Polysaccharide storage myopathy (PSSM) is a glycogen storage disorder in horses that causes muscles to cramp. It occurs primarily in horses with Quarter Horse bloodlines, like Paints and Appaloosas, but it can also occur in draft horses, draft crosses and Warmbloods.
In normal horses, insulin goes from the blood and is stored as glycogen in the muscle and liver cells. In horses affected by PSSM, a large amount of sugar (glycogen) is stored in the muscle and up to four times the typical amount of polysaccharide (an abnormal form of sugar) accumulates in the muscles.
Horses with PSSM are generally in good weight and have a quiet temperament. They often experience a PSSM episode when they begin training or go back into training after a layup, when their movement is restricted. The episode normally comes on after the horse has been walking and trotting for about 20 minutes.
When a horse has a PSSM episode, their muscles get very stiff and hard, especially over their hindquarters; they will sweat profusely, refuse to move, and their flanks may tremble. When they stop moving, the horse may stretch out as if urinating. Foals with PSSM often show signs of muscle pain and weakness when they have diarrhea or an infection like pneumonia.
Horses that have PSSM should not be fed sweet feed, wheat, oats, barley, molasses or feeds high in starch. They should also be exercised daily. Exercise encourages the body to use glucose properly and improves energy metabolism in skeletal muscle.
Researchers have found that if horses have only their diet adjusted, about 50 percent will improve. If both diet and exercise are adjusted, more than 75 percent of horses with PSSM will have few or no tying-up episodes. Though it may seem counter-intuitive, horses should be turned out and encouraged to move as soon as a PSSM episode has dissipated.
Read more at Horse Journals.
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