Dietary and management changes may help, but tying up might be hereditary

Traditionally, episodes of muscle pain and cramping after exercise were lumped together under the heading "Monday morning disease," or azoturia, with no particular distinction between their causes. But during the past 20 years, research has identified several forms and causes of what now is called exertional rhabdomyolysis, or tying up.

According to Stephanie Valberg, D.V.M., Ph.D., professor and the director of the Equine Center at the University of Minnesota, an early theory blamed the problem on too much lactic acid accumulation in muscles, but she said muscle biopsies of horses taken during tying up episodes showed this was not true. Valberg's research has found two main categories of tying up: sporadic and chronic. Nearly any horse can experience occasional tying up if muscles are stressed beyond fitness level, such as going on a long ride, a strenuous workout, or extreme exertion on a hot, humid day. Sporadic tying up also may occur if a horse exercises while suffering from influenza.

Horses that exhibit frequent episodes of tying up are considered to have chronic exertional rhabdomyolysis.

Chronic tying up can be broken down into two categories: polysaccharide storage myopathy (PSSM), which affects heavily muscled horses (Quarter Horses, warmbloods, and draft horses); and recurrent exertional rhabdomyolysis (RER), which occurs in fit Thoroughbred racehorses and, more commonly, fillies.

Horses with PSSM accumulate an abnormal amount of glycogen in the muscles and cannot regulate energy metabolism properly within the muscles. These horses may be doing mild exercise (walking and trotting) when they tie up, and they show signs within the first 20 minutes of exercise. PSSM can be managed with careful diet and exercise to get the horses gradually fit, Valberg said.

Tying up in Thoroughbreds usually is caused by excitement and involves a problem with how muscles contract.

"These horses usually don't start tying up until they are quite fit and in race training," said Valberg. "They tend to be two- and three-year-olds. It's usually the more nervous ones that are affected, with a high percentage of them being fillies."

Amy Gill, Ph.D., an equine nutritionist in Lexington, works with horsemen to design diets to help prevent or minimize episodes of tying up.

"When these horses tie up, stress is usually a factor," Gill said. "When they go out on the track to exercise, something may excite them or an unusual situation upsets them. When a horse gets loose at the track, for instance, someone sets off an alarm so exercise riders and track personnel will know there's a loose horse--and the alarm may precipitate a tying-up episode."

Excitability sets off various hormones. Cortisol and adrenaline levels go up; the animal is ready for fight or flight.

Calcium malfunction and diet

"One factor involved in muscle contracting and relaxing is movement of calcium within the cell," Valberg said.

To make the muscle contract, calcium is released from its storage site in the cell to interact with proteins, then it is pumped back into the storage site to relax the muscle. This happens many times each second when the horse is moving. "We think this is where the abnormality lies--when the muscle cell is moving the calcium back and forth," Valberg said.

The amount of calcium in muscle is not influenced by dietary calcium, so the problem cannot be controlled by altering the calcium level in the diet. It is, however, influenced by many factors and hormones within the muscle tissue.

"We think part of the reason why mares are more predisposed to the problem is because they are so much more easily stressed and upset, particularly when they are cycling [in estrus]," explained Valberg.

Minimizing carbohydrates in the diet can help the excitable horse. Nervous horses burn calories easily; it's hard to keep weight on them," Valberg said. "Yet, they get very excited on high-grain diets, and that much grain predisposes them to tying up. We've found that if you can substitute fat for part of the grain, it helps take the nervous edge off."

In working with horses during treadmill studies, Valberg found she could minimize episodes of tying up by placing them on a high-fat, low-starch diet. "That's one reason we created RE-LEVE," she said, referring to a special high-fat feed that provides adequate energy for athletic horses without containing an excess of carbohydrates.

"These Thoroughbreds need a specialized high-fat, low-starch feed to get enough calories into them, and very palatable feeds, because these horses become very finicky when they need to eat that much."

Gill said that when dietary levels of starch and sugar are reduced, it makes a difference.

"Many horses are intolerant of high blood levels of glucose," Gill said. "This increases excitability even in a horse that's not tying up. Horses, like people, can be very sensitive to sugar swings. There are many hormonal fluctuations that occur right after you feed a grain meal."

According to Gill, up to 20% of a horse's diet can be fat, though it generally does not need to be that high. "This includes forage and concentrates. Now that we're using rations that are high in fat and soluble fiber, most diets have about 10% to 15% [of total calories] coming from fat, and it can be supplied as vegetable oils, rice bran, etc."

Although the feed industry is developing low-starch, high-fat feeds for performance horses, Valberg warned that the starch content often is not listed on the label.

"There are many high-fat feeds available that are also high in starch, and those won't work for these horses," Valberg said. Carbohydrates are listed as NSC (nonstructural carbohydrates) on feed labels.

"We'd like to see the NSC level in the feed no higher than 15% for these horses," said Valberg. To prevent or minimize episodes of tying up, she suggested feeding a balanced diet that is 10% to 15% fat, less than 15% starch, and provides electrolytes, vitamin E, selenium, and minerals.

Feeding fat

Extra calories needed by a hardworking horse can be added to its feed as corn oil, soybean oil, wheat germ oil, or rice bran oil.

"Stay away from animal fats; most horses won't eat them and they are not as digestible in the horse," Gill said.

Commercial feeds formulated for horses that tie up are available if you prefer not to add fat to the horse's grain--Hallway Feeds's RE-LEVE and Agway's Respond. Nutrena manufactures XTN, a feed supplement that is a high-fat, high-soluble-fiber feed with a little bit of oats in it.

"Though XTN is about 25% starch, this supplement is only a portion of the diet; you're trying to get the total diet below 15% starch," explained Gill.

Some products formulated for horses that tie up contain rice bran, beet pulp, or alfalfa meal, but Gill cautioned, "Feed companies often add molasses to beet pulp, but you want to avoid simple sugars. Alfalfa meal is okay, but stay away from straight alfalfa because it's a little higher in sugar."

If the horse needs more calories, Gill advises feeding highly digestible, soluble fiber, such as soybean hulls, which are beneficial for the microbes in the gut, pick up water as they pass through the digestive tract, and do not cause the problems associated with a high-starch diet.

Medications that help

Valberg's research has defined medications that might be helpful. One of these, called Dantrium or Dantrolene, slows the release of calcium from storage sites in the muscle cells.

"We think that tying up occurs when there is too much calcium released all at once. Dantrium will slow that down and smooth it out," Valberg said.

There are important things trainers should know when using Dantrium.

"If you give it to horses that have been eating hay all night, they don't absorb it; Dantrium gets bound up in the feed," Valberg said. "It works best when given to horses before they eat."

Dantrium must be given on an empty stomach (after at least a three-hour fast) and about 90 minutes before exercise for it to work. Ideally, the horse should be trained early in the morning and before it is fed breakfast.

Unfortunately, horses are not permitted to race while receiving Dantrium, but it is helpful to use during training to calm a horse first introduced to the racetrack environment.

Management issues

Anything you can do to keep horses relaxed and minimize excitement can help prevent episodes of tying up. Place the horse in a stall next to horses it gets along with and in a quiet area where other horses are not traveling back and forth. Exercise this horse first so it does not become anxious while awaiting its turn to go out. If exercised with other horses, choose calm ones.

Valberg advised limiting the amount of time these horses are idle.

"In treadmill exercise trials, we found that horses tend to have more problems with tying up if they've been standing in a stall for two days before you take them out and exercise them," Valberg said. "It's better to get them out every day and not give them days off. Provide opportunities throughout the day to get them out of the stall."

If a horse has a really severe problem with tying up, take it away from the track and train at home in a different environment where it can spend more time out of the stall.

"If you can get them to start racing, we've found this doesn't affect their racing performance at all--if you can get them past the tying up episodes when starting out," Valberg said. "Much of getting past it is just getting them calmed down and used to being in training and at the track."

An inherited condition

Valberg has done several studies to find out why some horses tie up. She thinks they inherit an abnormality that makes excitement trigger the episodes.

"There are certain lines of Thoroughbreds that are more predisposed to this than others," Valberg said. "In the horses that have chronic problems, the regulation of muscle contraction and relaxation is abnormal.

"We've seen mares that tie up, and maybe they've had six foals and they all tie up. What that indicates to us is that in some horses, both genes are affected [inherited from both parents]. These individuals may be severely affected, and when you breed them, all their offspring will get one of these genes and be predisposed to tying up.

"Every once in a while you find a filly that's extraordinarily difficult to train," Valberg said. "You do everything you can, and she still ties up. Often these fillies are taken out of training and retired to breeding, and [they] produce offspring that have problems with tying up.

Valberg's research team is looking at the genetics of tying up in Thoroughbreds, and she is studying families of horses in which it occurs.

"If someone has closely related horses affected with this condition, we'd be interested in hearing from them," she said.

Call Valberg at (612) 625-5768 or e-mail her at valbe001@umn.edu.