Though the clinical signs of the disease are difficult to miss, the gold standard for diagnosis of PSSM remains muscle biopsy, as microscopic examination of muscle tissues reveals the presence of muscle damage with excessive normal glycogen (considered grade 1) or muscle damage with abnormal polysaccharide (grade 2).
Horses with extreme PSSM may be unable to stand. The most severe cases are characterized by profuse sweating, elevated respiration and heart rate, muscle twitching, refusal to move or only walk in a slow, wooden fashion, and off-colored, reddish-brown urine. As pain increases, gaits frequently change, becoming stilted with a shortened stride, and muscles of the hindquarters become firm and sore. Mild signs include unwillingness to work, reluctance to engage the hindquarters, shifting lameness, and stopping and stretching as if to urinate. The effects of PSSM manifest during exercise, usually 10 to 30 minutes after onset, and mirror many of those observed in other forms of tying-up. Researchers have not yet been able to identify the cause for different forms of the disease in spite of biochemical studies. Forms of PSSM that are not associated with this gene mutation are distinguished as type 2. Type 1 involves a mutation in the glycogen synthase-1 gene, which causes abnormal increased glycogen synthesis in skeletal muscles. PSSM can be divided into two distinct types. Second, the presence of an abnormal polysaccharide may also occur in some horses with the disorder. While skeletal muscle is a major reservoir of glycogen, too much causes problems, including PSSM. As a polysaccharide, glycogen serves as the primary storage form of glucose in horses. First, an abnormal accumulation of glycogen occurs in the muscle. Simply put, PSSM arises from two specific glitches in skeletal muscle metabolism. Advances in genetic research have made diagnosis straightforward in many cases, and feeding trials have fine-tuned nutritional approaches so that affected horses can lead normal, useful lives. In a recent survey, as many as 12% of healthy Quarter Horses are thought to be genetically predisposed to the muscle disorder which, at its worst, can be debilitating and often career-ending for ridden horses. Among light horses, polysaccharide storage myopathy (PSSM) primarily affects Quarter Horses and members of related breeds such as Paints and Appaloosas.
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