Page 115 - June 2022

Quarter Horses aged 4-5 years and in Warmblood and Draft horses aged 8–11 years.
Following exercise, normal horses slowly absorb glucose from the bowel and slowly replenish metabolized glycogen in muscle to normal resting levels. In contrast, a PSSM
horse forms an overly compensatory response that packs glycogen into the muscles to twice the normal level. Muscles of a PSSM horse
are highly sensitive to insulin as early as six months of age. This amplified insulin response transports more glucose into muscle cells, particularly when fed high-starch feeds. The Type 1 PSSM-affected horse absorbs glucose from the bloodstream and transports it to muscle tissues more quickly than the normal horse. If glucose is not immediately used, the overactive enzyme, glucose synthase, stores it as excess glycogen packets, which are visible in a stained biopsy sample viewed with a microscope.
Once a diagnosis is obtained, prevention
of future episodes relies on management with strict diet and exercise protocols. These horses
are always at risk of underlying muscle soreness, but at least 75% may improve with diet and exercise. It is best to give a PSSM horse turn out and resist the urge to stall or paddock confine
the horse. Confined PSSM horses experience
CK elevations, particularly within the first five minutes of exercise. Horses turned out to exercise at will, with the ability to walk about to forage, do not experience elevations in CK enzymes. Free exercise is a key component to keeping a PSSM horse comfortable over the long term. Additional exercise applied through careful conditioning programs facilitate adaptation to steadily increasing exercise demands.
DIETARY MANAGEMENT OF PSSM
An affected horse is not always one that consumes a high-grain diet, although high carbohydrate and high starch diets exacerbate episodes and amplify signs in both Type 1 and
2 PSSM. Dietary modifications essential to managing PSSM include feeding a low-sugar and low-starch (non-structural carbohydrate or NSC) diet, with less than 10% of a horse’s daily energy fed as sugar or starch. The NSC level of hay fed to PSSM horses shouldn’t exceed 12%
-- higher amounts stimulate greater insulin responses that exacerbate glycogen storage in muscle tissue. Small, frequent meals work best to eliminate the generation of high blood glucose surges from a meal. Avoid high-starch and high-
Daily exercise is critical for polysaccharide storage myopathy horses, especially Quarter Horses with low-oxidative enzymes which must be exercised and trained to efficiently burn fat.
sugar feeds, like sweet feed, grains, and molasses. Additional caloric needs are best offered
as fat sources (oil and/or rice bran) as energy substitutions. This lessens muscle uptake of glucose while providing free fatty acids for muscle use during aerobic exercise. As much as 10-15% fat can be added for more calories. Vegetable oils – safflower, corn, soybean, canola, flaxseed, linseed, fish, peanut and coconut oil – can be fed at amounts of 1/2 – 1 cup per day. Commercial rice bran products and low-starch and high-fat commercial feeds are also available as fat sources.
For a horse in fat body condition, take caution in feeding a high-fat diet – this horse needs caloric balance with restriction. Feed hay in amounts less than 1% of body weight and/or provide low-starch, fat-supplemented feed that is high in fiber. Use of a grazing muzzle during turnout allows some exercise yet restricts food intake to help with weight control. Rather than feeding an overweight horse fat for energy utilization, free fatty acids are amplified by fasting the horse for six hours prior to exercise.
Solely changing the diet will not achieve results in PSSM horses. Regardless of breed, a PSSM horse must be exercised regularly to utilize glucose and improve energy metabolism in muscles. Daily exercise is critical, especially in Quarter Horses with
low-oxidative enzymes – these horses must be exercised and trained to efficiently burn fat.
A safe approach gradually increases a horse’s exercise to at least 30 minutes daily, five days a week. To achieve the best results possible, follow dietary recommendations along with routine exercise and regular turnout. Dietary management combined with exercise can limit and possibly prevent bouts of tying-up in 75% of PSSM-affected horses. Results from these modifications are evident as early as three weeks but may take as long as four months
to be realized. An owner of a PSSM-affected horse needs to commit to maintaining a horse’s steady exercise and conditioning program.
TAKE HOME MESSAGE
To facilitate management of equine muscle disorders in the equine athlete, it is important to achieve an accurate diagnosis of the problem. This is accomplished via blood chemistries, genetic testing, muscle biopsy and muscle- nerve impulse testing along with understanding breed predilection for specific problems.
Once an accurate diagnosis is obtained,
then management and dietary strategies can minimize abnormal episodes related to muscle disease. Many significant equine muscle disorders respond, at least in part, to careful and aggressive dietary manipulation.
VETERINARY VIEWS
Dietary modifications essential to managing Polysaccharide Storage Myopathy include feeding a low-sugar and low-starch (non-structural carbohydrate or NSC) diet, with less than 10% of a horse’s daily energy fed as sugar or starch.
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© Susan Bachelor, Speedhorse