WRANGLING ON THE RANGE #124
FROM HORSE AND RIDER - JULY 2011
MUSCLE MALADIES
BY BARB CRABBE, DVM
HAS YOUR HORSE EVER "TIED UP" - NOT WITH A ROPE attached to his
halter, but with his large muscles so painfully locked that he
stops dead in his tracks, sweats profusely, and refuses to move?
If so, you know just how frightening an episode can be. Even the
veterinary treatment can be scary to witness, as it often
involves insertion of a long tube through the horse's nostril,
down to his stomach, in order to pump critical fluids directly
into his system.
The tying-up phenomenon isn't news. Horsemen have been plagued by
it since horse-and-buggy days. What is news is that researchers
have found a genetic component associated with tying up, and
continue to work hard to learn more, with new information
becoming available regularly.
In this article, I'm going to tell you what they've learned. I'll
explain about the different underlying muscle disorders that can
cause a horse to have repeated tie-up episodes, including
polysaccharide storage myopathy (PSSM) and recurrent exertional
rhabdonnyolysis (RER). I'll tell you what they are, how to tell
if your horse is affected, and what you can do about it if he is.
With this information in hand, you can minimize the chances that
you'll ever see another tie-up episode - and if you do, you'll
know how to protect your horse against potentially damaging
long-term effects.
MALADY: PSSM
What it is: Polysaccharide storage myopathy, a heritable disease,
is a result of abnormal storage of polysaccharide (sugars) within
the muscles. It's been suggested that there may be two types of
this disease.
The first, most stringent diagnosis can be made when there's an
increase in an abnormal form of polysaccharide that's resistant
to breakdown by the enzyme called amylase (amylaseresistant
polysaccharide), in addition to excessive amounts of
amylase-sensitive glycogen - a normal sugar that can be found in
muscles of normal horses. The muscles of horses with PSSM are
more sensitive to insulin - the hormone released to help with
sugar metabolismmeaning more sugar gets transported from the
bloodstream into the muscles after a high-carbohydrate meal, and
more will be stored as glycogen.
In an exciting new breakthrough, a specific gene that affects the
enzyme responsible for glycogen synthesis has been identified in
horses diagnosed with Type I PSSM. A genetic test for the gene's
presence is available.
A second type of PSSM has been described only where there's
excessive accumulation of amylase-sensitive glycogen, without the
abnormal amylaseresistant polysaccharide playing a role. Type 2
PSSM is also believed to be inherited, although a specific gene
has yet to be identified.
Who's at risk?
Type 1 PSSM is inherited as a dominant gene. This means that any
horse carrying the gene is likely to show clinical signs. Type 1
PSSM is seen most commonly in Quarter Horses, and breeds that
employ Quarter Horses as outcrosses, as well as draft horses and
warmbloods. If your horse is one of these breeds and has
experienced an episode of tying-up, he could be at risk. Other
signs of the disease include general muscle stiffness and
soreness, difficulty backing up, and visible muscle atrophy. Type
2 PSSM is also most common in the breeds named above, although it
can be seen in a wide variety of other breeds.
How is it diagnosed?
With the identification of its causative gene, Type 1 PSSM is
easy to diagnose with a genetic test. A blood or hair sample
submitted to the testing laboratory will confirm a diagnosis in
80 percent of affected horses. If the genetic test comes up
negative, a muscle biopsy is the next step to confirming a
diagnosis, and can be useful for determining whether your horse
has Type 2 PSSM or other muscle abnormalities.
Although muscle biopsy may sound scary, it's really a fairly
simple procedure. With your horse sedated, your vet will make an
incision just next to your horse's tail. He or she needs to
harvest a sample of muscle that's about the size of your little
finger, which requires an incision several inches in length. The
scars are barely visible once they've healed, and complications
with the incision site are rare. Most horses can continue their
normal activities immediately after the procedure. The muscle
sample will be cut into tiny sections and treated with special
stains that look for abnormal storage of glycogen as well as
amylase-resistant polysaccharide within the cells.
For genetic testing:
For detailed information on genetic testing, or to submit
samples, contact the University of Minnesota Equine Center,
Veterinary Diagnostic Laboratory, (800) 605-8787.
MALADY: RER
What it is:
If your horse has tied up SEVERAL TIMES BUT DOES NOT FIT YJR
PICTURE OF PSSM, it's possible he's effected with a different
underlying muscle disorder known as recurrent extertional
rhabdomyolysis (RER). This muscle disease is due to an
abnormality in calcium regulation within your horse's cells.
Unlike PSSM, it has nothing to do with polysaccharide storage.
Typically horses with RER will tie up when they become overly
exited.
***IF YOUR HORSE TIES UP
If your horse shows signs of tying up, it's important that you
know what to do. A severe tie-up can lead to muscle breakdown,
and release of a substance called myogtobin from the muscles into
the blood stream. Myogtobin is passed out in the urine, and can
be very damaging to your horse's kidneys, especially if he's at
all dehydrated. If your horse becomes suddenly stiff and
reluctant to move, pay attention to the following dos and don'ts:
* Don't make him move. His muscles need to rest and relax in
order to stop cramping, and further movement can make matters
worse.
* Do cover him with a cooler or blanket if it's cold outside.
Warming the muscles will help them to relax.
* Do call your vet. Many tie-ups will require veterinary care. In
fact, we prefer to see any horse that ties up, in order to
determine whether it's best to administer fluids to correct
dehydration, flush out muscle breakdown products, and protect the
kidneys. It's also a good idea to have your vet collect a blood
sample to detect enzymes that wilt confirm the diagnosis, and
help determine the severity.
* Don't administer bute, banamine, or any other non-steroidal
anti-inflammatory medications unless instructed to do so
by your veterinarian. These medications can be hard on your
horse's kidneys, particularly if he's dehydrated. And if the
tie-up is severe enough to involve myoglobin release, these two
factors combined can easily result in kidney failure-a potential
life-threatening complication.
* Do offer water or soaked hay if your horse is interested in
eating. Any fluids you can encourage him to take into his system
can be beneficial.
* Do administer a dose of acepromazine if you have one available.
This medication will relax your horse, help relieve his anxiety,
and also help dilate his blood vessels for improved blood flow to
the cramped muscles.
* Don't put him right back to work the following day, even if he
seems just fine. Small paddock turnout for several days is best,
with a careful schedule for a return to work. If you're not sure
what to do, ask your vet for help with a plan. He or she may
suggest rechecking your horse's blood for muscle enzymes, and
avoiding serious work until AST values return to normal-which
should take approximately two weeks. Be aware that it typically
takes between four and eight weeks for muscle tissue to repair,
even longer if damage is severe.
HIGH-FAT FEED OPTIONS
Here's a list of high-fat feed options that may help increase the
fat content in the diet of a horse prone to tying up.
* Re-Leve' by Hallway Feeds, re-leve.com.
* Ultium by Purina Mills, purinamills.com.
* Empower" by Nutrena, nutrenaworld.com.
* Ultimate Finish' by Buckeye Nutrition, buckeyenutrition.com.
* MOORGLO by ADM Alliance Nutrition, grostrong.com.***
BACK TO RER
Who is at risk?
RER is also believed to be inherited as a dominant trait in
Throughbreds, Standardbreds, and Arabians, although as of yet, no
gene has been itentified. If inheritance is indeed through a
dominant gene, then crossbred offspring of the affected breeds
(Half Arabians, for instance) can be expected to be effected as
well. Horses with more excitable temperament appear to be more at
risk, and fillies are more commonly affected than colts or
geldings. Younger horses with the contition are more likely to
tie up than their more mature, possibly calmer, barnmates. If you
have a high-strung 2-year-old filly of an above-named breed, with
history of tying up, it might be wise to consider RER.
How it is diagnosed?
Although researchers are working hard, no genetic test is yet
available. Muscle biopsies taken from horses with RER show subtle
abnormality where the nuclei of the muscle cells are more
centrally located than in normal horses.
Research request:
If you have a family of related horses with RER, Dr.Molly McCue
at the University of Minnesota would be interested in hearing
from you in her ongoing efforts to identify the genetic link.
Contact her at mccu0173@umn.edu.
MANAGEMENT TIPS
Whether your horse is diagnosed with PSSM, RER, or simply tied up
a time or two, there are things you can do to help prevent future
tie-up episodes and keep him performing at his best.
Diet:
By eliminating carbohydrates and adding fat, you'll limit the
amount of sugar available to be converted into glycogen in the
muscles. A low-starch/high-fat diet is critical for horses
diagnosed with PSSM, and can be beneficial for horses with RER as
well. Avoid high-starch feeds including corn, wheat, oats,
barley, and molasses, and feed 15 to 20 percent of the daily
calories as fat.
The most straightforward way to accomplish this goal is to feed a
lowstarch concentrate ration, with 2 to 3 cups of corn oil added
daily. Unfortunately, that's not always as easy as it seems. Your
horse may turn up his nose in disgust when offered this food
selection, and you may find the greasy goo a frustrating
challenge when it comes to feedroom maintenance. For a list of
optional sources of fat to add to your horse's diet, refer to the
sidebar up above.
Exercise suppresses uptake of sugars and enhances sugar
metabolism, which is beneficial for any horse that may tie up.
As an added benefit, PSSM horses are able to use excess muscle
glycogen as fuel-meaning increased exercise will result in
decreased accumulation. In general, regular daily exercise
without a day off is recommended for any horse diagnosed
with a muscle disease. Full-time pasture turn out is also
beneficial. At minimum, an affected horse's stall should have an
attached paddock that allows him to move freely throughout the
day. Studies have shown that 50 percent of horses diagnosed with
PSSM will improve with diet changes alone, while 90
percent show improvement with diet changes and exercise.
Supplements:
Blood levels of selenium and vitamin E are also important in
horses that tie up. Simple blood tests can be run to check for
vitamin E and selenium levels, and both can be supplemented
easily if levels are low. Because of the narrow toxicity range of
selenium, levels should be rechecked following diet adjustments
and reduced if needed. In some horses, selenium levels will
increase following diet changes, even without additional
supplementation.
Medications:
Other than treatment necessary during an actual tie-up episode,
there are no medications recommended for routine management of
PSSM. However, if your horse fits in the RER category of muscle
disease, there are several things that can be helpful. Fillies
may improve when maintained on progesterone, and light sedation
administered prior to an exercise session may reduce the chances
for a tie-up episode. In addition, dantrolene, a medication that
helps regulate calcium within the cells, can be administered
prior to exercise to reduce the risk of a tie-up episode.
..........
To be continued from time to time
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