Disclaimer: I am not a scientist studying these issues, I’m just a curious individual with enough of an equine science background to make some sense of the biology behind these topics. That, and they fascinate me. Check out this posts internal links if you want to read more into the science behind the tying up disorder.
Exertional rhabdomyolysis (ER) has been recognized in horses as a syndrome of muscle pain and cramping associated with exercise (in some form). This disorder is commonly known as “tying up” when the symptoms are evident. There are two general categories of ER: sporadic and chronic. Sporadic tying up is due to management issues including overwork, nutritional deficiencies, and other environmental triggers. Chronic ER arises from genetic abnormalities that are then triggered to cause episodes of tying up.
ER is being explored in Arabian and part-Arabian endurance horses right now, as much of the previous research has been done in other breeds and sports with differing demands on our equine partners. At the PNER Conference this year, Dr. McKenzie presented current research on the subject of tying up research in Arabian horses (and half Arabians). She is involved with the research herself and had a lot to say about the tying up disorder in general and the work determining the causes of tying up found in endurance horses. Generally, the research has focused on determining a cause of chronic ER in endurance horses, as the causes for sporadic ER are universal for horses (but still important to consider, of course).
I have never personally had a horse with any kind of tying up symptoms, but I’ve seen a good friend struggle with chronic tying up with their Quarter Horse. It can be devastating, particularly when you see a beloved horse in pain and can do very little to help them.
Tying up is the broad term for muscular breakdown and the symptoms that arise from that breakdown. Muscle damage releases potassium and myoglobin, which are then filtered through the kidneys. Since these byproducts are toxic, tying up can lead to kidney failure and death.
Symptomatically, a horse that is tying up will be stiff and sweaty, not want to move, and will display signs of acute discomfort.
What I found particularly interesting is that there is not a single cause for tying up. Different causes have taken on different names, but they are all related to the same kind of symptoms and results in the equine athlete. Research has already uncovered several routes to the disorder, and new studies point to even more possibilities that need further exploration.
One thing we do know with some certainly: tying up has a strong hereditary component, but the symptoms can also arise from overwork and other management issues that have already been mentioned.
The apparent causes of tying up in Quarter Horses and Thoroughbreds:
Most of the research into ER/tying up has been done in Thoroughbreds and Quarter horses. In Quarter Horses, chronic ER is usually due to a condition called polysaccharide storage myopathy (PSSM), while in Thoroughbreds it seems to be related to an abnormality in how their muscles contract.
In Thoroughbreds, where about 5-9% of the population has chronic ER, research has established that various stressors show a positive correlation with the disorder’s symptoms. The horse having a nervous temperament correlates to them being five times more likely to tie up. Having a high caloric intake (especially if it’s starchy intake), being lame, and having a period of rest in their recent history (even if it’s only two days) are other factors that trigger episodes of tying up. The chronic ER disorder in Thoroughbreds also appears to be genetically dominant and is therefore easily passed on to the offspring.
It’s suspected that chronic ER in Thoroughbreds is caused by a calcium regulation defect. Since one of the results of this abnormality (other than tying up symptoms) is that the horse can relax and contract their muscle faster than a normal horse, chronic ER in this breed has probably been selected for accidentally. Yes, it’s entirely possible that horses with ER may run faster!
Chronic ER affects around 9% of Quarter Horses and related breeds, like paints and appaloosas. In these types of horses tying up is caused by PSSM. PSSM is, in simplified terms, a muscle-glycogen storage disorder. 80% of affected Quarter Horses have a mutation of the Glycogen Synthase I gene (GYSI) which regulates this storage. The common complaint with PSSM is that symptoms of tying up appear when the horse is exercised after a period of rest – and it does not necessarily have to be hard exercise in these horses. (I think it’s interesting that around 36% Belgian draft horses have PSSM, as well, so it’s not just a disorder found in Quarter Horses and their related breeds.)
As with Thoroughbreds, PSSM is also genetically dominant. It was probably also selected for accidentally in these breeds, because it helps the horses gain weight quickly, and the trend in breeding Quarter Horses is for thick-bodied animals. Luckily, PSSM can now be tested for in breeding animals and hopefully avoided altogether.