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.
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