Page 64 - 2018 Barrel Stallion Register

Initially, research focused on finding one genetic marker for RER. “We haven’t been
able to find one specific genetic disorder in Thoroughbreds. In Quarter Horses one muta- tion has been found for PSSM 1 and another mutation causing malignant hyperthermia. But in Thoroughbreds, one genetic mutation has not been identified with genome mapping; we think RER is due to a number of factors interacting together to increase susceptibility to tying up. One factor seems to be susceptibility to stress,” she explains.
“When you take those same horses and turn them out or train in a less stressful environment, they seem to do well. But when you move them to a higher stress environment like a training facility, feed more carbohydrates (which makes them hyper and more easily stressed), and then have intermittent exercise without the ability to blow off a lot of steam, susceptible horses tend to develop RER,” says Valberg.
“Years ago when people were breeding fast horses, they tried to combine this amazing athletic ability (which might predispose them to be more easily inclined to tying up) with a more nervous, aggressive, hyper temperament (rather than mellow and laid-back) because that combination often produced more speed. One thing that inadvertently came along with that combination was higher susceptibility to tying up,” she explains.
“What we think is going wrong in the muscle in these situations is an abnormality inside the cell--within the small compartments where cal- cium is shuttling back and forth,” says Valberg. This is not a defect in terms of dietary calcium
and has nothing to do with blood calcium con- centration, but rather a glitch in the movement
of calcium back and forth inside those compart- ments in muscle cells. When too much calcium is released into one of the compartments where the contractile proteins are, the muscle contracts, but doesn’t relax. This eventually sets off a process that damages the muscle cell.
Calcium is stored in membranous sites within the cell. “When released, it interacts with the proteins to make the cell contract, and then it has to get pumped back into storage sites so the muscle can relax. This happens many times a second when the horse is moving. As the muscles contract and relax, the calcium is moving in and out of these storage sites. We think this is where the abnormality lies in these horses--when the muscle cell is moving calcium back and forth,” she says. This becomes a vicious cycle.
“We don’t have any one thing we can do
to prevent the tying up, but we try to break
the cycle—doing all the things we can for that individual horse to decrease the amount of stress. Often, if those horses are taken off the track and used for other purposes, they don’t have another episode of tying up,” she says. The lifestyle change helps. A new career like barrel racing, however, might be just as stressful as racing; the horse might still have tying-up episodes.
Management can help many of these horses. “Try to limit the amount of time they are standing still. We found in treadmill exercise trials, that susceptible horses tend to have more problems with tying up if they’ve been standing in a stall for 2 days and then are taken out for exercise. It’s better to get them out every day and
not have days off. Try to provide opportunities throughout the day to get them out of the stall, with as much time as they need to blow off steam.” All these things make a difference. Let them jog if they want on their way to and from training exercises or competitions when they
are hyper, instead of insisting that they stop the nervous behavior, creating a fight with the horse.
Diet changes also help. “These horses get very excited on high grain diets. They tend to be nervous horses, so they burn calories easily and it’s hard to keep weight on them. If you substitute fat for part of the grain, it helps take the nervous edge off and they are calmer,” Valberg says.
“In experimental trials, we found that if horses were fed less than 5 pounds of sweet feed, the grain didn’t seem to have an influ- ence on whether or not they tied up. But when these horses were fed up to 10 pounds or more of sweet feed to keep weight on and keep them going with enough energy for performance, the ones that were susceptible to RER started to have intermittent episodes of tying up. Some horses become ‘hotter’ and more excitable on high starch feeds. Providing the same amount of calories with feeds that contain less starch, and adding supplemental fat, can be helpful in some horses,” she says.
Genetics can be more complicated than our minds can comprehend. Good science is needed to ensure that tests are accurate. Know what you are testing for, and know the information that backs up the test. People want it simple and want answers quickly, and it’s not always that easy.
Genetics can be more complicated than our minds can comprehend. Good science is needed to ensure that tests are accurate.
More Research Needed
There is still a lot we don’t know about some forms of tying up, especially recurrent exertional rhabdomyolysis (RER). “We have a study looking at why horses are susceptible to RER. We’re also hoping to get funding for another study to figure out what is altered in the muscles of these horses. If we can pinpoint how stress is interacting with this disease, we might come up with better treatments and management strategies,” says Valberg.
The genetic factor requires more research because it’s not as simple as people had earlier assumed. “It does not appear to be one particular gene, with a horse getting one or two copies of a mutation in that gene, like happens with some of the other genetic diseases. There may be several genes that contribute a portion of the genetic susceptibility, but there has not been a genetic test identified to be associated with this, even though a lot of work has gone into it. Some large-scale studies looked at a lot of horses, but the interaction of diet, temperament and environment play such
a big role that it is difficult to tease out the genetic
factors.” Genetics is just part of the picture and in some situations a horse might never exhibit the problem, whereas in other situations it would.
“It’s easy now to sequence a horse’s genome. If you compare the genome of one horse to another, you can find 17 million variations—usually sim- ple, minor variations in the DNA code that may have no consequences, and some variations that may translate to a different protein sequence. It’s easy to find genetic variations from horse to horse, but takes an enormous amount of work to figure out if one of 17 million variations in sequence is consistently associated with the disease. In all the work we do, we try to show (if we find a variation) that it is strongly statistically associated with a well- defined specific disease,” says Valberg. The results get published so they can be verified by others.
“There are some commercial interests offer-
ing genetic tests for one or the other of those 17 million variations--without published research to identify the gene and the variant and clearly show a strong relationship with the disease.”
62 SPEEDHORSE
EQUINE HEALTH