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"Tying-Up", also known as "slow-onset rhabdomyolysis" usually occurs due
to an imbalance in the blood's calcium, magnesium, and chloride
concentration. Energy plays a part in this too... I'll get to that part
in a minute.
Anyway, when the horse sweats, he (or she) loses calcium, magnesium, and
chloride. A nasty chain of events can start at this point... when
enough chloride has been lost, a hypochloremic-induced alkalosis can
occur. This alkalosis binds calcium, reducing the plasma calcium
concentration even more. So, you have this sweating, exercising horse
who has a severely decreased concentration of calcium in his blood. If
on top of this, he is also stressed (too much excitement, trailer
anxiety, etc.), he will not be able to mobilize and use calcium from his
bones or absorb calcium from feed sitting in his intestine due to
increased corticosteriod levels produced by his body as a reaction to
the stress.
The horse's working muscles require energy to contract and RELAX. Dead
critters stiffen up because the muscle cells do not have the energy
needed to relax (rigor mortis). Now, with a decreased concentration of
calcium, magnesium, and available muscle energy, effective contraction
and relaxation is not going to occur as it should. The muscle fibers
may then produce the "tremors" you see in the horse who is "tying-up",
cramps, and a whole host of other symptoms which make you wonder if the
horse is really going to survive!
Now, how do high-calcium feeds play a part in the development of
"tying-up"? One might think that if the horse's plasma calcium levels
are too low, that he should be fed more calcium to "fill the blood" with
as much calcium as possible so that he doesn't get to such a low point
from sweat losses, binding, etc.
But what happens with high calcium intakes prior to the ride (fed high
calcium feeds for several weeks to his race date) is that the horse's
body "get's lazy". Since the feed is providing more than enough of all
of the calcium that the horse needs, there is a resulting decrease in
parathyroid hormone secretions, calcium absorbtion from the intestine,
and a reduced ability to pull calcium from the bones. In other words,
since the horse is getting all of the calcium he needs easily, his body
does not "condition" itself to extract calcium from the bones or from
the intestinal contents when he needs it the most (at a ride). As a
result, he is unable to replenish his calcium supply lost through sweat,
which messes up the muscles' contraction/relaxation duties, which
results in a horse that "ties-up".
A horse on a low calcium diet (which meets, but not significantly
exceeds his nutritional demands) has to utilize every bit of calcium he
can get from his feed. When ridden, his body will be more capable and
EFFECIENT at extracting calcium from the intestines and bones upon
demand.
But if you have a horse who just can't keep weight on without alfalfa (a
high calcium feed) and is eating his maximum safe allowance of fats/oils
and grain, what do you do??? Feed him alfalfa (or mixed with another
forage preferably) to keep his weight up, but remove the alfalfa 2 to 4
weeks before the next race and keep the calcium intake low enough to
just meet his nutritional needs. This should give him enough time to
"condition" his body to pull calcium when needed, and should reduce the
possibility of "tying-up". (Of course, we're keeping in mind to change
the ration slowing by incorporating some of the new ration with the old
;-> )
Since this is getting really long (my appologies to those who have to
pay for time with their service and/or long distance), anyone may e-mail
me for more info. or with questions. If I don't reply, try again... my
e-mail has not always been reliable. Disclaimer: I'm not a vet (yet...
working on it); my information is based strictly on research, other
vets, and sometimes my own experience (13 years, 270+ horses).
Hope this helps...
Kim ('Lee's mom)
aliakey@geocities.com
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