Page 61 - 2018 Barrel Stallion Register

aminotransferase) were checked 3 or 4 hours after the cramping, they would likely be normal. Folks at the track, for instance, will tell you their horse tied up and then they put it on the walker and the cramping goes away. In those circum- stances, the problem might be more related to muscle cramping and pain and not necessarily muscle damage,” says Valberg.
Genetic Muscle DisorDers
“On the other hand, horses that have exertional rhabdomyolysis, with significant elevations of CK and AST in their blood, get worse if you continue to exercise them. These two different things are easily confused—and the best way to distinguish them is to measure muscle enzymes CK and AST. These enzymes usually peak in the blood about 4 hours after an episode, so that’s the best time to check them to see if there has actually been muscle damage.” This narrows down what you are looking at.
“With classic tying up, the horses intermit- tently have peaks and elevations of CK that indicate muscle damage. There are some differ- ences between the Quarter Horses with PSSM that tie up repeatedly and Thoroughbreds that tie up repeatedly. One big difference is that Thoroughbreds usually don’t start tying up until they are in race training. They tend to be 2 and 3 year olds, as young horses have more problems than mature ones. It’s usually the more nervous individuals that are affected, and usually fillies, particularly if they have a nervous disposition,” Valberg says. This tying up is called recurrent exertional rhabdomyolysis (RER) and is often associated with excitement.
PolysacchariDe storaGe MyoPathy (PssM)
When she began her research on tying up, Valberg realized it wasn’t just one syndrome, so she used muscle biopsies, examining them under a microscope, and biochemical measurements to try to understand the differences. Donated horses from different breeds that had issues with tying up provided a research herd. High-speed tread- mills were used in these studies. Inherited muscle abnormalities were eventually broken down into two categories—polysaccharide storage myopathy (PSSM), often found in Quarter Horses, warm- bloods and draft horses, and recurrent exertional rhabdomyolysis (RER) in Thoroughbreds, Standardbreds and some Arabians.
Horses with PSSM accumulate an abnormal amount of sugar in the muscles that can be seen in muscle biopsies. “Many of these horses tie
up when they are not getting regular exercise. They develop this problem early in life, are very sensitive to starch and sugar, and don’t seem to be able to regulate energy metabolism properly in the muscles,” says Valberg.
Horses with PSSM (Polysaccharide Storage Myopathy) accumulate an abnormal amount of sugar in the muscles that can be seen in muscle biopsies. On the left is a biopsy from a horse with normal muscle tissue. On the right is a biopsy from a horse with PSSM type 1 showing abnormal amounts of sugar in the tissue.
“We’ve gained a lot more information in the past 15 years,” she continued. “In the past, the only way to make a diagnosis was with a biopsy to look at accumulation of sugar under the microscope. Now we know there’s more than one form of PSSM and classify them as type 1 and type 2. Type 1 affects more than 20 breeds and accounts for more than 80% of PSSM cases in some breeds, and is probably the classic azo- turia, or Monday morning disease, that people talked about in draft horses. We found a genetic mutation that causes this in the primary gene responsible for making the protein that makes glycogen. In horses with type 1 PSSM, the gene product glycogen synthase always seems turned on. No matter how much these horses eat, their muscles are always telling the body they need to keep making glycogen,” she says.
“They accumulate sugar in the muscles, but because the muscle thinks it always has to keep making it, it has a harder time accessing the glycogen when exercise begins. So when you start exercising the horse, when muscles are sup- posed to be able to access glycogen (to provide energy), they are unable to do that, and tie up,” she explains.
“If these horses were kept in more natural conditions they would do better—as they were in the wild where feed was not always abun- dant and they were always eating and traveling around to find food. They were able to access glycogen in the muscles because they didn’t have so much insulin being released from high starch feeds. Working muscles train themselves to access glycogen,” she says.
In earlier years, horsemen may have inad- vertently selected for horses with the genetic mutation and PSSM because they needed horses with high energy efficiency. This trait may
go hand in hand with being an easy keeper or having a calm, mellow temperament. There are some desirable things about these horses that led breeders to select for this type of animal.
“We looked at type 1 PSSM in European breeds and found the continental breeds that
were used as the foundation of some of our mod- ern breeds, like the Belgian draft horse, have an extremely high prevalence of PSSM. It was as high as 80% in some of the early European draft breeds,” says Valberg.
The Belgian is one of the oldest breeds, related to big horses of the Middle Ages. “We found that this genetic mutation probably started about 1200 years ago, about the time people were breeding the Great Horse, forerun- ner of several draft horse breeds, to carry knights in armor,” she explains.
“The Great Horse is probably where the mutation arose and it might have made these horses a little hardier (easy keepers). This may have been an advantage then, but now, when many horses are kept in stalls and don’t get much exercise, and getting more starch in their diet, this trait manifests as a disease,” explains Valberg.
“So, we manage PSSM type 1 horses with a low starch diet. This keeps insulin levels low and doesn’t stimulate the enzyme to keep pushing the muscles toward making glycogen. We tell people to keep these horses outside as much as
Horses with PSSM type 1 should be fed a low starch diet and should be outside as much as possible with constant low levels of exercise rather than sporadic exercise. Low level exercise trains the muscles to access the glycogen and use it for fuel, and trains the muscles to burn fat.
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equine health