize the added fat

Finally, make sure the horse is getting at least 10 grams of sodium daily by topdressing about 2 tablespoons of table salt with regular meals. When the horse is sweating heavily from intense exercise or hot temperatures, supplementing with additional sodium and/or a commercial electrolyte supplying potassium, calcium, and magnesium in addition to sodium and chloride may also be appropriate.

For many RER horses, the combination of stress and exercise may lead to an episode of tying up. The following suggestions from the Michigan State University College of Veterinary Medicine specifically target the Thoroughbred racehorse lifestyle but may be used to help avoid triggering an episode of tying up in any horse prone to recurrent exertional rhabdomyolysis:

Tying up in horses is not one single disease but a syndrome of muscle disorders affecting many breeds, ages, and disciplines of horses. In some cases, it is an inherited condition that may be diagnosed with a genetic test while in other cases, the underlying cause, method of diagnosis, and even the best ways to treat and prevent it from happening again are unclear. It is important for riders to understand there are different forms of tying up and there are different signs and symptoms associated with each form. Knowing when to call the veterinarian when there is an emergency as well as when a horse is just not performing as expected is key to ensuring your horse’s health.