Page 63 - 2018 Barrel Stallion Register

Malignant Hyperthermia (MH)
Malignant Hyperthermia (MH) is a very serious genetic disease in a small per- centage of Quarter Horses and can prove fatal if the horse undergoes anesthesia. This same problem has been identified in people, dogs and pigs. “It is due to a muta- tion in a gene that regulates a protein that releases calcium from storage sites within the muscle cell. If contractile filaments
in the muscle—the ones that enable the muscle to contract—are exposed to exces- sive amounts of calcium, the muscles stay in contraction and metabolism is greatly increased. This causes the body to become hot and muscles become rigid. The muscle damage is very severe,” she explains.
Things that trigger malignant hyper- thermia include anesthetics like halothane gas. For many years, horses occasionally showed severe reactions to anesthesia, but nobody knew it was due to this genetic mutation. “Then, Dr. Monica Aleman at UC-Davis had 2 Quarter Horses she was working with in an experimental situation that both developed a severe anesthetic reaction. She was able to identify the genetic mutation,” says Valberg.
“After Dr. Aleman’s discovery, we screened a large number of horses to try to see how common it is. We found it’s very uncommon--only in a small percentage of Quarter Horses (less than 1%) and Paints-- but its impact can be devastating,” she says.
“When we looked at all our muscle biopsies, we found some Quarter Horses that have both the type 1 PSSM mutation and the malignant hyperthermia (MH) mutation. This makes symptoms much worse when they tie up. We started looking at this because some owners told us their horses not only tied up following light exercise, but some horses died suddenly. This is not common with PSSM. When we l lo o o ok k e e d d a a t t t t h he e s s e e h ho or r s s e e s s w w e e f f o ou u n nd d t t h he e y y h h a a d d both mutations.”
Early signs of HYPP include muscle tremors and occasional prolapse of the third eyelid. As the condition progresses, the horse will collapse will not be able to get up, but may be able to rise after several hours. In this photo, the horse is lifted
by a hoist during a tying up episode where he is unable to stand on his own.
even if it’s inherited from only one parent. This is different from HERDA (Hereditary Equine Regional Dermal Asthenia) or GBED (Glycogen Branching Enzyme Deficiency), which are reces- sive—which means a horse must have two copies (one from each parent) to have the disease. The risk of affected offspring from a horse with PSSM is much higher.
“Our current recommendation, if a horse has PSSM, is to not breed it because of the high chance of passing it on since these horses can be a problem to manage,” she says. Most can be managed to minimize problems, but it takes a serious commitment, and not all horses can be adequately helped.
“There are horse owners who disagree with this,” says Valberg. “They discovered that their horse had PSSM through genetic testing, but the horse has never shown symptoms. This is possible in horses that get plenty of exercise, have large turn-out areas and are fed very little or no grain,” A horse may also never show symptoms just because a genetic mutation with a detrimental affect might not be expressed because of effects from all the other genes in the body. The impact of one genetic mutation is dependent upon characteristics of other genes on the body.
“Our research has shown that about 75%
of horses with PSSM type 1 respond com- pletely if you put them on the diet and exercise recommendations--turned out as much as pos- sible with an exercise schedule that is gradually increased--and limited access to lush pasture. A grazing muzzle can be used when grass is lush, since there is too much sugar in rapidly-growing grass,” she says.
One study she was involved with looked
at the type of hay that should be fed to these horses. “We found that Quarter Horses are par- ticularly sensitive to hay that is high in sugar,” says Valberg. Any hay fed to a horse with PSSM should have low sugar content, measured as NSC (non-structural carbohydrates).
“We usually recommend that the NSC content of hay be no higher than 12%. There are other alternatives if a horse needs more energy, such as complete feeds made specifically for horses for PSSM with low starch, high fiber, high fat and balanced for vitamins, minerals and protein,” she says.
Diet management must be combined with exercise. “Feeding fat without exercise won’t resolve the problem. We’ve done a trial in which we left horses in a stall and fed them fat, and it doesn’t improve their exercise tolerance at all. They must do the exercise in order to utilize it in the muscles.”
If the owner has no options for turnout and the horse lives in a stall, a daily exercise regime can help. A walk-trot for 15 or 20 minutes a day makes a big difference for these horses. Vitamin E is also important for healthy muscle function. “We’re seeing more horses that are low in vitamin E because they have less access to green grass. Hay may not be high in vitamin E, especially if it has been stored a long time,” she says.
RECURRENT EXTERNAL RHABDOMYOLYSIS (RER)
Recurrent exertional rhabdomyolysis (RER) occurs more often in Thoroughbreds, and Quarter Horses that have a lot of Thoroughbred breeding, than in more heavily-muscled horses. “There are strong links with that particular disorder with horses kept in stressful environ- ments. These horses, when they get out and start to exercise, are hyper and easily stressed. They become nervous and excited and tie up. At the race track, episodes of RER most commonly occur when these horses are just galloping, rather than racing or breezing, because they are being held back instead of allowed to run full out. They are upset at being held back--fighting the rider. If they want to jog coming back to the barn and you try to make them walk, they may tie up. Or if something happens that gets them really excited, they are likely to have problems,” says Valberg.
SPEEDHORSE 61
EQUINE HEALTH